resolve: Improve diagnostics for ambiguities in extern prelude

Author: petrochenkov

compiler/rustc_resolve/src/diagnostics.rs

@@ -1886,7 +1886,10 @@ 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 kind != AmbiguityKind::ExternPrelude
+                && b.is_extern_crate()
+                && ident.span.at_least_rust_2018()
+            {
                 help_msgs.push(format!("use `::{ident}` to refer to this {thing} unambiguously"))
             }
             match misc {

compiler/rustc_resolve/src/ident.rs

@@ -396,9 +396,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             struct Flags: u8 {
                 const MACRO_RULES          = 1 << 0;
                 const MODULE               = 1 << 1;
-                const MISC_SUGGEST_CRATE   = 1 << 2;
-                const MISC_SUGGEST_SELF    = 1 << 3;
-                const MISC_FROM_PRELUDE    = 1 << 4;
+                const EXTERN_PRELUDE       = 1 << 2;
+                const MISC_SUGGEST_CRATE   = 1 << 3;
+                const MISC_SUGGEST_SELF    = 1 << 4;
+                const MISC_FROM_PRELUDE    = 1 << 5;
             }
         }
 
@@ -564,15 +565,15 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     },
                     Scope::ExternPreludeItems => {
                         match this.extern_prelude_get_item(ident, finalize.is_some()) {
-                            Some(binding) => Ok((binding, Flags::empty())),
+                            Some(binding) => Ok((binding, Flags::EXTERN_PRELUDE)),
                             None => Err(Determinacy::determined(
                                 this.graph_root.unexpanded_invocations.borrow().is_empty(),
                             )),
                         }
                     }
                     Scope::ExternPreludeFlags => {
                         match this.extern_prelude_get_flag(ident, finalize.is_some()) {
-                            Some(binding) => Ok((binding, Flags::empty())),
+                            Some(binding) => Ok((binding, Flags::EXTERN_PRELUDE)),
                             None => Err(Determinacy::Determined),
                         }
                     }
@@ -684,7 +685,13 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                                 } else if innermost_binding
                                     .may_appear_after(parent_scope.expansion, binding)
                                 {
-                                    Some(AmbiguityKind::MoreExpandedVsOuter)
+                                    if flags.contains(Flags::EXTERN_PRELUDE)
+                                        && innermost_flags.contains(Flags::EXTERN_PRELUDE)
+                                    {
+                                        Some(AmbiguityKind::ExternPrelude)
+                                    } else {
+                                        Some(AmbiguityKind::MoreExpandedVsOuter)
+                                    }
                                 } else {
                                     None
                                 };

compiler/rustc_resolve/src/lib.rs

@@ -861,6 +861,7 @@ enum AmbiguityKind {
     GlobVsGlob,
     GlobVsExpanded,
     MoreExpandedVsOuter,
+    ExternPrelude,
 }
 
 impl AmbiguityKind {
@@ -881,6 +882,9 @@ impl AmbiguityKind {
             AmbiguityKind::MoreExpandedVsOuter => {
                 "a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution"
             }
+            AmbiguityKind::ExternPrelude => {
+                "a conflict between a macro-expanded `extern crate` and `--extern` flag during import or macro resolution"
+            }
         }
     }
 }

tests/ui/imports/issue-109148.stderr

@@ -36,9 +36,8 @@ error[E0659]: `std` is ambiguous
 LL | use ::std::mem as _;
    |       ^^^ ambiguous name
    |
-   = 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: ambiguous because of a conflict between a macro-expanded `extern crate` and `--extern` flag 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 +46,6 @@ 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

@@ -36,9 +36,8 @@ error[E0659]: `core` is ambiguous
 LL |     ::core::panic!();
    |       ^^^^ ambiguous name
    |
-   = 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: ambiguous because of a conflict between a macro-expanded `extern crate` and `--extern` flag 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 +46,6 @@ 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