feat: check for exposing multiple modules on publish

.github/workflows/ci.yaml

···
         run: make test
         working-directory: ./test/running_modules
 
+      - name: test/multi_namespace
+        run: ./test.sh
+        working-directory: ./test/multi_namespace
+
       - name: Test FFI in subdirectories
         run: make
         working-directory: ./test/subdir_ffi

compiler-cli/src/publish.rs

···
     build::{Codegen, Compile, Mode, Options, Package, Target},
     config::{PackageConfig, SpdxLicense},
     docs::DocContext,
-    error::SmallVersion,
+    error::{wrap, SmallVersion},
     hex,
     paths::{self, ProjectPaths},
     requirement::Requirement,
···
         } = do_build_hex_tarball(&paths, &mut config)?;
 
         check_for_name_squatting(&compile_result)?;
+        check_for_multiple_top_level_modules(&compile_result, i_am_sure)?;
 
         // Build HTML documentation
         let docs_tarball = fs::create_tar_archive(docs::build_documentation(
···
 
     if main.body.first().is_println() {
         return Err(Error::HexPackageSquatting);
+    }
+
+    Ok(())
+}
+
+fn check_for_multiple_top_level_modules(package: &Package, i_am_sure: bool) -> Result<(), Error> {
+    // Collect top-level module names
+    let mut top_level_module_names = package
+        .modules
+        .iter()
+        .filter_map(|module| module.name.split('/').next())
+        .collect::<Vec<_>>();
+
+    // Remove duplicates
+    top_level_module_names.sort_unstable();
+    top_level_module_names.dedup();
+
+    // If more than one top-level module name is found, prompt for confirmation
+    if top_level_module_names.len() > 1 {
+        let text = wrap(&format!(
+            "Your package defines multiple top-level modules: {}.
+
+Defining multiple top-level modules can lead to namespace pollution \
+and potential conflicts for consumers.
+
+To fix this, move all your modules under a single top-level module of your choice.
+
+For example:
+  src/{1}.gleam
+  src/{1}/module1.gleam
+  src/{1}/module2.gleam",
+            top_level_module_names.join(", "),
+            package.config.name
+        ));
+        println!("{text}\n");
+
+        let should_publish =
+            i_am_sure || cli::confirm("\nDo you wish to continue publishing this package?")?;
+        println!();
+
+        if !should_publish {
+            println!("Not publishing.");
+            std::process::exit(0);
+        }
     }
 
     Ok(())

test/multi_namespace/.gitignore

···
+*.beam
+*.ez
+build

test/multi_namespace/gleam.toml

···
+name = "multi_namespace"
+version = "1.0.0"
+description = "Test project for multi namespace"
+licences = ["Apache-2.0"]

test/multi_namespace/manifest.toml

···
+# This file was generated by Gleam
+# You typically do not need to edit this file
+
+packages = [
+]
+
+[requirements]

test/multi_namespace/src/multi_namespace.gleam

···
+pub fn main() {
+  "Hello from multi_namespace!"
+}

test/multi_namespace/src/second.gleam

···
+pub fn main() {
+  "Hello from second!"
+}

test/multi_namespace/test.sh

···
+#!/bin/sh
+
+set -eu
+
+GLEAM_COMMAND=${GLEAM_COMMAND:-"cargo run --quiet --"}
+
+g() {
+	echo "Running: $GLEAM_COMMAND $@"
+	$GLEAM_COMMAND "$@"
+}
+
+echo Resetting the build directory to get to a known state
+rm -fr build
+
+echo Running publish should not publish anything
+output=$(yes "n" | g publish)
+if echo "$output" | grep -q "Your package defines multiple top-level modules"; then
+    echo "Publish was correctly prevented with warning"
+else
+    echo "Expected publish to be aborted"
+    exit 1
+fi
+
+echo
+echo Success! 💖
+echo