Add a tidy check for GCC submodule version

src/tools/tidy/src/gcc_submodule.rs

@@ -0,0 +1,47 @@
+//! Tidy check to ensure that the commit SHA of the `src/gcc` submodule is the same as the
+//! required GCC version of the GCC codegen backend.
+
+use std::path::Path;
+use std::process::Command;
+
+pub fn check(root_path: &Path, compiler_path: &Path, bad: &mut bool) {
+    let cg_gcc_version_path = compiler_path.join("rustc_codegen_gcc/libgccjit.version");
+    let cg_gcc_version = std::fs::read_to_string(&cg_gcc_version_path)
+        .expect(&format!("Cannot read GCC version from {}", cg_gcc_version_path.display()))
+        .trim()
+        .to_string();
+
+    let git_output = Command::new("git")
+        .current_dir(root_path)
+        .arg("submodule")
+        .arg("status")
+        // --cached asks for the version that is actually committed in the repository, not the one
+        // that is currently checked out.
+        .arg("--cached")
+        .arg("src/gcc")
+        .output()
+        .expect("Cannot determine git SHA of the src/gcc checkout");
+
+    // This can return e.g.
+    // -e607be166673a8de9fc07f6f02c60426e556c5f2 src/gcc
+    //  e607be166673a8de9fc07f6f02c60426e556c5f2 src/gcc (master-e607be166673a8de9fc07f6f02c60426e556c5f2.e607be)
+    // +e607be166673a8de9fc07f6f02c60426e556c5f2 src/gcc (master-e607be166673a8de9fc07f6f02c60426e556c5f2.e607be)
+    let git_output = String::from_utf8_lossy(&git_output.stdout)
+        .trim()
+        .split_whitespace()
+        .next()
+        .unwrap_or_default()
+        .to_string();
+
+    // The SHA can start with + if the submodule is modified or - if it is not checked out.
+    let gcc_submodule_sha = git_output.trim_start_matches(&['+', '-']);
+    if gcc_submodule_sha != cg_gcc_version {
+        *bad = true;
+        eprintln!(
+            r#"Commit SHA of the src/gcc submodule (`{gcc_submodule_sha}`) does not match the required GCC version of the GCC codegen backend (`{cg_gcc_version}`).
+Make sure to set the src/gcc submodule to commit {cg_gcc_version}.
+The GCC codegen backend commit is configured at {}."#,
+            cg_gcc_version_path.display(),
+        );
+    }
+}

src/tools/tidy/src/lib.rs

@@ -75,6 +75,7 @@ pub mod features;
 pub mod fluent_alphabetical;
 pub mod fluent_period;
 mod fluent_used;
+pub mod gcc_submodule;
 pub(crate) mod iter_header;
 pub mod known_bug;
 pub mod mir_opt_tests;

src/tools/tidy/src/main.rs

@@ -116,6 +116,7 @@ fn main() {
         check!(fluent_alphabetical, &compiler_path, bless);
         check!(fluent_period, &compiler_path);
         check!(target_policy, &root_path);
+        check!(gcc_submodule, &root_path, &compiler_path);
 
         // Checks that only make sense for the std libs.
         check!(pal, &library_path);