resolve: Improve diagnostics for ambiguities in extern prelude

1 files changed, 9 insertions, 2 deletions

diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index e5321c7f3be..e420e68264b 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -1864,14 +1864,20 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         }
     }
 
-    fn ambiguity_diagnostics(&self, ambiguity_error: &AmbiguityError<'_>) -> AmbiguityErrorDiag {
+    fn ambiguity_diagnostics(&self, ambiguity_error: &AmbiguityError<'ra>) -> AmbiguityErrorDiag {
         let AmbiguityError { kind, ident, b1, b2, misc1, misc2, .. } = *ambiguity_error;
+        let extern_prelude_ambiguity = || {
+            self.extern_prelude.get(&Macros20NormalizedIdent::new(ident)).is_some_and(|entry| {
+                entry.item_binding == Some(b1) && entry.flag_binding.get() == Some(b2)
+            })
+        };
         let (b1, b2, misc1, misc2, swapped) = if b2.span.is_dummy() && !b1.span.is_dummy() {
             // We have to print the span-less alternative first, otherwise formatting looks bad.
             (b2, b1, misc2, misc1, true)
         } else {
             (b1, b2, misc1, misc2, false)
         };
+
         let could_refer_to = |b: NameBinding<'_>, misc: AmbiguityErrorMisc, also: &str| {
             let what = self.binding_description(b, ident, misc == AmbiguityErrorMisc::FromPrelude);
             let note_msg = format!("`{ident}` could{also} refer to {what}");
@@ -1887,7 +1893,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     "consider adding an explicit import of `{ident}` to disambiguate"
                 ))
             }
-            if b.is_extern_crate() && ident.span.at_least_rust_2018() {
+            if b.is_extern_crate() && ident.span.at_least_rust_2018() && !extern_prelude_ambiguity()
+            {
                 help_msgs.push(format!("use `::{ident}` to refer to this {thing} unambiguously"))
             }
             match misc {