Unconditionally capture tokens for attributes.

This allows us to avoid synthesizing tokens in `prepend_attr`, since we
have the original tokens available.

We still need to synthesize tokens when expanding `cfg_attr`,
but this is an unavoidable consequence of the syntax of `cfg_attr` -
the user does not supply the `#` and `[]` tokens that a `cfg_attr`
expands to.

compiler/rustc_ast/src/ast.rs

@@ -2423,6 +2423,7 @@ pub struct Attribute {
     /// or the construct this attribute is contained within (inner).
     pub style: AttrStyle,
     pub span: Span,
+    pub tokens: Option<LazyTokenStream>,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]

compiler/rustc_ast/src/attr/mod.rs

@@ -325,7 +325,7 @@ pub fn mk_attr(style: AttrStyle, path: Path, args: MacArgs, span: Span) -> Attri
 }
 
 pub fn mk_attr_from_item(style: AttrStyle, item: AttrItem, span: Span) -> Attribute {
-    Attribute { kind: AttrKind::Normal(item), id: mk_attr_id(), style, span }
+    Attribute { kind: AttrKind::Normal(item), id: mk_attr_id(), style, span, tokens: None }
 }
 
 /// Returns an inner attribute with the given value and span.
@@ -344,7 +344,13 @@ pub fn mk_doc_comment(
     data: Symbol,
     span: Span,
 ) -> Attribute {
-    Attribute { kind: AttrKind::DocComment(comment_kind, data), id: mk_attr_id(), style, span }
+    Attribute {
+        kind: AttrKind::DocComment(comment_kind, data),
+        id: mk_attr_id(),
+        style,
+        span,
+        tokens: None,
+    }
 }
 
 pub fn list_contains_name(items: &[NestedMetaItem], name: Symbol) -> bool {

compiler/rustc_ast/src/mut_visit.rs

@@ -577,7 +577,7 @@ pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
 }
 
 pub fn noop_visit_attribute<T: MutVisitor>(attr: &mut Attribute, vis: &mut T) {
-    let Attribute { kind, id: _, style: _, span } = attr;
+    let Attribute { kind, id: _, style: _, span, tokens: _ } = attr;
     match kind {
         AttrKind::Normal(AttrItem { path, args, tokens: _ }) => {
             vis.visit_path(path);