resolve: Improve diagnostics for ambiguities in extern prelude

Author: petrochenkov

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 {

tests/ui/imports/issue-109148.stderr

@@ -17,7 +17,6 @@ LL | use std::mem;
    |
    = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
    = note: `std` could refer to a built-in crate
-   = help: use `::std` to refer to this crate unambiguously
 note: `std` could also refer to the crate imported here
   --> $DIR/issue-109148.rs:6:9
    |
@@ -26,8 +25,7 @@ LL |         extern crate core as std;
 ...
 LL | m!();
    | ---- in this macro invocation
-   = help: use `::std` to refer to this crate unambiguously
-   = help: or use `crate::std` to refer to this crate unambiguously
+   = help: use `crate::std` to refer to this crate unambiguously
    = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0659]: `std` is ambiguous
@@ -38,7 +36,6 @@ LL | use ::std::mem as _;
    |
    = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
    = note: `std` could refer to a built-in crate
-   = help: use `::std` to refer to this crate unambiguously
 note: `std` could also refer to the crate imported here
   --> $DIR/issue-109148.rs:6:9
    |
@@ -47,7 +44,6 @@ LL |         extern crate core as std;
 ...
 LL | m!();
    | ---- in this macro invocation
-   = help: use `::std` to refer to this crate unambiguously
    = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 3 previous errors

tests/ui/macros/issue-78325-inconsistent-resolution.stderr

@@ -17,7 +17,6 @@ LL |     core::panic!();
    |
    = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
    = note: `core` could refer to a built-in crate
-   = help: use `::core` to refer to this crate unambiguously
 note: `core` could also refer to the crate imported here
   --> $DIR/issue-78325-inconsistent-resolution.rs:5:9
    |
@@ -26,8 +25,7 @@ LL |         extern crate std as core;
 ...
 LL | define_other_core!();
    | -------------------- in this macro invocation
-   = help: use `::core` to refer to this crate unambiguously
-   = help: or use `crate::core` to refer to this crate unambiguously
+   = help: use `crate::core` to refer to this crate unambiguously
    = note: this error originates in the macro `define_other_core` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0659]: `core` is ambiguous
@@ -38,7 +36,6 @@ LL |     ::core::panic!();
    |
    = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
    = note: `core` could refer to a built-in crate
-   = help: use `::core` to refer to this crate unambiguously
 note: `core` could also refer to the crate imported here
   --> $DIR/issue-78325-inconsistent-resolution.rs:5:9
    |
@@ -47,7 +44,6 @@ LL |         extern crate std as core;
 ...
 LL | define_other_core!();
    | -------------------- in this macro invocation
-   = help: use `::core` to refer to this crate unambiguously
    = note: this error originates in the macro `define_other_core` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 3 previous errors