rustdoc: linking to a local proc macro no longer warns

fixes #91274

tried to keep the fix general in case we ever have
any other kind of item that occupies
multiple namespaces simultaniously.

Author: lolbinarycat

src/librustdoc/lib.rs

@@ -17,6 +17,7 @@
 #![feature(test)]
 #![feature(type_alias_impl_trait)]
 #![feature(type_ascription)]
+#![feature(iterator_try_reduce)]
 #![recursion_limit = "256"]
 #![warn(rustc::internal)]
 #![allow(clippy::collapsible_if, clippy::collapsible_else_if)]

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -2219,6 +2219,16 @@ fn report_malformed_generics(
     );
 }
 
+fn refer_to_single_item(v: &[Res]) -> bool {
+    // proc macros can exist in multiple namespaces at once,
+    // so we need to compare DefIds
+    v.iter().try_reduce(|l, r| match (l, r) {
+        (Res::Def(_, lid), Res::Def(_, rid)) if lid == rid =>
+            Some(l),
+        _ => None,
+    }).is_some()
+}
+
 /// Report an ambiguity error, where there were multiple possible resolutions.
 ///
 /// If all `candidates` have the same kind, it's not possible to disambiguate so in this case,
@@ -2241,7 +2251,7 @@ fn ambiguity_error(
         )
         .filter(|res| descrs.insert(res.descr()))
         .collect::<Vec<_>>();
-    if descrs.len() == 1 {
+    if descrs.len() == 1 || refer_to_single_item(&kinds) {
         // There is no way for users to disambiguate at this point, so better return the first
         // candidate and not show a warning.
         return false;

tests/rustdoc/intra-doc/link-to-proc-macro.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: --crate-type=proc-macro
+//@ has 'foo/index.html' '//a[@href="macro.my_macro.html"]' 'my_macro'
+//! Link to [`my_macro`].
+#![crate_name = "foo"]
+
+// regression test for https://github.com/rust-lang/rust/issues/91274
+
+extern crate proc_macro;
+
+#[proc_macro]
+pub fn my_macro(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    input
+}