rustc_metadata: Store a flag telling whether an item may have doc links in its attributes

This should be cheap on rustc side, but it's significant optimization for rustdoc that won't need to decode and process attributes unnecessarily

compiler/rustc_ast/src/util/comments.rs

@@ -24,6 +24,14 @@ pub struct Comment {
     pub pos: BytePos,
 }
 
+/// A fast conservative estimate on whether the string can contain documentation links.
+/// A pair of square brackets `[]` must exist in the string, but we only search for the
+/// opening bracket because brackets always go in pairs in practice.
+#[inline]
+pub fn may_have_doc_links(s: &str) -> bool {
+    s.contains('[')
+}
+
 /// Makes a doc string more presentable to users.
 /// Used by rustdoc and perhaps other tools, but not by rustc.
 pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {