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··· 42 42 <tr> 43 43 <td class=table-post> 44 44 <div class="date"> 45 - 05/10 — 2021 45 + 26/01 — 2022 46 46 </div> 47 - <a href="/posts/novice_nix:_flake_templates" class="post-link"> 48 - <span class="post-link">Novice Nix: Flake Templates</span> 47 + <a href="/posts/lightweight_linting" class="post-link"> 48 + <span class="post-link">Lightweight Linting</span> 49 49 </a> 50 50 </td> 51 51 <td class=table-stats> 52 52 <span class="stats-number"> 53 - 5.5 53 + 8.5 54 54 </span> 55 55 <span class=stats-unit>min</span> 56 56 </td> ··· 59 59 <tr> 60 60 <td class=table-post> 61 61 <div class="date"> 62 - 11/04 — 2021 62 + 05/10 — 2021 63 63 </div> 64 - <a href="/posts/SDL2_devlog" class="post-link"> 65 - <span class="post-link">SDL2 Devlog</span> 64 + <a href="/posts/novice_nix:_flake_templates" class="post-link"> 65 + <span class="post-link">Novice Nix: Flake Templates</span> 66 66 </a> 67 67 </td> 68 68 <td class=table-stats> 69 69 <span class="stats-number"> 70 - 10.0 70 + 5.5 71 71 </span> 72 72 <span class=stats-unit>min</span> 73 73 </td>
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··· 12 12 <language>en-us</language> 13 13 <copyright>Creative Commons BY-NC-SA 4.0</copyright> 14 14 <item> 15 + <title>Lightweight Linting</title> 16 + <description>&lt;p&gt;&lt;a href="https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries"&gt;Tree-sitter&lt;/a&gt; queries allow you to search for patterns in syntax trees, much like a regex would, in text. Combine that with some Rust glue to write simple, custom linters.&lt;/p&gt; 17 + &lt;h3 id="tree-sitter-syntax-trees"&gt;Tree-sitter syntax trees&lt;/h3&gt; 18 + &lt;p&gt;Here is a quick crash course on syntax trees generated by tree-sitter. Syntax trees produced by tree-sitter are represented by S-expressions. The generated S-expression for the following Rust code,&lt;/p&gt; 19 + &lt;div class="sourceCode" id="cb1"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb1-1"&gt;&lt;a href="#cb1-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;fn&lt;/span&gt; main() &lt;span class="op"&gt;{&lt;/span&gt;&lt;/span&gt; 20 + &lt;span id="cb1-2"&gt;&lt;a href="#cb1-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; x &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="dv"&gt;2&lt;/span&gt;&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 21 + &lt;span id="cb1-3"&gt;&lt;a href="#cb1-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="op"&gt;}&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 22 + &lt;p&gt;would be:&lt;/p&gt; 23 + &lt;div class="sourceCode" id="cb2"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb2-1"&gt;&lt;a href="#cb2-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(source_file&lt;/span&gt; 24 + &lt;span id="cb2-2"&gt;&lt;a href="#cb2-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (function_item&lt;/span&gt; 25 + &lt;span id="cb2-3"&gt;&lt;a href="#cb2-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; name: (identifier)&lt;/span&gt; 26 + &lt;span id="cb2-4"&gt;&lt;a href="#cb2-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; parameters: (parameters)&lt;/span&gt; 27 + &lt;span id="cb2-5"&gt;&lt;a href="#cb2-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; body: &lt;/span&gt; 28 + &lt;span id="cb2-6"&gt;&lt;a href="#cb2-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (block&lt;/span&gt; 29 + &lt;span id="cb2-7"&gt;&lt;a href="#cb2-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (let_declaration &lt;/span&gt; 30 + &lt;span id="cb2-8"&gt;&lt;a href="#cb2-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier)&lt;/span&gt; 31 + &lt;span id="cb2-9"&gt;&lt;a href="#cb2-9" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (integer_literal)))))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 32 + &lt;p&gt;Syntax trees generated by tree-sitter have a couple of other cool properties: they are &lt;em&gt;lossless&lt;/em&gt; syntax trees. Given a lossless syntax tree, you can regenerate the original source code in its entirety. Consider the following addition to our example:&lt;/p&gt; 33 + &lt;div class="sourceCode" id="cb3"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb3-1"&gt;&lt;a href="#cb3-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;fn&lt;/span&gt; main() &lt;span class="op"&gt;{&lt;/span&gt;&lt;/span&gt; 34 + &lt;span id="cb3-2"&gt;&lt;a href="#cb3-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="op"&gt;+&lt;/span&gt; &lt;span class="co"&gt;// a comment goes here&lt;/span&gt;&lt;/span&gt; 35 + &lt;span id="cb3-3"&gt;&lt;a href="#cb3-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; x &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="dv"&gt;2&lt;/span&gt;&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 36 + &lt;span id="cb3-4"&gt;&lt;a href="#cb3-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;}&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 37 + &lt;p&gt;The tree-sitter syntax tree preserves the comment, while the typical abstract syntax tree wouldn’t:&lt;/p&gt; 38 + &lt;div class="sourceCode" id="cb4"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb4-1"&gt;&lt;a href="#cb4-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (source_file&lt;/span&gt; 39 + &lt;span id="cb4-2"&gt;&lt;a href="#cb4-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (function_item&lt;/span&gt; 40 + &lt;span id="cb4-3"&gt;&lt;a href="#cb4-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; name: (identifier)&lt;/span&gt; 41 + &lt;span id="cb4-4"&gt;&lt;a href="#cb4-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; parameters: (parameters)&lt;/span&gt; 42 + &lt;span id="cb4-5"&gt;&lt;a href="#cb4-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; body:&lt;/span&gt; 43 + &lt;span id="cb4-6"&gt;&lt;a href="#cb4-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (block&lt;/span&gt; 44 + &lt;span id="cb4-7"&gt;&lt;a href="#cb4-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="op"&gt;+&lt;/span&gt; (line_comment)&lt;/span&gt; 45 + &lt;span id="cb4-8"&gt;&lt;a href="#cb4-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (let_declaration&lt;/span&gt; 46 + &lt;span id="cb4-9"&gt;&lt;a href="#cb4-9" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier)&lt;/span&gt; 47 + &lt;span id="cb4-10"&gt;&lt;a href="#cb4-10" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (integer_literal)))))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 48 + &lt;h3 id="tree-sitter-queries"&gt;Tree-sitter queries&lt;/h3&gt; 49 + &lt;p&gt;Tree-sitter provides a DSL to match over CSTs. These queries resemble our S-expression syntax trees, here is a query to match all line comments in a Rust CST:&lt;/p&gt; 50 + &lt;div class="sourceCode" id="cb5"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb5-1"&gt;&lt;a href="#cb5-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(line_comment)&lt;/span&gt; 51 + &lt;span id="cb5-2"&gt;&lt;a href="#cb5-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 52 + &lt;span id="cb5-3"&gt;&lt;a href="#cb5-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; matches the following rust code&lt;/span&gt;&lt;/span&gt; 53 + &lt;span id="cb5-4"&gt;&lt;a href="#cb5-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; // a comment goes here&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 54 + &lt;p&gt;Neat, eh? But don’t take my word for it, give it a go on the &lt;a href="https://tree-sitter.github.io/tree-sitter/playground"&gt;tree-sitter playground&lt;/a&gt;. Type in a query like so:&lt;/p&gt; 55 + &lt;div class="sourceCode" id="cb6"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb6-1"&gt;&lt;a href="#cb6-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; the web playground requires you to specify a &amp;quot;capture&amp;quot;&lt;/span&gt;&lt;/span&gt; 56 + &lt;span id="cb6-2"&gt;&lt;a href="#cb6-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; you will notice the capture and the nodes it captured&lt;/span&gt;&lt;/span&gt; 57 + &lt;span id="cb6-3"&gt;&lt;a href="#cb6-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; turn blue&lt;/span&gt;&lt;/span&gt; 58 + &lt;span id="cb6-4"&gt;&lt;a href="#cb6-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(line_comment) @capture&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 59 + &lt;p&gt;Here’s another to match &lt;code&gt;let&lt;/code&gt; expressions that bind an integer to an identifier:&lt;/p&gt; 60 + &lt;div class="sourceCode" id="cb7"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb7-1"&gt;&lt;a href="#cb7-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(let_declaration&lt;/span&gt; 61 + &lt;span id="cb7-2"&gt;&lt;a href="#cb7-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier)&lt;/span&gt; 62 + &lt;span id="cb7-3"&gt;&lt;a href="#cb7-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (integer_literal))&lt;/span&gt; 63 + &lt;span id="cb7-4"&gt;&lt;a href="#cb7-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;/span&gt; 64 + &lt;span id="cb7-5"&gt;&lt;a href="#cb7-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; matches:&lt;/span&gt;&lt;/span&gt; 65 + &lt;span id="cb7-6"&gt;&lt;a href="#cb7-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = 2;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 66 + &lt;p&gt;We can &lt;em&gt;capture&lt;/em&gt; nodes into variables:&lt;/p&gt; 67 + &lt;div class="sourceCode" id="cb8"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb8-1"&gt;&lt;a href="#cb8-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(let_declaration &lt;/span&gt; 68 + &lt;span id="cb8-2"&gt;&lt;a href="#cb8-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier) @my-capture&lt;/span&gt; 69 + &lt;span id="cb8-3"&gt;&lt;a href="#cb8-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (integer_literal))&lt;/span&gt; 70 + &lt;span id="cb8-4"&gt;&lt;a href="#cb8-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;/span&gt; 71 + &lt;span id="cb8-5"&gt;&lt;a href="#cb8-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; matches:&lt;/span&gt;&lt;/span&gt; 72 + &lt;span id="cb8-6"&gt;&lt;a href="#cb8-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = 2;&lt;/span&gt;&lt;/span&gt; 73 + &lt;span id="cb8-7"&gt;&lt;a href="#cb8-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 74 + &lt;span id="cb8-8"&gt;&lt;a href="#cb8-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; captures:&lt;/span&gt;&lt;/span&gt; 75 + &lt;span id="cb8-9"&gt;&lt;a href="#cb8-9" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; foo&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 76 + &lt;p&gt;And apply certain &lt;em&gt;predicates&lt;/em&gt; to captures:&lt;/p&gt; 77 + &lt;div class="sourceCode" id="cb9"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb9-1"&gt;&lt;a href="#cb9-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((let_declaration&lt;/span&gt; 78 + &lt;span id="cb9-2"&gt;&lt;a href="#cb9-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier) @my-capture&lt;/span&gt; 79 + &lt;span id="cb9-3"&gt;&lt;a href="#cb9-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (integer_literal))&lt;/span&gt; 80 + &lt;span id="cb9-4"&gt;&lt;a href="#cb9-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (&lt;span class="sc"&gt;#e&lt;/span&gt;q? @my-capture &lt;span class="st"&gt;&amp;quot;foo&amp;quot;&lt;/span&gt;))&lt;/span&gt; 81 + &lt;span id="cb9-5"&gt;&lt;a href="#cb9-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;/span&gt; 82 + &lt;span id="cb9-6"&gt;&lt;a href="#cb9-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; matches:&lt;/span&gt;&lt;/span&gt; 83 + &lt;span id="cb9-7"&gt;&lt;a href="#cb9-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = 2;&lt;/span&gt;&lt;/span&gt; 84 + &lt;span id="cb9-8"&gt;&lt;a href="#cb9-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 85 + &lt;span id="cb9-9"&gt;&lt;a href="#cb9-9" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; and not:&lt;/span&gt;&lt;/span&gt; 86 + &lt;span id="cb9-10"&gt;&lt;a href="#cb9-10" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let bar = 2;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 87 + &lt;p&gt;The &lt;code&gt;#match?&lt;/code&gt; predicate checks if a capture matches a regex:&lt;/p&gt; 88 + &lt;div class="sourceCode" id="cb10"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb10-1"&gt;&lt;a href="#cb10-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((let_declaration&lt;/span&gt; 89 + &lt;span id="cb10-2"&gt;&lt;a href="#cb10-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier) @my-capture&lt;/span&gt; 90 + &lt;span id="cb10-3"&gt;&lt;a href="#cb10-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (integer_literal))&lt;/span&gt; 91 + &lt;span id="cb10-4"&gt;&lt;a href="#cb10-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (#match? @my-capture &lt;span class="st"&gt;&amp;quot;foo|bar&amp;quot;&lt;/span&gt;))&lt;/span&gt; 92 + &lt;span id="cb10-5"&gt;&lt;a href="#cb10-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;/span&gt; 93 + &lt;span id="cb10-6"&gt;&lt;a href="#cb10-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; matches both `foo` and `bar`:&lt;/span&gt;&lt;/span&gt; 94 + &lt;span id="cb10-7"&gt;&lt;a href="#cb10-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = 2;&lt;/span&gt;&lt;/span&gt; 95 + &lt;span id="cb10-8"&gt;&lt;a href="#cb10-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let bar = 2;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 96 + &lt;p&gt;Exhibit indifference, as a stoic programmer would, with the &lt;em&gt;wildcard&lt;/em&gt; pattern:&lt;/p&gt; 97 + &lt;div class="sourceCode" id="cb11"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb11-1"&gt;&lt;a href="#cb11-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(let_declaration&lt;/span&gt; 98 + &lt;span id="cb11-2"&gt;&lt;a href="#cb11-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; pattern: (identifier)&lt;/span&gt; 99 + &lt;span id="cb11-3"&gt;&lt;a href="#cb11-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; value: (&lt;span class="op"&gt;_&lt;/span&gt;))&lt;/span&gt; 100 + &lt;span id="cb11-4"&gt;&lt;a href="#cb11-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;/span&gt; 101 + &lt;span id="cb11-5"&gt;&lt;a href="#cb11-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; matches:&lt;/span&gt;&lt;/span&gt; 102 + &lt;span id="cb11-6"&gt;&lt;a href="#cb11-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = &amp;quot;foo&amp;quot;;&lt;/span&gt;&lt;/span&gt; 103 + &lt;span id="cb11-7"&gt;&lt;a href="#cb11-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = 42;&lt;/span&gt;&lt;/span&gt; 104 + &lt;span id="cb11-8"&gt;&lt;a href="#cb11-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;; let foo = bar;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 105 + &lt;p&gt;&lt;a href="https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries"&gt;The documentation&lt;/a&gt; does the tree-sitter query DSL more justice, but we now know enough to write our first lint.&lt;/p&gt; 106 + &lt;h3 id="write-you-a-tree-sitter-lint"&gt;Write you a tree-sitter lint&lt;/h3&gt; 107 + &lt;p&gt;Strings in &lt;code&gt;std::env&lt;/code&gt; functions are error prone:&lt;/p&gt; 108 + &lt;div class="sourceCode" id="cb12"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb12-1"&gt;&lt;a href="#cb12-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="pp"&gt;std::env::&lt;/span&gt;remove_var(&lt;span class="st"&gt;&amp;quot;RUST_BACKTACE&amp;quot;&lt;/span&gt;)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 109 + &lt;span id="cb12-2"&gt;&lt;a href="#cb12-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="co"&gt;// ^^^^ &amp;quot;TACE&amp;quot; instead of &amp;quot;TRACE&amp;quot;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 110 + &lt;p&gt;I prefer this instead:&lt;/p&gt; 111 + &lt;div class="sourceCode" id="cb13"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb13-1"&gt;&lt;a href="#cb13-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;// somewhere in a module that is well spellchecked&lt;/span&gt;&lt;/span&gt; 112 + &lt;span id="cb13-2"&gt;&lt;a href="#cb13-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;static&lt;/span&gt; BACKTRACE&lt;span class="op"&gt;:&lt;/span&gt; &lt;span class="op"&gt;&amp;amp;&lt;/span&gt;&lt;span class="dt"&gt;str&lt;/span&gt; &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="st"&gt;&amp;quot;RUST_BACKTRACE&amp;quot;&lt;/span&gt;&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 113 + &lt;span id="cb13-3"&gt;&lt;a href="#cb13-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 114 + &lt;span id="cb13-4"&gt;&lt;a href="#cb13-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;// rest of the codebase&lt;/span&gt;&lt;/span&gt; 115 + &lt;span id="cb13-5"&gt;&lt;a href="#cb13-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="pp"&gt;std::env::&lt;/span&gt;remove_var(BACKTRACE)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 116 + &lt;p&gt;Let’s write a lint to find &lt;code&gt;std::env&lt;/code&gt; functions that use strings. Put aside the effectiveness of this lint for the moment, and take a stab at writing a tree-sitter query. For reference, a function call like so:&lt;/p&gt; 117 + &lt;div class="sourceCode" id="cb14"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb14-1"&gt;&lt;a href="#cb14-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;remove_var(&lt;span class="st"&gt;&amp;quot;RUST_BACKTRACE&amp;quot;&lt;/span&gt;)&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 118 + &lt;p&gt;Produces the following S-expression:&lt;/p&gt; 119 + &lt;div class="sourceCode" id="cb15"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb15-1"&gt;&lt;a href="#cb15-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(call_expression&lt;/span&gt; 120 + &lt;span id="cb15-2"&gt;&lt;a href="#cb15-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; function: (identifier)&lt;/span&gt; 121 + &lt;span id="cb15-3"&gt;&lt;a href="#cb15-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; arguments: (arguments (string_literal)))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 122 + &lt;p&gt;We are definitely looking for a &lt;code&gt;call_expression&lt;/code&gt;:&lt;/p&gt; 123 + &lt;div class="sourceCode" id="cb16"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb16-1"&gt;&lt;a href="#cb16-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;(call_expression) @raise&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 124 + &lt;p&gt;Whose function name matches &lt;code&gt;std::env::var&lt;/code&gt; or &lt;code&gt;std::env::remove_var&lt;/code&gt; at the very least (I know, I know, this isn’t the most optimal regex):&lt;/p&gt; 125 + &lt;div class="sourceCode" id="cb17"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb17-1"&gt;&lt;a href="#cb17-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((call_expression&lt;/span&gt; 126 + &lt;span id="cb17-2"&gt;&lt;a href="#cb17-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; function: (&lt;span class="op"&gt;_&lt;/span&gt;) @fn-name) @raise&lt;/span&gt; 127 + &lt;span id="cb17-3"&gt;&lt;a href="#cb17-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (#match? @fn-name &lt;span class="st"&gt;&amp;quot;std::env::(var|remove_var)&amp;quot;&lt;/span&gt;))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 128 + &lt;p&gt;Let’s turn that &lt;code&gt;std::&lt;/code&gt; prefix optional:&lt;/p&gt; 129 + &lt;div class="sourceCode" id="cb18"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb18-1"&gt;&lt;a href="#cb18-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((call_expression&lt;/span&gt; 130 + &lt;span id="cb18-2"&gt;&lt;a href="#cb18-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; function: (&lt;span class="op"&gt;_&lt;/span&gt;) @fn-name) @raise&lt;/span&gt; 131 + &lt;span id="cb18-3"&gt;&lt;a href="#cb18-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (#match? @fn-name &lt;span class="st"&gt;&amp;quot;(std::|)env::(var|remove_var)&amp;quot;&lt;/span&gt;))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 132 + &lt;p&gt;And ensure that &lt;code&gt;arguments&lt;/code&gt; is a string:&lt;/p&gt; 133 + &lt;div class="sourceCode" id="cb19"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb19-1"&gt;&lt;a href="#cb19-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((call_expression&lt;/span&gt; 134 + &lt;span id="cb19-2"&gt;&lt;a href="#cb19-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; function: (&lt;span class="op"&gt;_&lt;/span&gt;) @fn-name&lt;/span&gt; 135 + &lt;span id="cb19-3"&gt;&lt;a href="#cb19-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; arguments: (arguments (string_literal)))&lt;/span&gt; 136 + &lt;span id="cb19-4"&gt;&lt;a href="#cb19-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (#match? @fn-name &lt;span class="st"&gt;&amp;quot;(std::|)env::(var|remove_var)&amp;quot;&lt;/span&gt;))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 137 + &lt;h3 id="running-our-linter"&gt;Running our linter&lt;/h3&gt; 138 + &lt;p&gt;We could always plug our query into the web playground, but let’s go a step further:&lt;/p&gt; 139 + &lt;div class="sourceCode" id="cb20"&gt;&lt;pre class="sourceCode bash"&gt;&lt;code class="sourceCode bash"&gt;&lt;span id="cb20-1"&gt;&lt;a href="#cb20-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="ex"&gt;cargo&lt;/span&gt; new &lt;span class="at"&gt;--bin&lt;/span&gt; toy-lint&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 140 + &lt;p&gt;Add &lt;code&gt;tree-sitter&lt;/code&gt; and &lt;code&gt;tree-sitter-rust&lt;/code&gt; to your dependencies:&lt;/p&gt; 141 + &lt;div class="sourceCode" id="cb21"&gt;&lt;pre class="sourceCode toml"&gt;&lt;code class="sourceCode toml"&gt;&lt;span id="cb21-1"&gt;&lt;a href="#cb21-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;# within Cargo.toml&lt;/span&gt;&lt;/span&gt; 142 + &lt;span id="cb21-2"&gt;&lt;a href="#cb21-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;[&lt;/span&gt;&lt;span class="dt"&gt;dependencies&lt;/span&gt;&lt;span class="kw"&gt;]&lt;/span&gt;&lt;/span&gt; 143 + &lt;span id="cb21-3"&gt;&lt;a href="#cb21-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="dt"&gt;tree-sitter&lt;/span&gt; &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="st"&gt;&amp;quot;0.20&amp;quot;&lt;/span&gt;&lt;/span&gt; 144 + &lt;span id="cb21-4"&gt;&lt;a href="#cb21-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 145 + &lt;span id="cb21-5"&gt;&lt;a href="#cb21-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;[&lt;/span&gt;&lt;span class="dt"&gt;dependencies&lt;/span&gt;&lt;span class="kw"&gt;.&lt;/span&gt;&lt;span class="dt"&gt;tree-sitter-rust&lt;/span&gt;&lt;span class="kw"&gt;]&lt;/span&gt;&lt;/span&gt; 146 + &lt;span id="cb21-6"&gt;&lt;a href="#cb21-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="dt"&gt;git&lt;/span&gt; &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="st"&gt;&amp;quot;https://github.com/tree-sitter/tree-sitter-rust&amp;quot;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 147 + &lt;p&gt;Let’s load in some Rust code to work with. As &lt;a href="https://en.wikipedia.org/wiki/Self-reference"&gt;an ode to Gödel&lt;/a&gt; (G&lt;code&gt;ode&lt;/code&gt;l?), why not load in our linter itself:&lt;/p&gt; 148 + &lt;div class="sourceCode" id="cb22"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb22-1"&gt;&lt;a href="#cb22-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;fn&lt;/span&gt; main() &lt;span class="op"&gt;{&lt;/span&gt;&lt;/span&gt; 149 + &lt;span id="cb22-2"&gt;&lt;a href="#cb22-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; src &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="pp"&gt;include_str!&lt;/span&gt;(&lt;span class="st"&gt;&amp;quot;main.rs&amp;quot;&lt;/span&gt;)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 150 + &lt;span id="cb22-3"&gt;&lt;a href="#cb22-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="op"&gt;}&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 151 + &lt;p&gt;Most tree-sitter APIs require a reference to a &lt;code&gt;Language&lt;/code&gt; struct, we will be working with Rust if you haven’t already guessed:&lt;/p&gt; 152 + &lt;div class="sourceCode" id="cb23"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb23-1"&gt;&lt;a href="#cb23-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;use&lt;/span&gt; &lt;span class="pp"&gt;tree_sitter::&lt;/span&gt;Language&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 153 + &lt;span id="cb23-2"&gt;&lt;a href="#cb23-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 154 + &lt;span id="cb23-3"&gt;&lt;a href="#cb23-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; rust_lang&lt;span class="op"&gt;:&lt;/span&gt; Language &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="pp"&gt;tree_sitter_rust::&lt;/span&gt;language()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 155 + &lt;p&gt;Enough scaffolding, let’s parse some Rust:&lt;/p&gt; 156 + &lt;div class="sourceCode" id="cb24"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb24-1"&gt;&lt;a href="#cb24-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;use&lt;/span&gt; &lt;span class="pp"&gt;tree_sitter::&lt;/span&gt;Parser&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 157 + &lt;span id="cb24-2"&gt;&lt;a href="#cb24-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 158 + &lt;span id="cb24-3"&gt;&lt;a href="#cb24-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; &lt;span class="kw"&gt;mut&lt;/span&gt; parser &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="pp"&gt;Parser::&lt;/span&gt;new()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 159 + &lt;span id="cb24-4"&gt;&lt;a href="#cb24-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;parser&lt;span class="op"&gt;.&lt;/span&gt;set_language(rust_lang)&lt;span class="op"&gt;.&lt;/span&gt;unwrap()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 160 + &lt;span id="cb24-5"&gt;&lt;a href="#cb24-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 161 + &lt;span id="cb24-6"&gt;&lt;a href="#cb24-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; parse_tree &lt;span class="op"&gt;=&lt;/span&gt; parser&lt;span class="op"&gt;.&lt;/span&gt;parse(&lt;span class="op"&gt;&amp;amp;&lt;/span&gt;src&lt;span class="op"&gt;,&lt;/span&gt; &lt;span class="cn"&gt;None&lt;/span&gt;)&lt;span class="op"&gt;.&lt;/span&gt;unwrap()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 162 + &lt;p&gt;The second argument to &lt;code&gt;Parser::parse&lt;/code&gt; may be of interest. Tree-sitter has this cool feature that allows for quick reparsing of existing parse trees if they contain edits. If you do happen to want to reparse a source file, you can pass in the old tree:&lt;/p&gt; 163 + &lt;div class="sourceCode" id="cb25"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb25-1"&gt;&lt;a href="#cb25-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;// if you wish to reparse instead of parse&lt;/span&gt;&lt;/span&gt; 164 + &lt;span id="cb25-2"&gt;&lt;a href="#cb25-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;old_tree&lt;span class="op"&gt;.&lt;/span&gt;edit(&lt;span class="co"&gt;/* redacted */&lt;/span&gt;)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 165 + &lt;span id="cb25-3"&gt;&lt;a href="#cb25-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 166 + &lt;span id="cb25-4"&gt;&lt;a href="#cb25-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;// generate shiny new reparsed tree&lt;/span&gt;&lt;/span&gt; 167 + &lt;span id="cb25-5"&gt;&lt;a href="#cb25-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; new_tree &lt;span class="op"&gt;=&lt;/span&gt; parser&lt;span class="op"&gt;.&lt;/span&gt;parse(&lt;span class="op"&gt;&amp;amp;&lt;/span&gt;src&lt;span class="op"&gt;,&lt;/span&gt; &lt;span class="cn"&gt;Some&lt;/span&gt;(old_tree))&lt;span class="op"&gt;.&lt;/span&gt;unwrap()&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 168 + &lt;p&gt;Anyhow (&lt;a href="http://github.com/dtolnay/anyhow"&gt;hah!&lt;/a&gt;), now that we have a parse tree, we can inspect it:&lt;/p&gt; 169 + &lt;div class="sourceCode" id="cb26"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb26-1"&gt;&lt;a href="#cb26-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="pp"&gt;println!&lt;/span&gt;(&lt;span class="st"&gt;&amp;quot;{}&amp;quot;&lt;/span&gt;&lt;span class="op"&gt;,&lt;/span&gt; parse_tree&lt;span class="op"&gt;.&lt;/span&gt;root_node()&lt;span class="op"&gt;.&lt;/span&gt;to_sexp())&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 170 + &lt;p&gt;Or better yet, run a query on it:&lt;/p&gt; 171 + &lt;div class="sourceCode" id="cb27"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb27-1"&gt;&lt;a href="#cb27-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;use&lt;/span&gt; &lt;span class="pp"&gt;tree_sitter::&lt;/span&gt;Query&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 172 + &lt;span id="cb27-2"&gt;&lt;a href="#cb27-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 173 + &lt;span id="cb27-3"&gt;&lt;a href="#cb27-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; query &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="pp"&gt;Query::&lt;/span&gt;new(&lt;/span&gt; 174 + &lt;span id="cb27-4"&gt;&lt;a href="#cb27-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; rust_lang&lt;span class="op"&gt;,&lt;/span&gt;&lt;/span&gt; 175 + &lt;span id="cb27-5"&gt;&lt;a href="#cb27-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="st"&gt;r#&amp;quot;&lt;/span&gt;&lt;/span&gt; 176 + &lt;span id="cb27-6"&gt;&lt;a href="#cb27-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="st"&gt; ((call_expression&lt;/span&gt;&lt;/span&gt; 177 + &lt;span id="cb27-7"&gt;&lt;a href="#cb27-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="st"&gt; function: (_) @fn-name&lt;/span&gt;&lt;/span&gt; 178 + &lt;span id="cb27-8"&gt;&lt;a href="#cb27-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="st"&gt; arguments: (arguments (string_literal))) @raise&lt;/span&gt;&lt;/span&gt; 179 + &lt;span id="cb27-9"&gt;&lt;a href="#cb27-9" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="st"&gt; (#match? @fn-name &amp;quot;(std::|)env::(var|remove_var)&amp;quot;))&lt;/span&gt;&lt;/span&gt; 180 + &lt;span id="cb27-10"&gt;&lt;a href="#cb27-10" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="st"&gt; &amp;quot;#&lt;/span&gt;&lt;/span&gt; 181 + &lt;span id="cb27-11"&gt;&lt;a href="#cb27-11" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;)&lt;/span&gt; 182 + &lt;span id="cb27-12"&gt;&lt;a href="#cb27-12" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="op"&gt;.&lt;/span&gt;unwrap()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 183 + &lt;p&gt;A &lt;code&gt;QueryCursor&lt;/code&gt; is tree-sitter’s way of maintaining state as we iterate through the matches or captures produced by running a query on the parse tree. Observe:&lt;/p&gt; 184 + &lt;div class="sourceCode" id="cb28"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb28-1"&gt;&lt;a href="#cb28-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;use&lt;/span&gt; &lt;span class="pp"&gt;tree_sitter::&lt;/span&gt;QueryCursor&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 185 + &lt;span id="cb28-2"&gt;&lt;a href="#cb28-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 186 + &lt;span id="cb28-3"&gt;&lt;a href="#cb28-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; &lt;span class="kw"&gt;mut&lt;/span&gt; query_cursor &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="pp"&gt;QueryCursor::&lt;/span&gt;new()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 187 + &lt;span id="cb28-4"&gt;&lt;a href="#cb28-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; all_matches &lt;span class="op"&gt;=&lt;/span&gt; query_cursor&lt;span class="op"&gt;.&lt;/span&gt;matches(&lt;/span&gt; 188 + &lt;span id="cb28-5"&gt;&lt;a href="#cb28-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;&amp;amp;&lt;/span&gt;query&lt;span class="op"&gt;,&lt;/span&gt;&lt;/span&gt; 189 + &lt;span id="cb28-6"&gt;&lt;a href="#cb28-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; parse_tree&lt;span class="op"&gt;.&lt;/span&gt;root_node()&lt;span class="op"&gt;,&lt;/span&gt;&lt;/span&gt; 190 + &lt;span id="cb28-7"&gt;&lt;a href="#cb28-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; src&lt;span class="op"&gt;.&lt;/span&gt;as_bytes()&lt;span class="op"&gt;,&lt;/span&gt;&lt;/span&gt; 191 + &lt;span id="cb28-8"&gt;&lt;a href="#cb28-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 192 + &lt;p&gt;We begin by passing our query to the cursor, followed by the “root node”, which is another way of saying, “start from the top”, and lastly, the source itself. If you have already taken a look at the C API, you will notice that the last argument, the source (known as the &lt;code&gt;TextProvider&lt;/code&gt;), is not required. The Rust bindings seem to require this argument to provide predicate functionality such as &lt;code&gt;#match?&lt;/code&gt; and &lt;code&gt;#eq?&lt;/code&gt;.&lt;/p&gt; 193 + &lt;p&gt;Do something with the matches:&lt;/p&gt; 194 + &lt;div class="sourceCode" id="cb29"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb29-1"&gt;&lt;a href="#cb29-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;// get the index of the capture named &amp;quot;raise&amp;quot;&lt;/span&gt;&lt;/span&gt; 195 + &lt;span id="cb29-2"&gt;&lt;a href="#cb29-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;let&lt;/span&gt; raise_idx &lt;span class="op"&gt;=&lt;/span&gt; query&lt;span class="op"&gt;.&lt;/span&gt;capture_index_for_name(&lt;span class="st"&gt;&amp;quot;raise&amp;quot;&lt;/span&gt;)&lt;span class="op"&gt;.&lt;/span&gt;unwrap()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 196 + &lt;span id="cb29-3"&gt;&lt;a href="#cb29-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;/span&gt; 197 + &lt;span id="cb29-4"&gt;&lt;a href="#cb29-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="kw"&gt;for&lt;/span&gt; each_match &lt;span class="kw"&gt;in&lt;/span&gt; all_matches &lt;span class="op"&gt;{&lt;/span&gt;&lt;/span&gt; 198 + &lt;span id="cb29-5"&gt;&lt;a href="#cb29-5" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="co"&gt;// iterate over all captures called &amp;quot;raise&amp;quot;&lt;/span&gt;&lt;/span&gt; 199 + &lt;span id="cb29-6"&gt;&lt;a href="#cb29-6" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="co"&gt;// ignore captures such as &amp;quot;fn-name&amp;quot;&lt;/span&gt;&lt;/span&gt; 200 + &lt;span id="cb29-7"&gt;&lt;a href="#cb29-7" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;for&lt;/span&gt; capture &lt;span class="kw"&gt;in&lt;/span&gt; each_match&lt;/span&gt; 201 + &lt;span id="cb29-8"&gt;&lt;a href="#cb29-8" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;.&lt;/span&gt;captures&lt;/span&gt; 202 + &lt;span id="cb29-9"&gt;&lt;a href="#cb29-9" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;.&lt;/span&gt;iter()&lt;/span&gt; 203 + &lt;span id="cb29-10"&gt;&lt;a href="#cb29-10" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;.&lt;/span&gt;filter(&lt;span class="op"&gt;|&lt;/span&gt;c&lt;span class="op"&gt;|&lt;/span&gt; c&lt;span class="op"&gt;.&lt;/span&gt;idx &lt;span class="op"&gt;==&lt;/span&gt; raise_idx)&lt;/span&gt; 204 + &lt;span id="cb29-11"&gt;&lt;a href="#cb29-11" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;{&lt;/span&gt;&lt;/span&gt; 205 + &lt;span id="cb29-12"&gt;&lt;a href="#cb29-12" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; range &lt;span class="op"&gt;=&lt;/span&gt; capture&lt;span class="op"&gt;.&lt;/span&gt;node&lt;span class="op"&gt;.&lt;/span&gt;range()&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 206 + &lt;span id="cb29-13"&gt;&lt;a href="#cb29-13" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; text &lt;span class="op"&gt;=&lt;/span&gt; &lt;span class="op"&gt;&amp;amp;&lt;/span&gt;src[range&lt;span class="op"&gt;.&lt;/span&gt;start_byte&lt;span class="op"&gt;..&lt;/span&gt;range&lt;span class="op"&gt;.&lt;/span&gt;end_byte]&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 207 + &lt;span id="cb29-14"&gt;&lt;a href="#cb29-14" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; line &lt;span class="op"&gt;=&lt;/span&gt; range&lt;span class="op"&gt;.&lt;/span&gt;start_point&lt;span class="op"&gt;.&lt;/span&gt;row&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 208 + &lt;span id="cb29-15"&gt;&lt;a href="#cb29-15" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="kw"&gt;let&lt;/span&gt; col &lt;span class="op"&gt;=&lt;/span&gt; range&lt;span class="op"&gt;.&lt;/span&gt;start_point&lt;span class="op"&gt;.&lt;/span&gt;column&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 209 + &lt;span id="cb29-16"&gt;&lt;a href="#cb29-16" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="pp"&gt;println!&lt;/span&gt;(&lt;/span&gt; 210 + &lt;span id="cb29-17"&gt;&lt;a href="#cb29-17" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="st"&gt;&amp;quot;[Line: {}, Col: {}] Offending source code: `{}`&amp;quot;&lt;/span&gt;&lt;span class="op"&gt;,&lt;/span&gt;&lt;/span&gt; 211 + &lt;span id="cb29-18"&gt;&lt;a href="#cb29-18" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; line&lt;span class="op"&gt;,&lt;/span&gt; col&lt;span class="op"&gt;,&lt;/span&gt; text&lt;/span&gt; 212 + &lt;span id="cb29-19"&gt;&lt;a href="#cb29-19" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; )&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt; 213 + &lt;span id="cb29-20"&gt;&lt;a href="#cb29-20" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; &lt;span class="op"&gt;}&lt;/span&gt;&lt;/span&gt; 214 + &lt;span id="cb29-21"&gt;&lt;a href="#cb29-21" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="op"&gt;}&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 215 + &lt;p&gt;Lastly, add the following line to your source code, to get the linter to catch something:&lt;/p&gt; 216 + &lt;div class="sourceCode" id="cb30"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb30-1"&gt;&lt;a href="#cb30-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="pp"&gt;env::&lt;/span&gt;remove_var(&lt;span class="st"&gt;&amp;quot;RUST_BACKTRACE&amp;quot;&lt;/span&gt;)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 217 + &lt;p&gt;And &lt;code&gt;cargo run&lt;/code&gt;:&lt;/p&gt; 218 + &lt;pre class="shell"&gt;&lt;code&gt;λ cargo run 219 + Compiling toy-lint v0.1.0 (/redacted/path/to/toy-lint) 220 + Finished dev [unoptimized + debuginfo] target(s) in 0.74s 221 + Running `target/debug/toy-lint` 222 + [Line: 40, Col: 4] Offending source code: `env::remove_var(&amp;quot;RUST_BACKTRACE&amp;quot;)`&lt;/code&gt;&lt;/pre&gt; 223 + &lt;p&gt;Thank you tree-sitter!&lt;/p&gt; 224 + &lt;h3 id="bonus"&gt;Bonus&lt;/h3&gt; 225 + &lt;p&gt;Keen readers will notice that I avoided &lt;code&gt;std::env::set_var&lt;/code&gt;. Because &lt;code&gt;set_var&lt;/code&gt; is called with two arguments, a “key” and a “value”, unlike &lt;code&gt;env::var&lt;/code&gt; and &lt;code&gt;env::remove_var&lt;/code&gt;. As a result, it requires more juggling:&lt;/p&gt; 226 + &lt;div class="sourceCode" id="cb32"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb32-1"&gt;&lt;a href="#cb32-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((call_expression&lt;/span&gt; 227 + &lt;span id="cb32-2"&gt;&lt;a href="#cb32-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; function: (&lt;span class="op"&gt;_&lt;/span&gt;) @fn-name&lt;/span&gt; 228 + &lt;span id="cb32-3"&gt;&lt;a href="#cb32-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; arguments: (arguments &lt;span class="op"&gt;.&lt;/span&gt; (string_literal)&lt;span class="op"&gt;?&lt;/span&gt; &lt;span class="op"&gt;.&lt;/span&gt; (string_literal) &lt;span class="op"&gt;.&lt;/span&gt;)) @raise&lt;/span&gt; 229 + &lt;span id="cb32-4"&gt;&lt;a href="#cb32-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (#match? @fn-name &lt;span class="st"&gt;&amp;quot;(std::|)env::(var|remove_var|set_var)&amp;quot;&lt;/span&gt;))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 230 + &lt;p&gt;The interesting part of this query is the humble &lt;code&gt;.&lt;/code&gt;, the &lt;em&gt;anchor&lt;/em&gt; operator. Anchors help constrain child nodes in certain ways. In this case, it ensures that we match exactly two &lt;code&gt;string_literal&lt;/code&gt;s who are siblings or exactly one &lt;code&gt;string_literal&lt;/code&gt; with no siblings. Unfortunately, this query also matches the following invalid Rust code:&lt;/p&gt; 231 + &lt;div class="sourceCode" id="cb33"&gt;&lt;pre class="sourceCode rust"&gt;&lt;code class="sourceCode rust"&gt;&lt;span id="cb33-1"&gt;&lt;a href="#cb33-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="co"&gt;// remove_var accepts only 1 arg!&lt;/span&gt;&lt;/span&gt; 232 + &lt;span id="cb33-2"&gt;&lt;a href="#cb33-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;&lt;span class="pp"&gt;std::env::&lt;/span&gt;remove_var(&lt;span class="st"&gt;&amp;quot;RUST_BACKTRACE&amp;quot;&lt;/span&gt;&lt;span class="op"&gt;,&lt;/span&gt; &lt;span class="st"&gt;&amp;quot;1&amp;quot;&lt;/span&gt;)&lt;span class="op"&gt;;&lt;/span&gt;&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt; 233 + &lt;h3 id="notes"&gt;Notes&lt;/h3&gt; 234 + &lt;p&gt;All-in-all, the query DSL does a great job in lowering the bar to writing language tools. The knowledge gained from mastering the query DSL can be applied to other languages that have tree-sitter grammars too. This query detects &lt;code&gt;to_json&lt;/code&gt; methods that do not accept additional arguments, in Ruby:&lt;/p&gt; 235 + &lt;div class="sourceCode" id="cb34"&gt;&lt;pre class="sourceCode scheme"&gt;&lt;code class="sourceCode scheme"&gt;&lt;span id="cb34-1"&gt;&lt;a href="#cb34-1" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt;((method&lt;/span&gt; 236 + &lt;span id="cb34-2"&gt;&lt;a href="#cb34-2" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; name: (identifier) @fn&lt;/span&gt; 237 + &lt;span id="cb34-3"&gt;&lt;a href="#cb34-3" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; !parameters)&lt;/span&gt; 238 + &lt;span id="cb34-4"&gt;&lt;a href="#cb34-4" aria-hidden="true" tabindex="-1"&gt;&lt;/a&gt; (&lt;span class="sc"&gt;#i&lt;/span&gt;s? @fn &lt;span class="st"&gt;&amp;quot;to_json&amp;quot;&lt;/span&gt;))&lt;/span&gt;&lt;/code&gt;&lt;/pre&gt;&lt;/div&gt;</description> 239 + <link>https://peppe.rs/posts/lightweight_linting/</link> 240 + <pubDate>Wed, 26 Jan 2022 12:52:00 +0000</pubDate> 241 + <guid>https://peppe.rs/posts/lightweight_linting/</guid> 242 + </item> 243 + <item> 15 244 <title>Novice Nix: Flake Templates</title> 16 245 <description>&lt;p&gt;Flakes are very handy to setup entirely pure, project-specific dependencies (not just dependencies, but build steps, shell environments and more) in a declarative way. Writing Flake expressions can get repetitive though, oftentimes, you’d much rather start off with a skeleton. Luckily, &lt;code&gt;nix&lt;/code&gt; already supports templates!&lt;/p&gt; 17 246 &lt;p&gt;You might already be familiar with &lt;code&gt;nix flake init&lt;/code&gt;, that drops a “default” flake expression into your current working directory. If you head over to the manpage:&lt;/p&gt;
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··· 1 + <!DOCTYPE html> 2 + <html lang="en"> 3 + <head> 4 + <link rel="stylesheet" href="/style.css"> 5 + <link rel="stylesheet" href="/syntax.css"> 6 + <meta charset="UTF-8"> 7 + <meta name="viewport" content="initial-scale=1"> 8 + <meta content="#ffffff" name="theme-color"> 9 + <meta name="HandheldFriendly" content="true"> 10 + <meta property="og:title" content="Lightweight Linting"> 11 + <meta property="og:type" content="website"> 12 + <meta property="og:description" content="a static site {for, by, about} me "> 13 + <meta property="og:url" content="https://peppe.rs"> 14 + <link rel="icon" type="image/x-icon" href="/favicon.png"> 15 + <title>Lightweight Linting · peppe.rs</title> 16 + <body> 17 + <div class="posts"> 18 + <div class="post"> 19 + <a href="/" class="post-end-link">Home</a> 20 + <span>/</span> 21 + <a href="/posts" class="post-end-link">Posts</a> 22 + <span>/</span> 23 + <a class="post-end-link">Lightweight Linting</a> 24 + <a class="stats post-end-link" href="https://git.peppe.rs/web/site/plain/posts/lightweight_linting.md 25 + ">View Raw</a> 26 + <div class="separator"></div> 27 + <div class="date"> 28 + 26/01 — 2022 29 + <div class="stats"> 30 + <span class="stats-number"> 31 + 170.62 32 + </span> 33 + <span class="stats-unit">cm</span> 34 + &nbsp 35 + <span class="stats-number"> 36 + 8.5 37 + </span> 38 + <span class="stats-unit">min</span> 39 + </div> 40 + </div> 41 + <h1> 42 + Lightweight Linting 43 + </h1> 44 + <div class="post-text"> 45 + <p><a href="https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries">Tree-sitter</a> queries allow you to search for patterns in syntax trees, much like a regex would, in text. Combine that with some Rust glue to write simple, custom linters.</p> 46 + <h3 id="tree-sitter-syntax-trees">Tree-sitter syntax trees</h3> 47 + <p>Here is a quick crash course on syntax trees generated by tree-sitter. Syntax trees produced by tree-sitter are represented by S-expressions. The generated S-expression for the following Rust code,</p> 48 + <div class="sourceCode" id="cb1"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">fn</span> main() <span class="op">{</span></span> 49 + <span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> x <span class="op">=</span> <span class="dv">2</span><span class="op">;</span></span> 50 + <span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div> 51 + <p>would be:</p> 52 + <div class="sourceCode" id="cb2"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>(source_file</span> 53 + <span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a> (function_item</span> 54 + <span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> name: (identifier)</span> 55 + <span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a> parameters: (parameters)</span> 56 + <span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a> body: </span> 57 + <span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a> (block</span> 58 + <span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a> (let_declaration </span> 59 + <span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span> 60 + <span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a> value: (integer_literal)))))</span></code></pre></div> 61 + <p>Syntax trees generated by tree-sitter have a couple of other cool properties: they are <em>lossless</em> syntax trees. Given a lossless syntax tree, you can regenerate the original source code in its entirety. Consider the following addition to our example:</p> 62 + <div class="sourceCode" id="cb3"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a> <span class="kw">fn</span> main() <span class="op">{</span></span> 63 + <span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="op">+</span> <span class="co">// a comment goes here</span></span> 64 + <span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> x <span class="op">=</span> <span class="dv">2</span><span class="op">;</span></span> 65 + <span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span></code></pre></div> 66 + <p>The tree-sitter syntax tree preserves the comment, while the typical abstract syntax tree wouldn’t:</p> 67 + <div class="sourceCode" id="cb4"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a> (source_file</span> 68 + <span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a> (function_item</span> 69 + <span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a> name: (identifier)</span> 70 + <span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a> parameters: (parameters)</span> 71 + <span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a> body:</span> 72 + <span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a> (block</span> 73 + <span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="op">+</span> (line_comment)</span> 74 + <span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a> (let_declaration</span> 75 + <span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span> 76 + <span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a> value: (integer_literal)))))</span></code></pre></div> 77 + <h3 id="tree-sitter-queries">Tree-sitter queries</h3> 78 + <p>Tree-sitter provides a DSL to match over CSTs. These queries resemble our S-expression syntax trees, here is a query to match all line comments in a Rust CST:</p> 79 + <div class="sourceCode" id="cb5"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>(line_comment)</span> 80 + <span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a></span> 81 + <span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="co">; matches the following rust code</span></span> 82 + <span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a><span class="co">; // a comment goes here</span></span></code></pre></div> 83 + <p>Neat, eh? But don’t take my word for it, give it a go on the <a href="https://tree-sitter.github.io/tree-sitter/playground">tree-sitter playground</a>. Type in a query like so:</p> 84 + <div class="sourceCode" id="cb6"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="co">; the web playground requires you to specify a &quot;capture&quot;</span></span> 85 + <span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="co">; you will notice the capture and the nodes it captured</span></span> 86 + <span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a><span class="co">; turn blue</span></span> 87 + <span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a>(line_comment) @capture</span></code></pre></div> 88 + <p>Here’s another to match <code>let</code> expressions that bind an integer to an identifier:</p> 89 + <div class="sourceCode" id="cb7"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>(let_declaration</span> 90 + <span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span> 91 + <span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span> 92 + <span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a> </span> 93 + <span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span> 94 + <span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span></code></pre></div> 95 + <p>We can <em>capture</em> nodes into variables:</p> 96 + <div class="sourceCode" id="cb8"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>(let_declaration </span> 97 + <span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier) @my-capture</span> 98 + <span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span> 99 + <span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a> </span> 100 + <span id="cb8-5"><a href="#cb8-5" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span> 101 + <span id="cb8-6"><a href="#cb8-6" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span> 102 + <span id="cb8-7"><a href="#cb8-7" aria-hidden="true" tabindex="-1"></a></span> 103 + <span id="cb8-8"><a href="#cb8-8" aria-hidden="true" tabindex="-1"></a><span class="co">; captures:</span></span> 104 + <span id="cb8-9"><a href="#cb8-9" aria-hidden="true" tabindex="-1"></a><span class="co">; foo</span></span></code></pre></div> 105 + <p>And apply certain <em>predicates</em> to captures:</p> 106 + <div class="sourceCode" id="cb9"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a>((let_declaration</span> 107 + <span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier) @my-capture</span> 108 + <span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span> 109 + <span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a> (<span class="sc">#e</span>q? @my-capture <span class="st">&quot;foo&quot;</span>))</span> 110 + <span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a> </span> 111 + <span id="cb9-6"><a href="#cb9-6" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span> 112 + <span id="cb9-7"><a href="#cb9-7" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span> 113 + <span id="cb9-8"><a href="#cb9-8" aria-hidden="true" tabindex="-1"></a></span> 114 + <span id="cb9-9"><a href="#cb9-9" aria-hidden="true" tabindex="-1"></a><span class="co">; and not:</span></span> 115 + <span id="cb9-10"><a href="#cb9-10" aria-hidden="true" tabindex="-1"></a><span class="co">; let bar = 2;</span></span></code></pre></div> 116 + <p>The <code>#match?</code> predicate checks if a capture matches a regex:</p> 117 + <div class="sourceCode" id="cb10"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a>((let_declaration</span> 118 + <span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier) @my-capture</span> 119 + <span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a> value: (integer_literal))</span> 120 + <span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a> (#match? @my-capture <span class="st">&quot;foo|bar&quot;</span>))</span> 121 + <span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a> </span> 122 + <span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a><span class="co">; matches both `foo` and `bar`:</span></span> 123 + <span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 2;</span></span> 124 + <span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a><span class="co">; let bar = 2;</span></span></code></pre></div> 125 + <p>Exhibit indifference, as a stoic programmer would, with the <em>wildcard</em> pattern:</p> 126 + <div class="sourceCode" id="cb11"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a>(let_declaration</span> 127 + <span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> pattern: (identifier)</span> 128 + <span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> value: (<span class="op">_</span>))</span> 129 + <span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> </span> 130 + <span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a><span class="co">; matches:</span></span> 131 + <span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = &quot;foo&quot;;</span></span> 132 + <span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = 42;</span></span> 133 + <span id="cb11-8"><a href="#cb11-8" aria-hidden="true" tabindex="-1"></a><span class="co">; let foo = bar;</span></span></code></pre></div> 134 + <p><a href="https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries">The documentation</a> does the tree-sitter query DSL more justice, but we now know enough to write our first lint.</p> 135 + <h3 id="write-you-a-tree-sitter-lint">Write you a tree-sitter lint</h3> 136 + <p>Strings in <code>std::env</code> functions are error prone:</p> 137 + <div class="sourceCode" id="cb12"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a><span class="pp">std::env::</span>remove_var(<span class="st">&quot;RUST_BACKTACE&quot;</span>)<span class="op">;</span></span> 138 + <span id="cb12-2"><a href="#cb12-2" aria-hidden="true" tabindex="-1"></a> <span class="co">// ^^^^ &quot;TACE&quot; instead of &quot;TRACE&quot;</span></span></code></pre></div> 139 + <p>I prefer this instead:</p> 140 + <div class="sourceCode" id="cb13"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="co">// somewhere in a module that is well spellchecked</span></span> 141 + <span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a><span class="kw">static</span> BACKTRACE<span class="op">:</span> <span class="op">&amp;</span><span class="dt">str</span> <span class="op">=</span> <span class="st">&quot;RUST_BACKTRACE&quot;</span><span class="op">;</span></span> 142 + <span id="cb13-3"><a href="#cb13-3" aria-hidden="true" tabindex="-1"></a></span> 143 + <span id="cb13-4"><a href="#cb13-4" aria-hidden="true" tabindex="-1"></a><span class="co">// rest of the codebase</span></span> 144 + <span id="cb13-5"><a href="#cb13-5" aria-hidden="true" tabindex="-1"></a><span class="pp">std::env::</span>remove_var(BACKTRACE)<span class="op">;</span></span></code></pre></div> 145 + <p>Let’s write a lint to find <code>std::env</code> functions that use strings. Put aside the effectiveness of this lint for the moment, and take a stab at writing a tree-sitter query. For reference, a function call like so:</p> 146 + <div class="sourceCode" id="cb14"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a>remove_var(<span class="st">&quot;RUST_BACKTRACE&quot;</span>)</span></code></pre></div> 147 + <p>Produces the following S-expression:</p> 148 + <div class="sourceCode" id="cb15"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a>(call_expression</span> 149 + <span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a> function: (identifier)</span> 150 + <span id="cb15-3"><a href="#cb15-3" aria-hidden="true" tabindex="-1"></a> arguments: (arguments (string_literal)))</span></code></pre></div> 151 + <p>We are definitely looking for a <code>call_expression</code>:</p> 152 + <div class="sourceCode" id="cb16"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb16-1"><a href="#cb16-1" aria-hidden="true" tabindex="-1"></a>(call_expression) @raise</span></code></pre></div> 153 + <p>Whose function name matches <code>std::env::var</code> or <code>std::env::remove_var</code> at the very least (I know, I know, this isn’t the most optimal regex):</p> 154 + <div class="sourceCode" id="cb17"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span> 155 + <span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name) @raise</span> 156 + <span id="cb17-3"><a href="#cb17-3" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">&quot;std::env::(var|remove_var)&quot;</span>))</span></code></pre></div> 157 + <p>Let’s turn that <code>std::</code> prefix optional:</p> 158 + <div class="sourceCode" id="cb18"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb18-1"><a href="#cb18-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span> 159 + <span id="cb18-2"><a href="#cb18-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name) @raise</span> 160 + <span id="cb18-3"><a href="#cb18-3" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">&quot;(std::|)env::(var|remove_var)&quot;</span>))</span></code></pre></div> 161 + <p>And ensure that <code>arguments</code> is a string:</p> 162 + <div class="sourceCode" id="cb19"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span> 163 + <span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name</span> 164 + <span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a> arguments: (arguments (string_literal)))</span> 165 + <span id="cb19-4"><a href="#cb19-4" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">&quot;(std::|)env::(var|remove_var)&quot;</span>))</span></code></pre></div> 166 + <h3 id="running-our-linter">Running our linter</h3> 167 + <p>We could always plug our query into the web playground, but let’s go a step further:</p> 168 + <div class="sourceCode" id="cb20"><pre class="sourceCode bash"><code class="sourceCode bash"><span id="cb20-1"><a href="#cb20-1" aria-hidden="true" tabindex="-1"></a><span class="ex">cargo</span> new <span class="at">--bin</span> toy-lint</span></code></pre></div> 169 + <p>Add <code>tree-sitter</code> and <code>tree-sitter-rust</code> to your dependencies:</p> 170 + <div class="sourceCode" id="cb21"><pre class="sourceCode toml"><code class="sourceCode toml"><span id="cb21-1"><a href="#cb21-1" aria-hidden="true" tabindex="-1"></a><span class="co"># within Cargo.toml</span></span> 171 + <span id="cb21-2"><a href="#cb21-2" aria-hidden="true" tabindex="-1"></a><span class="kw">[</span><span class="dt">dependencies</span><span class="kw">]</span></span> 172 + <span id="cb21-3"><a href="#cb21-3" aria-hidden="true" tabindex="-1"></a><span class="dt">tree-sitter</span> <span class="op">=</span> <span class="st">&quot;0.20&quot;</span></span> 173 + <span id="cb21-4"><a href="#cb21-4" aria-hidden="true" tabindex="-1"></a></span> 174 + <span id="cb21-5"><a href="#cb21-5" aria-hidden="true" tabindex="-1"></a><span class="kw">[</span><span class="dt">dependencies</span><span class="kw">.</span><span class="dt">tree-sitter-rust</span><span class="kw">]</span></span> 175 + <span id="cb21-6"><a href="#cb21-6" aria-hidden="true" tabindex="-1"></a><span class="dt">git</span> <span class="op">=</span> <span class="st">&quot;https://github.com/tree-sitter/tree-sitter-rust&quot;</span></span></code></pre></div> 176 + <p>Let’s load in some Rust code to work with. As <a href="https://en.wikipedia.org/wiki/Self-reference">an ode to Gödel</a> (G<code>ode</code>l?), why not load in our linter itself:</p> 177 + <div class="sourceCode" id="cb22"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb22-1"><a href="#cb22-1" aria-hidden="true" tabindex="-1"></a><span class="kw">fn</span> main() <span class="op">{</span></span> 178 + <span id="cb22-2"><a href="#cb22-2" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> src <span class="op">=</span> <span class="pp">include_str!</span>(<span class="st">&quot;main.rs&quot;</span>)<span class="op">;</span></span> 179 + <span id="cb22-3"><a href="#cb22-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div> 180 + <p>Most tree-sitter APIs require a reference to a <code>Language</code> struct, we will be working with Rust if you haven’t already guessed:</p> 181 + <div class="sourceCode" id="cb23"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb23-1"><a href="#cb23-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>Language<span class="op">;</span></span> 182 + <span id="cb23-2"><a href="#cb23-2" aria-hidden="true" tabindex="-1"></a></span> 183 + <span id="cb23-3"><a href="#cb23-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> rust_lang<span class="op">:</span> Language <span class="op">=</span> <span class="pp">tree_sitter_rust::</span>language()<span class="op">;</span></span></code></pre></div> 184 + <p>Enough scaffolding, let’s parse some Rust:</p> 185 + <div class="sourceCode" id="cb24"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb24-1"><a href="#cb24-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>Parser<span class="op">;</span></span> 186 + <span id="cb24-2"><a href="#cb24-2" aria-hidden="true" tabindex="-1"></a></span> 187 + <span id="cb24-3"><a href="#cb24-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> <span class="kw">mut</span> parser <span class="op">=</span> <span class="pp">Parser::</span>new()<span class="op">;</span></span> 188 + <span id="cb24-4"><a href="#cb24-4" aria-hidden="true" tabindex="-1"></a>parser<span class="op">.</span>set_language(rust_lang)<span class="op">.</span>unwrap()<span class="op">;</span></span> 189 + <span id="cb24-5"><a href="#cb24-5" aria-hidden="true" tabindex="-1"></a></span> 190 + <span id="cb24-6"><a href="#cb24-6" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> parse_tree <span class="op">=</span> parser<span class="op">.</span>parse(<span class="op">&amp;</span>src<span class="op">,</span> <span class="cn">None</span>)<span class="op">.</span>unwrap()<span class="op">;</span></span></code></pre></div> 191 + <p>The second argument to <code>Parser::parse</code> may be of interest. Tree-sitter has this cool feature that allows for quick reparsing of existing parse trees if they contain edits. If you do happen to want to reparse a source file, you can pass in the old tree:</p> 192 + <div class="sourceCode" id="cb25"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb25-1"><a href="#cb25-1" aria-hidden="true" tabindex="-1"></a><span class="co">// if you wish to reparse instead of parse</span></span> 193 + <span id="cb25-2"><a href="#cb25-2" aria-hidden="true" tabindex="-1"></a>old_tree<span class="op">.</span>edit(<span class="co">/* redacted */</span>)<span class="op">;</span></span> 194 + <span id="cb25-3"><a href="#cb25-3" aria-hidden="true" tabindex="-1"></a></span> 195 + <span id="cb25-4"><a href="#cb25-4" aria-hidden="true" tabindex="-1"></a><span class="co">// generate shiny new reparsed tree</span></span> 196 + <span id="cb25-5"><a href="#cb25-5" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> new_tree <span class="op">=</span> parser<span class="op">.</span>parse(<span class="op">&amp;</span>src<span class="op">,</span> <span class="cn">Some</span>(old_tree))<span class="op">.</span>unwrap()</span></code></pre></div> 197 + <p>Anyhow (<a href="http://github.com/dtolnay/anyhow">hah!</a>), now that we have a parse tree, we can inspect it:</p> 198 + <div class="sourceCode" id="cb26"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb26-1"><a href="#cb26-1" aria-hidden="true" tabindex="-1"></a><span class="pp">println!</span>(<span class="st">&quot;{}&quot;</span><span class="op">,</span> parse_tree<span class="op">.</span>root_node()<span class="op">.</span>to_sexp())<span class="op">;</span></span></code></pre></div> 199 + <p>Or better yet, run a query on it:</p> 200 + <div class="sourceCode" id="cb27"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb27-1"><a href="#cb27-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>Query<span class="op">;</span></span> 201 + <span id="cb27-2"><a href="#cb27-2" aria-hidden="true" tabindex="-1"></a></span> 202 + <span id="cb27-3"><a href="#cb27-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> query <span class="op">=</span> <span class="pp">Query::</span>new(</span> 203 + <span id="cb27-4"><a href="#cb27-4" aria-hidden="true" tabindex="-1"></a> rust_lang<span class="op">,</span></span> 204 + <span id="cb27-5"><a href="#cb27-5" aria-hidden="true" tabindex="-1"></a> <span class="st">r#&quot;</span></span> 205 + <span id="cb27-6"><a href="#cb27-6" aria-hidden="true" tabindex="-1"></a><span class="st"> ((call_expression</span></span> 206 + <span id="cb27-7"><a href="#cb27-7" aria-hidden="true" tabindex="-1"></a><span class="st"> function: (_) @fn-name</span></span> 207 + <span id="cb27-8"><a href="#cb27-8" aria-hidden="true" tabindex="-1"></a><span class="st"> arguments: (arguments (string_literal))) @raise</span></span> 208 + <span id="cb27-9"><a href="#cb27-9" aria-hidden="true" tabindex="-1"></a><span class="st"> (#match? @fn-name &quot;(std::|)env::(var|remove_var)&quot;))</span></span> 209 + <span id="cb27-10"><a href="#cb27-10" aria-hidden="true" tabindex="-1"></a><span class="st"> &quot;#</span></span> 210 + <span id="cb27-11"><a href="#cb27-11" aria-hidden="true" tabindex="-1"></a>)</span> 211 + <span id="cb27-12"><a href="#cb27-12" aria-hidden="true" tabindex="-1"></a><span class="op">.</span>unwrap()<span class="op">;</span></span></code></pre></div> 212 + <p>A <code>QueryCursor</code> is tree-sitter’s way of maintaining state as we iterate through the matches or captures produced by running a query on the parse tree. Observe:</p> 213 + <div class="sourceCode" id="cb28"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb28-1"><a href="#cb28-1" aria-hidden="true" tabindex="-1"></a><span class="kw">use</span> <span class="pp">tree_sitter::</span>QueryCursor<span class="op">;</span></span> 214 + <span id="cb28-2"><a href="#cb28-2" aria-hidden="true" tabindex="-1"></a></span> 215 + <span id="cb28-3"><a href="#cb28-3" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> <span class="kw">mut</span> query_cursor <span class="op">=</span> <span class="pp">QueryCursor::</span>new()<span class="op">;</span></span> 216 + <span id="cb28-4"><a href="#cb28-4" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> all_matches <span class="op">=</span> query_cursor<span class="op">.</span>matches(</span> 217 + <span id="cb28-5"><a href="#cb28-5" aria-hidden="true" tabindex="-1"></a> <span class="op">&amp;</span>query<span class="op">,</span></span> 218 + <span id="cb28-6"><a href="#cb28-6" aria-hidden="true" tabindex="-1"></a> parse_tree<span class="op">.</span>root_node()<span class="op">,</span></span> 219 + <span id="cb28-7"><a href="#cb28-7" aria-hidden="true" tabindex="-1"></a> src<span class="op">.</span>as_bytes()<span class="op">,</span></span> 220 + <span id="cb28-8"><a href="#cb28-8" aria-hidden="true" tabindex="-1"></a>)<span class="op">;</span></span></code></pre></div> 221 + <p>We begin by passing our query to the cursor, followed by the “root node”, which is another way of saying, “start from the top”, and lastly, the source itself. If you have already taken a look at the C API, you will notice that the last argument, the source (known as the <code>TextProvider</code>), is not required. The Rust bindings seem to require this argument to provide predicate functionality such as <code>#match?</code> and <code>#eq?</code>.</p> 222 + <p>Do something with the matches:</p> 223 + <div class="sourceCode" id="cb29"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb29-1"><a href="#cb29-1" aria-hidden="true" tabindex="-1"></a><span class="co">// get the index of the capture named &quot;raise&quot;</span></span> 224 + <span id="cb29-2"><a href="#cb29-2" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span> raise_idx <span class="op">=</span> query<span class="op">.</span>capture_index_for_name(<span class="st">&quot;raise&quot;</span>)<span class="op">.</span>unwrap()<span class="op">;</span></span> 225 + <span id="cb29-3"><a href="#cb29-3" aria-hidden="true" tabindex="-1"></a></span> 226 + <span id="cb29-4"><a href="#cb29-4" aria-hidden="true" tabindex="-1"></a><span class="kw">for</span> each_match <span class="kw">in</span> all_matches <span class="op">{</span></span> 227 + <span id="cb29-5"><a href="#cb29-5" aria-hidden="true" tabindex="-1"></a> <span class="co">// iterate over all captures called &quot;raise&quot;</span></span> 228 + <span id="cb29-6"><a href="#cb29-6" aria-hidden="true" tabindex="-1"></a> <span class="co">// ignore captures such as &quot;fn-name&quot;</span></span> 229 + <span id="cb29-7"><a href="#cb29-7" aria-hidden="true" tabindex="-1"></a> <span class="kw">for</span> capture <span class="kw">in</span> each_match</span> 230 + <span id="cb29-8"><a href="#cb29-8" aria-hidden="true" tabindex="-1"></a> <span class="op">.</span>captures</span> 231 + <span id="cb29-9"><a href="#cb29-9" aria-hidden="true" tabindex="-1"></a> <span class="op">.</span>iter()</span> 232 + <span id="cb29-10"><a href="#cb29-10" aria-hidden="true" tabindex="-1"></a> <span class="op">.</span>filter(<span class="op">|</span>c<span class="op">|</span> c<span class="op">.</span>idx <span class="op">==</span> raise_idx)</span> 233 + <span id="cb29-11"><a href="#cb29-11" aria-hidden="true" tabindex="-1"></a> <span class="op">{</span></span> 234 + <span id="cb29-12"><a href="#cb29-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> range <span class="op">=</span> capture<span class="op">.</span>node<span class="op">.</span>range()<span class="op">;</span></span> 235 + <span id="cb29-13"><a href="#cb29-13" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> text <span class="op">=</span> <span class="op">&amp;</span>src[range<span class="op">.</span>start_byte<span class="op">..</span>range<span class="op">.</span>end_byte]<span class="op">;</span></span> 236 + <span id="cb29-14"><a href="#cb29-14" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> line <span class="op">=</span> range<span class="op">.</span>start_point<span class="op">.</span>row<span class="op">;</span></span> 237 + <span id="cb29-15"><a href="#cb29-15" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span> col <span class="op">=</span> range<span class="op">.</span>start_point<span class="op">.</span>column<span class="op">;</span></span> 238 + <span id="cb29-16"><a href="#cb29-16" aria-hidden="true" tabindex="-1"></a> <span class="pp">println!</span>(</span> 239 + <span id="cb29-17"><a href="#cb29-17" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;[Line: {}, Col: {}] Offending source code: `{}`&quot;</span><span class="op">,</span></span> 240 + <span id="cb29-18"><a href="#cb29-18" aria-hidden="true" tabindex="-1"></a> line<span class="op">,</span> col<span class="op">,</span> text</span> 241 + <span id="cb29-19"><a href="#cb29-19" aria-hidden="true" tabindex="-1"></a> )<span class="op">;</span></span> 242 + <span id="cb29-20"><a href="#cb29-20" aria-hidden="true" tabindex="-1"></a> <span class="op">}</span></span> 243 + <span id="cb29-21"><a href="#cb29-21" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div> 244 + <p>Lastly, add the following line to your source code, to get the linter to catch something:</p> 245 + <div class="sourceCode" id="cb30"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb30-1"><a href="#cb30-1" aria-hidden="true" tabindex="-1"></a><span class="pp">env::</span>remove_var(<span class="st">&quot;RUST_BACKTRACE&quot;</span>)<span class="op">;</span></span></code></pre></div> 246 + <p>And <code>cargo run</code>:</p> 247 + <pre class="shell"><code>λ cargo run 248 + Compiling toy-lint v0.1.0 (/redacted/path/to/toy-lint) 249 + Finished dev [unoptimized + debuginfo] target(s) in 0.74s 250 + Running `target/debug/toy-lint` 251 + [Line: 40, Col: 4] Offending source code: `env::remove_var(&quot;RUST_BACKTRACE&quot;)`</code></pre> 252 + <p>Thank you tree-sitter!</p> 253 + <h3 id="bonus">Bonus</h3> 254 + <p>Keen readers will notice that I avoided <code>std::env::set_var</code>. Because <code>set_var</code> is called with two arguments, a “key” and a “value”, unlike <code>env::var</code> and <code>env::remove_var</code>. As a result, it requires more juggling:</p> 255 + <div class="sourceCode" id="cb32"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb32-1"><a href="#cb32-1" aria-hidden="true" tabindex="-1"></a>((call_expression</span> 256 + <span id="cb32-2"><a href="#cb32-2" aria-hidden="true" tabindex="-1"></a> function: (<span class="op">_</span>) @fn-name</span> 257 + <span id="cb32-3"><a href="#cb32-3" aria-hidden="true" tabindex="-1"></a> arguments: (arguments <span class="op">.</span> (string_literal)<span class="op">?</span> <span class="op">.</span> (string_literal) <span class="op">.</span>)) @raise</span> 258 + <span id="cb32-4"><a href="#cb32-4" aria-hidden="true" tabindex="-1"></a> (#match? @fn-name <span class="st">&quot;(std::|)env::(var|remove_var|set_var)&quot;</span>))</span></code></pre></div> 259 + <p>The interesting part of this query is the humble <code>.</code>, the <em>anchor</em> operator. Anchors help constrain child nodes in certain ways. In this case, it ensures that we match exactly two <code>string_literal</code>s who are siblings or exactly one <code>string_literal</code> with no siblings. Unfortunately, this query also matches the following invalid Rust code:</p> 260 + <div class="sourceCode" id="cb33"><pre class="sourceCode rust"><code class="sourceCode rust"><span id="cb33-1"><a href="#cb33-1" aria-hidden="true" tabindex="-1"></a><span class="co">// remove_var accepts only 1 arg!</span></span> 261 + <span id="cb33-2"><a href="#cb33-2" aria-hidden="true" tabindex="-1"></a><span class="pp">std::env::</span>remove_var(<span class="st">&quot;RUST_BACKTRACE&quot;</span><span class="op">,</span> <span class="st">&quot;1&quot;</span>)<span class="op">;</span></span></code></pre></div> 262 + <h3 id="notes">Notes</h3> 263 + <p>All-in-all, the query DSL does a great job in lowering the bar to writing language tools. The knowledge gained from mastering the query DSL can be applied to other languages that have tree-sitter grammars too. This query detects <code>to_json</code> methods that do not accept additional arguments, in Ruby:</p> 264 + <div class="sourceCode" id="cb34"><pre class="sourceCode scheme"><code class="sourceCode scheme"><span id="cb34-1"><a href="#cb34-1" aria-hidden="true" tabindex="-1"></a>((method</span> 265 + <span id="cb34-2"><a href="#cb34-2" aria-hidden="true" tabindex="-1"></a> name: (identifier) @fn</span> 266 + <span id="cb34-3"><a href="#cb34-3" aria-hidden="true" tabindex="-1"></a> !parameters)</span> 267 + <span id="cb34-4"><a href="#cb34-4" aria-hidden="true" tabindex="-1"></a> (<span class="sc">#i</span>s? @fn <span class="st">&quot;to_json&quot;</span>))</span></code></pre></div> 268 + 269 + </div> 270 + 271 + <div class="intro"> 272 + Hi. 273 + <div class="hot-links"> 274 + <a href="https://peppe.rs/index.xml" class="feed-button">Subscribe</a> 275 + <a href="https://liberapay.com/nerdypepper/donate" class="donate-button">Donate</a> 276 + </div> 277 + <p>I'm Akshay, I go by nerd or nerdypepper on the internet.</p> 278 + <p> 279 + I am a compsci undergrad, Rust programmer and an enthusiastic Vimmer. 280 + I write <a href="https://git.peppe.rs">open-source stuff</a> to pass time. 281 + I also design fonts: 282 + <a href="https://git.peppe.rs/fonts/scientifica">scientifica</a>, 283 + <a href="https://git.peppe.rs/fonts/curie">curie</a>. 284 + </p> 285 + <p>Send me a mail at nerdy@peppe.rs or a message at nerdypepper@irc.rizon.net.</p> 286 + </div> 287 + 288 + <a href="/" class="post-end-link">Home</a> 289 + <span>/</span> 290 + <a href="/posts" class="post-end-link">Posts</a> 291 + <span>/</span> 292 + <a class="post-end-link">Lightweight Linting</a> 293 + <a class="stats post-end-link" href="https://git.peppe.rs/web/site/plain/posts/lightweight_linting.md 294 + ">View Raw</a> 295 + </div> 296 + </div> 297 + </body> 298 + </html>
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··· 1 + [Tree-sitter](https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries) 2 + queries allow you to search for patterns in syntax trees, 3 + much like a regex would, in text. Combine that with some Rust 4 + glue to write simple, custom linters. 5 + 6 + ### Tree-sitter syntax trees 7 + 8 + Here is a quick crash course on syntax trees generated by 9 + tree-sitter. Syntax trees produced by tree-sitter are 10 + represented by S-expressions. The generated S-expression for 11 + the following Rust code, 12 + 13 + ```rust 14 + fn main() { 15 + let x = 2; 16 + } 17 + ``` 18 + 19 + would be: 20 + 21 + ```scheme 22 + (source_file 23 + (function_item 24 + name: (identifier) 25 + parameters: (parameters) 26 + body: 27 + (block 28 + (let_declaration 29 + pattern: (identifier) 30 + value: (integer_literal))))) 31 + ``` 32 + 33 + Syntax trees generated by tree-sitter have a couple of other 34 + cool properties: they are _lossless_ syntax trees. Given a 35 + lossless syntax tree, you can regenerate the original source 36 + code in its entirety. Consider the following addition to our 37 + example: 38 + 39 + ```rust 40 + fn main() { 41 + + // a comment goes here 42 + let x = 2; 43 + } 44 + ``` 45 + 46 + The tree-sitter syntax tree preserves the comment, while the 47 + typical abstract syntax tree wouldn't: 48 + 49 + ```scheme 50 + (source_file 51 + (function_item 52 + name: (identifier) 53 + parameters: (parameters) 54 + body: 55 + (block 56 + + (line_comment) 57 + (let_declaration 58 + pattern: (identifier) 59 + value: (integer_literal))))) 60 + ``` 61 + 62 + ### Tree-sitter queries 63 + 64 + Tree-sitter provides a DSL to match over CSTs. These queries 65 + resemble our S-expression syntax trees, here is a query to 66 + match all line comments in a Rust CST: 67 + 68 + ```scheme 69 + (line_comment) 70 + 71 + ; matches the following rust code 72 + ; // a comment goes here 73 + ``` 74 + 75 + Neat, eh? But don't take my word for it, give it a go on the 76 + [tree-sitter 77 + playground](https://tree-sitter.github.io/tree-sitter/playground). 78 + Type in a query like so: 79 + 80 + ```scheme 81 + ; the web playground requires you to specify a "capture" 82 + ; you will notice the capture and the nodes it captured 83 + ; turn blue 84 + (line_comment) @capture 85 + ``` 86 + 87 + Here's another to match `let` expressions that 88 + bind an integer to an identifier: 89 + 90 + ```scheme 91 + (let_declaration 92 + pattern: (identifier) 93 + value: (integer_literal)) 94 + 95 + ; matches: 96 + ; let foo = 2; 97 + ``` 98 + 99 + We can _capture_ nodes into variables: 100 + 101 + ```scheme 102 + (let_declaration 103 + pattern: (identifier) @my-capture 104 + value: (integer_literal)) 105 + 106 + ; matches: 107 + ; let foo = 2; 108 + 109 + ; captures: 110 + ; foo 111 + ``` 112 + 113 + And apply certain _predicates_ to captures: 114 + 115 + ```scheme 116 + ((let_declaration 117 + pattern: (identifier) @my-capture 118 + value: (integer_literal)) 119 + (#eq? @my-capture "foo")) 120 + 121 + ; matches: 122 + ; let foo = 2; 123 + 124 + ; and not: 125 + ; let bar = 2; 126 + ``` 127 + 128 + The `#match?` predicate checks if a capture matches a regex: 129 + 130 + ```scheme 131 + ((let_declaration 132 + pattern: (identifier) @my-capture 133 + value: (integer_literal)) 134 + (#match? @my-capture "foo|bar")) 135 + 136 + ; matches both `foo` and `bar`: 137 + ; let foo = 2; 138 + ; let bar = 2; 139 + ``` 140 + 141 + Exhibit indifference, as a stoic programmer would, with the 142 + _wildcard_ pattern: 143 + 144 + ```scheme 145 + (let_declaration 146 + pattern: (identifier) 147 + value: (_)) 148 + 149 + ; matches: 150 + ; let foo = "foo"; 151 + ; let foo = 42; 152 + ; let foo = bar; 153 + ``` 154 + 155 + [The 156 + documentation](https://tree-sitter.github.io/tree-sitter/using-parsers#pattern-matching-with-queries) 157 + does the tree-sitter query DSL more justice, but we now know 158 + enough to write our first lint. 159 + 160 + ### Write you a tree-sitter lint 161 + 162 + Strings in `std::env` functions are error prone: 163 + 164 + ```rust 165 + std::env::remove_var("RUST_BACKTACE"); 166 + // ^^^^ "TACE" instead of "TRACE" 167 + ``` 168 + 169 + I prefer this instead: 170 + 171 + ```rust 172 + // somewhere in a module that is well spellchecked 173 + static BACKTRACE: &str = "RUST_BACKTRACE"; 174 + 175 + // rest of the codebase 176 + std::env::remove_var(BACKTRACE); 177 + ``` 178 + 179 + Let's write a lint to find `std::env` functions that use 180 + strings. Put aside the effectiveness of this lint for the 181 + moment, and take a stab at writing a tree-sitter query. For 182 + reference, a function call like so: 183 + 184 + ```rust 185 + remove_var("RUST_BACKTRACE") 186 + ``` 187 + 188 + Produces the following S-expression: 189 + 190 + ```scheme 191 + (call_expression 192 + function: (identifier) 193 + arguments: (arguments (string_literal))) 194 + ``` 195 + 196 + We are definitely looking for a `call_expression`: 197 + 198 + ```scheme 199 + (call_expression) @raise 200 + ``` 201 + 202 + Whose function name matches `std::env::var` or 203 + `std::env::remove_var` at the very least (I know, I know, 204 + this isn't the most optimal regex): 205 + 206 + ```scheme 207 + ((call_expression 208 + function: (_) @fn-name) @raise 209 + (#match? @fn-name "std::env::(var|remove_var)")) 210 + ``` 211 + 212 + Let's turn that `std::` prefix optional: 213 + 214 + ```scheme 215 + ((call_expression 216 + function: (_) @fn-name) @raise 217 + (#match? @fn-name "(std::|)env::(var|remove_var)")) 218 + ``` 219 + 220 + And ensure that `arguments` is a string: 221 + 222 + ```scheme 223 + ((call_expression 224 + function: (_) @fn-name 225 + arguments: (arguments (string_literal))) 226 + (#match? @fn-name "(std::|)env::(var|remove_var)")) 227 + ``` 228 + 229 + ### Running our linter 230 + 231 + We could always plug our query into the web playground, but 232 + let's go a step further: 233 + 234 + ```bash 235 + cargo new --bin toy-lint 236 + ``` 237 + 238 + Add `tree-sitter` and `tree-sitter-rust` to your 239 + dependencies: 240 + 241 + ```toml 242 + # within Cargo.toml 243 + [dependencies] 244 + tree-sitter = "0.20" 245 + 246 + [dependencies.tree-sitter-rust] 247 + git = "https://github.com/tree-sitter/tree-sitter-rust" 248 + ``` 249 + 250 + Let's load in some Rust code to work with. As [an ode to 251 + Gödel](https://en.wikipedia.org/wiki/Self-reference) 252 + (G`ode`l?), why not load in our linter itself: 253 + 254 + ```rust 255 + fn main() { 256 + let src = include_str!("main.rs"); 257 + } 258 + ``` 259 + 260 + Most tree-sitter APIs require a reference to a `Language` 261 + struct, we will be working with Rust if you haven't 262 + already guessed: 263 + 264 + ```rust 265 + use tree_sitter::Language; 266 + 267 + let rust_lang: Language = tree_sitter_rust::language(); 268 + ``` 269 + 270 + Enough scaffolding, let's parse some Rust: 271 + 272 + ```rust 273 + use tree_sitter::Parser; 274 + 275 + let mut parser = Parser::new(); 276 + parser.set_language(rust_lang).unwrap(); 277 + 278 + let parse_tree = parser.parse(&src, None).unwrap(); 279 + ``` 280 + 281 + The second argument to `Parser::parse` may be of interest. 282 + Tree-sitter has this cool feature that allows for quick 283 + reparsing of existing parse trees if they contain edits. If 284 + you do happen to want to reparse a source file, you can pass 285 + in the old tree: 286 + 287 + ```rust 288 + // if you wish to reparse instead of parse 289 + old_tree.edit(/* redacted */); 290 + 291 + // generate shiny new reparsed tree 292 + let new_tree = parser.parse(&src, Some(old_tree)).unwrap() 293 + ``` 294 + 295 + Anyhow ([hah!](http://github.com/dtolnay/anyhow)), now that we have a parse tree, we can inspect it: 296 + 297 + ```rust 298 + println!("{}", parse_tree.root_node().to_sexp()); 299 + ``` 300 + 301 + Or better yet, run a query on it: 302 + 303 + ```rust 304 + use tree_sitter::Query; 305 + 306 + let query = Query::new( 307 + rust_lang, 308 + r#" 309 + ((call_expression 310 + function: (_) @fn-name 311 + arguments: (arguments (string_literal))) @raise 312 + (#match? @fn-name "(std::|)env::(var|remove_var)")) 313 + "# 314 + ) 315 + .unwrap(); 316 + ``` 317 + 318 + A `QueryCursor` is tree-sitter's way of maintaining state as 319 + we iterate through the matches or captures produced by 320 + running a query on the parse tree. Observe: 321 + 322 + ```rust 323 + use tree_sitter::QueryCursor; 324 + 325 + let mut query_cursor = QueryCursor::new(); 326 + let all_matches = query_cursor.matches( 327 + &query, 328 + parse_tree.root_node(), 329 + src.as_bytes(), 330 + ); 331 + ``` 332 + 333 + We begin by passing our query to the cursor, followed by the 334 + "root node", which is another way of saying, "start from the 335 + top", and lastly, the source itself. If you have already 336 + taken a look at the C API, you will notice that the last 337 + argument, the source (known as the `TextProvider`), is not 338 + required. The Rust bindings seem to require this argument to 339 + provide predicate functionality such as `#match?` and 340 + `#eq?`. 341 + 342 + Do something with the matches: 343 + 344 + ```rust 345 + // get the index of the capture named "raise" 346 + let raise_idx = query.capture_index_for_name("raise").unwrap(); 347 + 348 + for each_match in all_matches { 349 + // iterate over all captures called "raise" 350 + // ignore captures such as "fn-name" 351 + for capture in each_match 352 + .captures 353 + .iter() 354 + .filter(|c| c.idx == raise_idx) 355 + { 356 + let range = capture.node.range(); 357 + let text = &src[range.start_byte..range.end_byte]; 358 + let line = range.start_point.row; 359 + let col = range.start_point.column; 360 + println!( 361 + "[Line: {}, Col: {}] Offending source code: `{}`", 362 + line, col, text 363 + ); 364 + } 365 + } 366 + ``` 367 + 368 + Lastly, add the following line to your source code, to get 369 + the linter to catch something: 370 + 371 + ```rust 372 + env::remove_var("RUST_BACKTRACE"); 373 + ``` 374 + 375 + And `cargo run`: 376 + 377 + ```shell 378 + λ cargo run 379 + Compiling toy-lint v0.1.0 (/redacted/path/to/toy-lint) 380 + Finished dev [unoptimized + debuginfo] target(s) in 0.74s 381 + Running `target/debug/toy-lint` 382 + [Line: 40, Col: 4] Offending source code: `env::remove_var("RUST_BACKTRACE")` 383 + ``` 384 + 385 + Thank you tree-sitter! 386 + 387 + ### Bonus 388 + 389 + Keen readers will notice that I avoided `std::env::set_var`. 390 + Because `set_var` is called with two arguments, a "key" and 391 + a "value", unlike `env::var` and `env::remove_var`. As a 392 + result, it requires more juggling: 393 + 394 + ```scheme 395 + ((call_expression 396 + function: (_) @fn-name 397 + arguments: (arguments . (string_literal)? . (string_literal) .)) @raise 398 + (#match? @fn-name "(std::|)env::(var|remove_var|set_var)")) 399 + ``` 400 + 401 + The interesting part of this query is the humble `.`, the 402 + _anchor_ operator. Anchors help constrain child nodes in 403 + certain ways. In this case, it ensures that we match exactly 404 + two `string_literal`s who are siblings or exactly one 405 + `string_literal` with no siblings. Unfortunately, this query 406 + also matches the following invalid Rust code: 407 + 408 + ```rust 409 + // remove_var accepts only 1 arg! 410 + std::env::remove_var("RUST_BACKTRACE", "1"); 411 + ``` 412 + 413 + ### Notes 414 + 415 + All-in-all, the query DSL does a great job in lowering the 416 + bar to writing language tools. The knowledge gained from 417 + mastering the query DSL can be applied to other languages 418 + that have tree-sitter grammars too. This query 419 + detects `to_json` methods that do not accept additional 420 + arguments, in Ruby: 421 + 422 + ```scheme 423 + ((method 424 + name: (identifier) @fn 425 + !parameters) 426 + (#is? @fn "to_json")) 427 + ```