Support parsing Meta followed by fat arrow

Author: dtolnay

src/attr.rs

@@ -721,7 +721,7 @@ pub(crate) mod parsing {
     pub(crate) fn parse_meta_after_path(path: Path, input: ParseStream) -> Result<Meta> {
         if input.peek(token::Paren) || input.peek(token::Bracket) || input.peek(token::Brace) {
             parse_meta_list_after_path(path, input).map(Meta::List)
-        } else if input.peek(Token![=]) {
+        } else if input.peek(Token![=]) && !input.peek(Token![==]) && !input.peek(Token![=>]) {
             parse_meta_name_value_after_path(path, input).map(Meta::NameValue)
         } else {
             Ok(Meta::Path(path))