2 files changed, 65 insertions, 0 deletions
diff --git a/scripts/linters/lint b/scripts/linters/lint
index 150e1f1a..7e7c5fe6 100755
--- a/scripts/linters/lint
+++ b/scripts/linters/lint
@@ -6,6 +6,7 @@ declare(strict_types=1);
 require __DIR__ . '/vendor/autoload.php';
 
 use Shirabe\Lint\Linters\CargoWorkspaceDependencies;
+use Shirabe\Lint\Linters\ComposerPin;
 use Shirabe\Lint\Linters\ContiguousUseBlock;
 use Shirabe\Lint\Linters\NoBannedUse;
 use Shirabe\Lint\Linters\NoDecorativeSectionComment;
@@ -24,6 +25,7 @@ $rootDir = dirname(__DIR__, 2);
 
 $runner = new Runner($rootDir, [
     [new CargoWorkspaceDependencies(), []],
+    [new ComposerPin(), []],
     [new ContiguousUseBlock(), []],
     [new NoBannedUse(), []],
     [new NoDecorativeSectionComment(), [
diff --git a/scripts/linters/src/Linters/ComposerPin.php b/scripts/linters/src/Linters/ComposerPin.php
new file mode 100644
index 00000000..d1db55fe
--- /dev/null
+++ b/scripts/linters/src/Linters/ComposerPin.php
@@ -0,0 +1,63 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Shirabe\Lint\Linters;
+
+use Shirabe\Lint\Linter;
+
+/**
+ * The Composer sources are pinned twice: as the `composer` submodule, which a `cargo` build reads,
+ * and as the `composer` flake input, which a `nix build` reads in its place because a flake source
+ * carries no submodule. Both have to name the same commit, or the two builds embed different
+ * Composer runtimes.
+ */
+final class ComposerPin implements Linter
+{
+    public function name(): string
+    {
+        return 'composer_pin';
+    }
+
+    public function failureIntro(): string
+    {
+        return "The `composer` submodule and the `composer` flake input name different commits.\n"
+            . 'Point the input at the submodule commit and run `nix flake lock`:';
+    }
+
+    public function check(string $rootDir, array $excludes): array
+    {
+        $submoduleRev = $this->submoduleRev($rootDir);
+        $inputRev = $this->inputRev($rootDir);
+
+        if ($submoduleRev === $inputRev) {
+            return [];
+        }
+
+        return [
+            "submodule:   {$submoduleRev}",
+            "flake input: {$inputRev}",
+        ];
+    }
+
+    private function submoduleRev(string $rootDir): string
+    {
+        $command = sprintf('git -C %s ls-tree HEAD composer', escapeshellarg($rootDir));
+        exec($command, $output, $status);
+        if ($status !== 0 || $output === []) {
+            return '(unreadable: git ls-tree HEAD composer failed)';
+        }
+
+        // `<mode> commit <rev>\tcomposer`
+        $fields = preg_split('/\s+/', $output[0]);
+
+        return $fields[2] ?? '(unreadable: unexpected git ls-tree output)';
+    }
+
+    private function inputRev(string $rootDir): string
+    {
+        $lock = json_decode((string) file_get_contents("{$rootDir}/flake.lock"), true);
+
+        return $lock['nodes']['composer']['locked']['rev'] ?? '(missing from flake.lock)';
+    }
+}