From 3d035505a3e24830e6da84d31d7f63b02b6d5ae0 Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Sun, 2 Nov 2025 12:33:38 +0000
Subject: [PATCH] Better warning message for crate type unsupported by codegen
 backend

---
 compiler/rustc_driver_impl/src/lib.rs  |  8 ++++++--
 compiler/rustc_interface/src/passes.rs |  1 +
 compiler/rustc_session/messages.ftl    |  3 +++
 compiler/rustc_session/src/errors.rs   |  7 +++++++
 compiler/rustc_session/src/output.rs   | 14 ++++++++------
 5 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index c926a7c742a0a..462ca98c21151 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -686,8 +686,12 @@ fn print_crate_info(
                 };
                 let t_outputs = rustc_interface::util::build_output_filenames(attrs, sess);
                 let crate_name = passes::get_crate_name(sess, attrs);
-                let crate_types =
-                    collect_crate_types(sess, &codegen_backend.supported_crate_types(sess), attrs);
+                let crate_types = collect_crate_types(
+                    sess,
+                    &codegen_backend.supported_crate_types(sess),
+                    codegen_backend.name(),
+                    attrs,
+                );
                 for &style in &crate_types {
                     let fname = rustc_session::output::filename_for_input(
                         sess, style, crate_name, &t_outputs,
diff --git a/compiler/rustc_interface/src/passes.rs b/compiler/rustc_interface/src/passes.rs
index 58ed71a970776..e68db4f44ca48 100644
--- a/compiler/rustc_interface/src/passes.rs
+++ b/compiler/rustc_interface/src/passes.rs
@@ -929,6 +929,7 @@ pub fn create_and_enter_global_ctxt<T, F: for<'tcx> FnOnce(TyCtxt<'tcx>) -> T>(
     let crate_types = collect_crate_types(
         sess,
         &compiler.codegen_backend.supported_crate_types(sess),
+        compiler.codegen_backend.name(),
         &pre_configured_attrs,
     );
     let stable_crate_id = StableCrateId::new(
diff --git a/compiler/rustc_session/messages.ftl b/compiler/rustc_session/messages.ftl
index b46e8ab4fdcbe..a4166951e350a 100644
--- a/compiler/rustc_session/messages.ftl
+++ b/compiler/rustc_session/messages.ftl
@@ -140,6 +140,9 @@ session_unleashed_feature_help_unnamed = skipping check that does not even have
 
 session_unstable_virtual_function_elimination = `-Zvirtual-function-elimination` requires `-Clto`
 
+session_unsupported_crate_type_for_codegen_backend =
+    dropping unsupported crate type `{$crate_type}` for codegen backend `{$codegen_backend}`
+
 session_unsupported_crate_type_for_target =
     dropping unsupported crate type `{$crate_type}` for target `{$target_triple}`
 
diff --git a/compiler/rustc_session/src/errors.rs b/compiler/rustc_session/src/errors.rs
index 50bc7348dc9ef..07b41a3de714d 100644
--- a/compiler/rustc_session/src/errors.rs
+++ b/compiler/rustc_session/src/errors.rs
@@ -371,6 +371,13 @@ struct BinaryFloatLiteralNotSupported {
     span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(session_unsupported_crate_type_for_codegen_backend)]
+pub(crate) struct UnsupportedCrateTypeForCodegenBackend {
+    pub(crate) crate_type: CrateType,
+    pub(crate) codegen_backend: &'static str,
+}
+
 #[derive(Diagnostic)]
 #[diag(session_unsupported_crate_type_for_target)]
 pub(crate) struct UnsupportedCrateTypeForTarget<'a> {
diff --git a/compiler/rustc_session/src/output.rs b/compiler/rustc_session/src/output.rs
index a48a4f649da1d..99c736c3b1a5e 100644
--- a/compiler/rustc_session/src/output.rs
+++ b/compiler/rustc_session/src/output.rs
@@ -177,6 +177,7 @@ pub fn categorize_crate_type(s: Symbol) -> Option<CrateType> {
 pub fn collect_crate_types(
     session: &Session,
     backend_crate_types: &[CrateType],
+    codegen_backend_name: &'static str,
     attrs: &[ast::Attribute],
 ) -> Vec<CrateType> {
     // If we're generating a test executable, then ignore all other output
@@ -223,17 +224,18 @@ pub fn collect_crate_types(
     }
 
     base.retain(|crate_type| {
-        if invalid_output_for_target(session, *crate_type)
-            || !backend_crate_types.contains(crate_type)
-        {
-            // FIXME provide a better warning for the case where the codegen
-            // backend doesn't support it once cargo doesn't hard code this
-            // warning message.
+        if invalid_output_for_target(session, *crate_type) {
             session.dcx().emit_warn(errors::UnsupportedCrateTypeForTarget {
                 crate_type: *crate_type,
                 target_triple: &session.opts.target_triple,
             });
             false
+        } else if !backend_crate_types.contains(crate_type) {
+            session.dcx().emit_warn(errors::UnsupportedCrateTypeForCodegenBackend {
+                crate_type: *crate_type,
+                codegen_backend: codegen_backend_name,
+            });
+            false
         } else {
             true
         }