resolve: Cleanup ::name suggestions for ambiguities

using the now available scope information

Author: petrochenkov

compiler/rustc_resolve/src/diagnostics.rs

@@ -1996,22 +1996,27 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
     fn ambiguity_diagnostic(&self, ambiguity_error: &AmbiguityError<'ra>) -> errors::Ambiguity {
         let AmbiguityError { kind, ident, b1, b2, scope1, scope2, .. } = *ambiguity_error;
-        let extern_prelude_ambiguity = || {
-            // Note: b1 may come from a module scope, as an extern crate item in module.
-            matches!(scope2, Scope::ExternPreludeFlags)
-                && self
-                    .extern_prelude
-                    .get(&Macros20NormalizedIdent::new(ident))
-                    .is_some_and(|entry| entry.item_decl.map(|(b, _)| b) == Some(b1))
+        let extern_prelude_flag = |scope| matches!(scope, Scope::ExternPreludeFlags);
+        let extern_prelude_item = |scope, decl| {
+            // Declaration comes from extern prelude item scope,
+            matches!(scope, Scope::ExternPreludeItems)
+                // or from a module scope, as an extern crate item in the root module.
+                || matches!(scope, Scope::ModuleNonGlobs(..))
+                    && self
+                        .extern_prelude
+                        .get(&Macros20NormalizedIdent::new(ident))
+                        .is_some_and(|entry| entry.item_decl.map(|(d, _)| d) == Some(decl))
         };
+        let extern_prelude_ambiguity =
+            extern_prelude_flag(scope2) && extern_prelude_item(scope1, b1);
         let (b1, b2, scope1, scope2, 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, scope2, scope1, true)
         } else {
             (b1, b2, scope1, scope2, false)
         };
 
-        let could_refer_to = |b: Decl<'_>, scope: Scope<'ra>, also: &str| {
+        let could_refer_to = |b: Decl<'ra>, scope: Scope<'ra>, also: &str| {
             let what = self.decl_description(b, ident, scope);
             let note_msg = format!("`{ident}` could{also} refer to {what}");
 
@@ -2026,7 +2031,9 @@ 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() && !extern_prelude_ambiguity()
+
+            if !extern_prelude_ambiguity
+                && (extern_prelude_flag(scope) || extern_prelude_item(scope, b))
             {
                 help_msgs.push(format!("use `::{ident}` to refer to this {thing} unambiguously"))
             }

tests/ui/imports/extern-prelude-extern-crate-restricted-shadowing.stderr

@@ -24,6 +24,7 @@ LL |         extern crate std as Vec;
 ...
 LL | define_vec!();
    | ------------- in this macro invocation
+   = help: use `::Vec` to refer to this crate unambiguously
 note: `Vec` could also refer to a struct from prelude
   --> $SRC_DIR/std/src/prelude/mod.rs:LL:COL
    = note: this error originates in the macro `define_vec` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/ui/imports/issue-114682-5.stderr

@@ -45,7 +45,6 @@ note: `issue_114682_5_extern_1` could also refer to the crate defined here
    |
 LL |     pub use crate::*;
    |             ^^^^^
-   = help: use `::issue_114682_5_extern_1` to refer to this crate unambiguously
    = note: `#[deny(ambiguous_glob_imports)]` (part of `#[deny(future_incompatible)]`) on by default
 
 error: aborting due to 3 previous errors
@@ -72,6 +71,5 @@ note: `issue_114682_5_extern_1` could also refer to the crate defined here
    |
 LL |     pub use crate::*;
    |             ^^^^^
-   = help: use `::issue_114682_5_extern_1` to refer to this crate unambiguously
    = note: `#[deny(ambiguous_glob_imports)]` (part of `#[deny(future_incompatible)]`) on by default
 

tests/ui/macros/macro-path-prelude-shadowing.stderr

@@ -6,6 +6,7 @@ LL |         std::panic!();
    |
    = note: ambiguous because of a conflict between a name from a glob import and an 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 module imported here
   --> $DIR/macro-path-prelude-shadowing.rs:28:9
    |