use cherry-picked commit from #100754 to emit note without error

Author: CleanCut

compiler/rustc_error_messages/locales/en-US/passes.ftl

@@ -364,9 +364,11 @@ passes_unknown_external_lang_item =
 passes_missing_panic_handler =
     `#[panic_handler]` function required, but not found
 
-passes_missing_alloc_error_handler =
+passes_alloc_func_required =
     `#[alloc_error_handler]` function required, but not found
-    .note = use `#![feature(default_alloc_error_handler)]` for a default error handler
+
+passes_missing_alloc_error_handler =
+    use `#![feature(default_alloc_error_handler)]` for a default error handler
 
 passes_missing_lang_item =
     language item required, but not found: `{$name}`

compiler/rustc_passes/src/errors.rs

@@ -690,9 +690,12 @@ pub struct UnknownExternLangItem {
 #[diag(passes::missing_panic_handler)]
 pub struct MissingPanicHandler;
 
+#[derive(Diagnostic)]
+#[diag(passes::alloc_func_required)]
+pub struct AllocFuncRequired;
+
 #[derive(Diagnostic)]
 #[diag(passes::missing_alloc_error_handler)]
-#[note]
 pub struct MissingAllocErrorHandler;
 
 #[derive(Diagnostic)]

compiler/rustc_passes/src/weak_lang_items.rs

@@ -8,7 +8,8 @@ use rustc_middle::ty::TyCtxt;
 use rustc_session::config::CrateType;
 
 use crate::errors::{
-    MissingAllocErrorHandler, MissingLangItem, MissingPanicHandler, UnknownExternLangItem,
+    AllocFuncRequired, MissingAllocErrorHandler, MissingLangItem, MissingPanicHandler,
+    UnknownExternLangItem,
 };
 
 /// Checks the crate for usage of weak lang items, returning a vector of all the
@@ -70,7 +71,8 @@ fn verify<'tcx>(tcx: TyCtxt<'tcx>, items: &lang_items::LanguageItems) {
                 tcx.sess.emit_err(MissingPanicHandler);
             } else if item == LangItem::Oom {
                 if !tcx.features().default_alloc_error_handler {
-                    tcx.sess.emit_err(MissingAllocErrorHandler);
+                    tcx.sess.emit_err(AllocFuncRequired);
+                    tcx.sess.emit_note(MissingAllocErrorHandler);
                 }
             } else {
                 tcx.sess.emit_err(MissingLangItem { name: *name });