Make doc comments cheaper with `AttrKind`.

`AttrKind` is a new type with two variants, `Normal` and `DocComment`. It's a
big performance win (over 10% in some cases) because `DocComment` lets doc
comments (which are common) be represented very cheaply.

`Attribute` gets some new helper methods to ease the transition:
- `has_name()`: check if the attribute name matches a single `Symbol`; for
  `DocComment` variants it succeeds if the symbol is `sym::doc`.
- `is_doc_comment()`: check if it has a `DocComment` kind.
- `{get,unwrap}_normal_item()`: extract the item from a `Normal` variant;
  panic otherwise.

Fixes #60935.

1 files changed, 13 insertions, 9 deletions

diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index 6cfa0dfad82..b688dce87c1 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -287,7 +287,7 @@ pub fn parse_in_attr<'a, T>(
 ) -> PResult<'a, T> {
     let mut parser = Parser::new(
         sess,
-        attr.item.tokens.clone(),
+        attr.get_normal_item().tokens.clone(),
         None,
         false,
         false,
@@ -393,18 +393,22 @@ fn prepend_attrs(
 
         let source = pprust::attribute_to_string(attr);
         let macro_filename = FileName::macro_expansion_source_code(&source);
-        if attr.is_sugared_doc {
-            let stream = parse_stream_from_source_str(macro_filename, source, sess, Some(span));
-            builder.push(stream);
-            continue
-        }
+
+        let item = match attr.kind {
+            ast::AttrKind::Normal(ref item) => item,
+            ast::AttrKind::DocComment(_) => {
+                let stream = parse_stream_from_source_str(macro_filename, source, sess, Some(span));
+                builder.push(stream);
+                continue
+            }
+        };
 
         // synthesize # [ $path $tokens ] manually here
         let mut brackets = tokenstream::TokenStreamBuilder::new();
 
         // For simple paths, push the identifier directly
-        if attr.item.path.segments.len() == 1 && attr.item.path.segments[0].args.is_none() {
-            let ident = attr.item.path.segments[0].ident;
+        if item.path.segments.len() == 1 && item.path.segments[0].args.is_none() {
+            let ident = item.path.segments[0].ident;
             let token = token::Ident(ident.name, ident.as_str().starts_with("r#"));
             brackets.push(tokenstream::TokenTree::token(token, ident.span));
 
@@ -415,7 +419,7 @@ fn prepend_attrs(
             brackets.push(stream);
         }
 
-        brackets.push(attr.item.tokens.clone());
+        brackets.push(item.tokens.clone());
 
         // The span we list here for `#` and for `[ ... ]` are both wrong in
         // that it encompasses more than each token, but it hopefully is "good