make codegen-backend config warning less noisy

If `codegen-backends` is missing "cranelift" and "gcc" (which is common),
bootstrap will now only show this warning during `dist` and `install` steps,
or if codegen-backends was explicitly called for build.

Signed-off-by: onur-ozkan <[email protected]>

src/bootstrap/src/core/build_steps/compile.rs

@@ -1290,15 +1290,21 @@ fn needs_codegen_config(run: &RunConfig<'_>) -> bool {
 pub(crate) const CODEGEN_BACKEND_PREFIX: &str = "rustc_codegen_";
 
 fn is_codegen_cfg_needed(path: &TaskPath, run: &RunConfig<'_>) -> bool {
-    if path.path.to_str().unwrap().contains(CODEGEN_BACKEND_PREFIX) {
+    let path = path.path.to_str().unwrap();
+
+    let is_explicitly_called = |p| -> bool { run.builder.paths.contains(p) };
+    let should_enforce = run.builder.kind == Kind::Dist || run.builder.kind == Kind::Install;
+
+    if path.contains(CODEGEN_BACKEND_PREFIX) {
         let mut needs_codegen_backend_config = true;
         for backend in run.builder.config.codegen_backends(run.target) {
-            if path.path.to_str().unwrap().ends_with(&(CODEGEN_BACKEND_PREFIX.to_owned() + backend))
-            {
+            if path.ends_with(&(CODEGEN_BACKEND_PREFIX.to_owned() + backend)) {
                 needs_codegen_backend_config = false;
             }
         }
-        if needs_codegen_backend_config {
+        if (is_explicitly_called(&PathBuf::from(path)) || should_enforce)
+            && needs_codegen_backend_config
+        {
             run.builder.info(
                 "WARNING: no codegen-backends config matched the requested path to build a codegen backend. \
                 HELP: add backend to codegen-backends in config.toml.",