Make CodegenBackend::join_codegen infallible.

Because they all are, in practice.

Author: nnethercote

src/lib.rs

@@ -217,13 +217,11 @@ impl CodegenBackend for GccCodegenBackend {
         Box::new(res)
     }
 
-    fn join_codegen(&self, ongoing_codegen: Box<dyn Any>, sess: &Session, _outputs: &OutputFilenames) -> Result<(CodegenResults, FxIndexMap<WorkProductId, WorkProduct>), ErrorGuaranteed> {
-        let (codegen_results, work_products) = ongoing_codegen
+    fn join_codegen(&self, ongoing_codegen: Box<dyn Any>, sess: &Session, _outputs: &OutputFilenames) -> (CodegenResults, FxIndexMap<WorkProductId, WorkProduct>) {
+        ongoing_codegen
             .downcast::<rustc_codegen_ssa::back::write::OngoingCodegen<GccCodegenBackend>>()
             .expect("Expected GccCodegenBackend's OngoingCodegen, found Box<Any>")
-            .join(sess);
-
-        Ok((codegen_results, work_products))
+            .join(sess)
     }
 
     fn link(&self, sess: &Session, codegen_results: CodegenResults, outputs: &OutputFilenames) -> Result<(), ErrorGuaranteed> {