compiler: Avoid double-errors on ABIs during hir_analysis

Author: workingjubilee

compiler/rustc_ast_lowering/src/item.rs

@@ -1621,13 +1621,21 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         // we can't do codegen for unsupported ABIs, so error now so we won't get farther
         if !tcx.sess.target.is_abi_supported(extern_abi) {
-            struct_span_code_err!(
+            let mut err = struct_span_code_err!(
                 tcx.dcx(),
                 span,
                 E0570,
                 "`{extern_abi}` is not a supported ABI for the current target",
-            )
-            .emit();
+            );
+
+            if let ExternAbi::Stdcall { unwind } = extern_abi {
+                let c_abi = ExternAbi::C { unwind };
+                let system_abi = ExternAbi::System { unwind };
+                err.help(format!("if you need `extern {extern_abi}` on win32 and `extern {c_abi}` everywhere else, \
+                use `extern {system_abi}`"
+                ));
+            }
+            err.emit();
         }
         // stability gate even things that are already errored on
         gate_unstable_abi(tcx.sess, tcx.features(), span, extern_abi);

compiler/rustc_hir_analysis/src/check/check.rs

@@ -56,16 +56,8 @@ pub fn check_abi(tcx: TyCtxt<'_>, hir_id: hir::HirId, span: Span, abi: ExternAbi
 
     match AbiMap::from_target(&tcx.sess.target).canonize_abi(abi, false) {
         AbiMapping::Direct(..) => (),
-        AbiMapping::Invalid => {
-            let mut err = struct_span_code_err!(
-                tcx.dcx(),
-                span,
-                E0570,
-                "`{abi}` is not a supported ABI for the current target",
-            );
-            add_help(abi, &mut err);
-            err.emit();
-        }
+        // already erred in rustc_ast_lowering
+        AbiMapping::Invalid => (),
         AbiMapping::Deprecated(..) => {
             tcx.node_span_lint(UNSUPPORTED_CALLING_CONVENTIONS, hir_id, span, |lint| {
                 lint.primary_message("use of calling convention not supported on this target");