resolve: Improve diagnostics for ambiguities in extern prelude

Author: petrochenkov

compiler/rustc_resolve/src/diagnostics.rs

@@ -1865,14 +1865,20 @@ 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}");
@@ -1888,7 +1894,8 @@ 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 {

compiler/rustc_resolve/src/ident.rs

@@ -436,6 +436,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         // to detect potential ambiguities.
         let mut innermost_result: Option<(NameBinding<'_>, Flags)> = None;
         let mut determinacy = Determinacy::Determined;
+        // Shadowed bindings don't need to be marked as used or non-speculatively loaded.
+        macro finalize_scope() {
+            if innermost_result.is_none() { finalize } else { None }
+        }
 
         // Go through all the scopes and try to resolve the name.
         let break_result = self.visit_scopes(
@@ -501,13 +505,13 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         _ => Err(Determinacy::Determined),
                     },
                     Scope::Module(module, derive_fallback_lint_id) => {
-                        let (adjusted_parent_scope, finalize) =
+                        let (adjusted_parent_scope, adjusted_finalize) =
                             if matches!(scope_set, ScopeSet::ModuleAndExternPrelude(..)) {
-                                (parent_scope, finalize)
+                                (parent_scope, finalize_scope!())
                             } else {
                                 (
                                     &ParentScope { module, ..*parent_scope },
-                                    finalize.map(|f| Finalize { used: Used::Scope, ..f }),
+                                    finalize_scope!().map(|f| Finalize { used: Used::Scope, ..f }),
                                 )
                             };
                         let binding = this.reborrow().resolve_ident_in_module_unadjusted(
@@ -520,7 +524,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             } else {
                                 Shadowing::Restricted
                             },
-                            finalize,
+                            adjusted_finalize,
                             ignore_binding,
                             ignore_import,
                         );
@@ -569,15 +573,18 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         None => Err(Determinacy::Determined),
                     },
                     Scope::ExternPreludeItems => {
-                        match this.reborrow().extern_prelude_get_item(ident, finalize.is_some()) {
+                        match this
+                            .reborrow()
+                            .extern_prelude_get_item(ident, finalize_scope!().is_some())
+                        {
                             Some(binding) => Ok((binding, Flags::empty())),
                             None => Err(Determinacy::determined(
                                 this.graph_root.unexpanded_invocations.borrow().is_empty(),
                             )),
                         }
                     }
                     Scope::ExternPreludeFlags => {
-                        match this.extern_prelude_get_flag(ident, finalize.is_some()) {
+                        match this.extern_prelude_get_flag(ident, finalize_scope!().is_some()) {
                             Some(binding) => Ok((binding, Flags::empty())),
                             None => Err(Determinacy::Determined),
                         }
@@ -612,8 +619,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             if matches!(ident.name, sym::f16)
                                 && !this.tcx.features().f16()
                                 && !ident.span.allows_unstable(sym::f16)
-                                && finalize.is_some()
-                                && innermost_result.is_none()
+                                && finalize_scope!().is_some()
                             {
                                 feature_err(
                                     this.tcx.sess,
@@ -626,8 +632,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             if matches!(ident.name, sym::f128)
                                 && !this.tcx.features().f128()
                                 && !ident.span.allows_unstable(sym::f128)
-                                && finalize.is_some()
-                                && innermost_result.is_none()
+                                && finalize_scope!().is_some()
                             {
                                 feature_err(
                                     this.tcx.sess,

compiler/rustc_resolve/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(arbitrary_self_types)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
+#![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(rustc_attrs)]

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

tests/ui/resolve/extern-prelude-speculative.rs

@@ -0,0 +1,10 @@
+// Non-existent path in `--extern` doesn't result in an error if it's shadowed by `extern crate`.
+
+//@ check-pass
+//@ compile-flags: --extern something=/path/to/nowhere
+
+extern crate std as something;
+
+fn main() {
+    something::println!();
+}

tests/ui/rust-2018/uniform-paths/deadlock.rs

@@ -2,7 +2,7 @@
 //@ compile-flags:--extern foo --extern bar
 
 use bar::foo; //~ ERROR can't find crate for `bar`
-use foo::bar; //~ ERROR can't find crate for `foo`
+use foo::bar;
 //~^^ ERROR unresolved imports `bar::foo`, `foo::bar`
 
 fn main() {}

tests/ui/rust-2018/uniform-paths/deadlock.stderr

@@ -4,12 +4,6 @@ error[E0463]: can't find crate for `bar`
 LL | use bar::foo;
    |     ^^^ can't find crate
 
-error[E0463]: can't find crate for `foo`
-  --> $DIR/deadlock.rs:5:5
-   |
-LL | use foo::bar;
-   |     ^^^ can't find crate
-
 error[E0432]: unresolved imports `bar::foo`, `foo::bar`
   --> $DIR/deadlock.rs:4:5
    |
@@ -18,7 +12,7 @@ LL | use bar::foo;
 LL | use foo::bar;
    |     ^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0432, E0463.
 For more information about an error, try `rustc --explain E0432`.