resolve: Reject ambiguity built-in attr vs different built-in attr

compiler/rustc_hir/src/def.rs

@@ -5,6 +5,7 @@ use rustc_ast as ast;
 use rustc_ast::NodeId;
 use rustc_macros::HashStable_Generic;
 use rustc_span::hygiene::MacroKind;
+use rustc_span::Symbol;
 
 use std::array::IntoIter;
 use std::fmt::Debug;
@@ -34,7 +35,7 @@ pub enum CtorKind {
 #[derive(HashStable_Generic)]
 pub enum NonMacroAttrKind {
     /// Single-segment attribute defined by the language (`#[inline]`)
-    Builtin,
+    Builtin(Symbol),
     /// Multi-segment custom attribute living in a "tool module" (`#[rustfmt::skip]`).
     Tool,
     /// Single-segment custom attribute registered by a derive macro (`#[serde(default)]`).
@@ -371,7 +372,7 @@ impl CtorKind {
 impl NonMacroAttrKind {
     pub fn descr(self) -> &'static str {
         match self {
-            NonMacroAttrKind::Builtin => "built-in attribute",
+            NonMacroAttrKind::Builtin(..) => "built-in attribute",
             NonMacroAttrKind::Tool => "tool attribute",
             NonMacroAttrKind::DeriveHelper | NonMacroAttrKind::DeriveHelperCompat => {
                 "derive helper attribute"
@@ -393,7 +394,7 @@ impl NonMacroAttrKind {
             NonMacroAttrKind::Tool
             | NonMacroAttrKind::DeriveHelper
             | NonMacroAttrKind::DeriveHelperCompat => true,
-            NonMacroAttrKind::Builtin | NonMacroAttrKind::Registered => false,
+            NonMacroAttrKind::Builtin(..) | NonMacroAttrKind::Registered => false,
         }
     }
 }

compiler/rustc_resolve/src/diagnostics.rs

@@ -683,7 +683,7 @@ impl<'a> Resolver<'a> {
                     ));
                 }
                 Scope::BuiltinAttrs => {
-                    let res = Res::NonMacroAttr(NonMacroAttrKind::Builtin);
+                    let res = Res::NonMacroAttr(NonMacroAttrKind::Builtin(kw::Empty));
                     if filter_fn(res) {
                         suggestions.extend(
                             BUILTIN_ATTRIBUTES

compiler/rustc_resolve/src/macros.rs

@@ -757,7 +757,11 @@ impl<'a> Resolver<'a> {
                     }
                     Scope::BuiltinAttrs => {
                         if is_builtin_attr_name(ident.name) {
-                            ok(Res::NonMacroAttr(NonMacroAttrKind::Builtin), DUMMY_SP, this.arenas)
+                            ok(
+                                Res::NonMacroAttr(NonMacroAttrKind::Builtin(ident.name)),
+                                DUMMY_SP,
+                                this.arenas,
+                            )
                         } else {
                             Err(Determinacy::Determined)
                         }
@@ -810,13 +814,15 @@ impl<'a> Resolver<'a> {
                             // Found another solution, if the first one was "weak", report an error.
                             let (res, innermost_res) = (binding.res(), innermost_binding.res());
                             if res != innermost_res {
-                                let builtin = Res::NonMacroAttr(NonMacroAttrKind::Builtin);
+                                let is_builtin = |res| {
+                                    matches!(res, Res::NonMacroAttr(NonMacroAttrKind::Builtin(..)))
+                                };
                                 let derive_helper_compat =
                                     Res::NonMacroAttr(NonMacroAttrKind::DeriveHelperCompat);
 
                                 let ambiguity_error_kind = if is_import {
                                     Some(AmbiguityKind::Import)
-                                } else if innermost_res == builtin || res == builtin {
+                                } else if is_builtin(innermost_res) || is_builtin(res) {
                                     Some(AmbiguityKind::BuiltinAttr)
                                 } else if innermost_res == derive_helper_compat
                                     || res == derive_helper_compat

src/test/ui/proc-macro/ambiguous-builtin-attrs.rs

@@ -1,5 +1,5 @@
+// edition:2018
 // aux-build:builtin-attrs.rs
-
 #![feature(decl_macro)] //~ ERROR `feature` is ambiguous
 
 extern crate builtin_attrs;
@@ -31,3 +31,7 @@ fn main() {
     Bench;
     NonExistent; //~ ERROR cannot find value `NonExistent` in this scope
 }
+
+use deny as allow;
+#[allow(unused)] //~ ERROR `allow` is ambiguous (built-in attribute vs any other name)
+fn builtin_renamed() {}

src/test/ui/proc-macro/ambiguous-builtin-attrs.stderr

@@ -60,6 +60,20 @@ LL | use builtin_attrs::*;
    |     ^^^^^^^^^^^^^^^^
    = help: use `crate::repr` to refer to this attribute macro unambiguously
 
+error[E0659]: `allow` is ambiguous (built-in attribute vs any other name)
+  --> $DIR/ambiguous-builtin-attrs.rs:36:3
+   |
+LL | #[allow(unused)]
+   |   ^^^^^ ambiguous name
+   |
+   = note: `allow` could refer to a built-in attribute
+note: `allow` could also refer to the built-in attribute imported here
+  --> $DIR/ambiguous-builtin-attrs.rs:35:5
+   |
+LL | use deny as allow;
+   |     ^^^^^^^^^^^^^
+   = help: use `crate::allow` to refer to this built-in attribute unambiguously
+
 error[E0659]: `feature` is ambiguous (built-in attribute vs any other name)
   --> $DIR/ambiguous-builtin-attrs.rs:3:4
    |
@@ -80,7 +94,7 @@ error[E0517]: attribute should be applied to a struct, enum, or union
 LL | fn non_macro_expanded_location<#[repr(C)] T>() {
    |                                       ^   - not a struct, enum, or union
 
-error: aborting due to 7 previous errors
+error: aborting due to 8 previous errors
 
 Some errors have detailed explanations: E0425, E0517, E0659.
 For more information about an error, try `rustc --explain E0425`.