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, 7 insertions, 5 deletions

diff --git a/src/libsyntax/parse/parser/item.rs b/src/libsyntax/parse/parser/item.rs
index 5b60e7e6dba..cc6235c6fc7 100644
--- a/src/libsyntax/parse/parser/item.rs
+++ b/src/libsyntax/parse/parser/item.rs
@@ -3,8 +3,8 @@ use super::diagnostics::{Error, dummy_arg, ConsumeClosingDelim};
 
 use crate::maybe_whole;
 use crate::ptr::P;
-use crate::ast::{self, DUMMY_NODE_ID, Ident, Attribute, AttrStyle, AnonConst, Item, ItemKind};
-use crate::ast::{ImplItem, ImplItemKind, TraitItem, TraitItemKind, UseTree, UseTreeKind};
+use crate::ast::{self, DUMMY_NODE_ID, Ident, Attribute, AttrKind, AttrStyle, AnonConst, Item};
+use crate::ast::{ItemKind, ImplItem, ImplItemKind, TraitItem, TraitItemKind, UseTree, UseTreeKind};
 use crate::ast::{PathSegment, IsAuto, Constness, IsAsync, Unsafety, Defaultness};
 use crate::ast::{Visibility, VisibilityKind, Mutability, FnHeader, ForeignItem, ForeignItemKind};
 use crate::ast::{Ty, TyKind, Generics, GenericBounds, TraitRef, EnumDef, VariantData, StructField};
@@ -483,12 +483,14 @@ impl<'a> Parser<'a> {
     /// Emits an expected-item-after-attributes error.
     fn expected_item_err(&mut self, attrs: &[Attribute]) -> PResult<'a,  ()> {
         let message = match attrs.last() {
-            Some(&Attribute { is_sugared_doc: true, .. }) => "expected item after doc comment",
-            _ => "expected item after attributes",
+            Some(&Attribute { kind: AttrKind::DocComment(_), .. }) =>
+                "expected item after doc comment",
+            _ =>
+                "expected item after attributes",
         };
 
         let mut err = self.diagnostic().struct_span_err(self.prev_span, message);
-        if attrs.last().unwrap().is_sugared_doc {
+        if attrs.last().unwrap().is_doc_comment() {
             err.span_label(self.prev_span, "this doc comment doesn't document anything");
         }
         Err(err)