ci/parse: init

The nix-parse workflow can now be run locally the same way as in CI.

To do this, the CI's workflow was slightly adjusted. Instead of testing
only the changed files, we're now testing all files in the repository.

This is possible in two ways:

1. By calling nix-instantiate once with all files as arguments. This
will be rather fast, but only the first error is shown before it errors
out.
2. By calling nix-instantiate once for each file. This will be much
slower, but has the advantage that we see all errors at once.

To avoid running the long variant every time, we first do a quick check
with the fast version. If that fails, we run the slower one to report
the errors. This gives us the best of both.

.github/workflows/nix-parse-v2.yml

···
     needs: get-merge-commit
     if: "needs.get-merge-commit.outputs.mergedSha && !contains(github.event.pull_request.title, '[skip treewide]')"
     steps:
-      - name: Get list of changed files from PR
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-        run: |
-          gh api \
-            repos/${{ github.repository }}/pulls/${{github.event.number}}/files --paginate \
-            | jq --raw-output '.[] | select(.status != "removed" and (.filename | endswith(".nix"))) | .filename' \
-            > "$HOME/changed_files"
-          if [[ -s "$HOME/changed_files" ]]; then
-            echo "CHANGED_FILES=$HOME/changed_files" > "$GITHUB_ENV"
-          fi
-
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
         with:
           ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
···
           extra_nix_config: sandbox = true
           nix_path: nixpkgs=channel:nixpkgs-unstable
 
-      - name: Parse all changed or added nix files
+      - name: Parse all nix files
         run: |
-          ret=0
-          while IFS= read -r file; do
-            out="$(nix-instantiate --parse "$file")" || { echo "$out" && ret=1; }
-          done < "$HOME/changed_files"
-          exit "$ret"
-        if: ${{ env.CHANGED_FILES && env.CHANGED_FILES != '' }}
+          nix-build ci -A parse

ci/default.nix

···
   manual-nixos = (import ../nixos/release.nix { }).manual.${system} or null;
   manual-nixpkgs = (import ../pkgs/top-level/release.nix { }).manual;
   manual-nixpkgs-tests = (import ../pkgs/top-level/release.nix { }).manual.tests;
+  parse = pkgs.lib.recurseIntoAttrs {
+    latest = pkgs.callPackage ./parse.nix { nix = pkgs.nixVersions.latest; };
+  };
   shell = import ../shell.nix { inherit nixpkgs system; };
 }

ci/parse.nix

···
+{
+  lib,
+  nix,
+  runCommand,
+}:
+let
+  nixpkgs =
+    with lib.fileset;
+    toSource {
+      root = ../.;
+      fileset = (fileFilter (file: file.hasExt "nix") ../.);
+    };
+in
+runCommand "nix-parse-${nix.name}"
+  {
+    nativeBuildInputs = [
+      nix
+    ];
+  }
+  ''
+    export NIX_STORE_DIR=$TMPDIR/store
+    export NIX_STATE_DIR=$TMPDIR/state
+
+    cd "${nixpkgs}"
+
+    # Passes all files to nix-instantiate at once.
+    # Much faster, but will only show first error.
+    parse-all() {
+      find . -type f -iname '*.nix' | xargs -P $(nproc) nix-instantiate --parse >/dev/null 2>/dev/null
+    }
+
+    # Passes each file separately to nix-instantiate with -n1.
+    # Much slower, but will show all errors.
+    parse-each() {
+      find . -type f -iname '*.nix' | xargs -n1 -P $(nproc) nix-instantiate --parse >/dev/null
+    }
+
+    if ! parse-all; then
+      parse-each
+    fi
+
+    touch $out
+  ''