Remove `ast::Token::take`.

Instead of replacing `TokenTreesReader::token` in two steps, we can just
do it in one, which is both simpler and faster.

compiler/rustc_ast/src/token.rs

@@ -13,7 +13,7 @@ use rustc_span::symbol::{kw, sym};
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::{self, edition::Edition, Span, DUMMY_SP};
 use std::borrow::Cow;
-use std::{fmt, mem};
+use std::fmt;
 
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub enum CommentKind {
@@ -335,11 +335,6 @@ impl Token {
         Token::new(Ident(ident.name, ident.is_raw_guess()), ident.span)
     }
 
-    /// Return this token by value and leave a dummy token in its place.
-    pub fn take(&mut self) -> Self {
-        mem::replace(self, Token::dummy())
-    }
-
     /// For interpolated tokens, returns a span of the fragment to which the interpolated
     /// token refers. For all other tokens this is just a regular span.
     /// It is particularly important to use this for identifiers and lifetimes

compiler/rustc_parse/src/lexer/tokentrees.rs

@@ -247,14 +247,13 @@ impl<'a> TokenTreesReader<'a> {
     fn parse_token_tree_other(&mut self) -> TokenTree {
         // `spacing` for the returned token is determined by the next token:
         // its kind and its `preceded_by_whitespace` status.
-        let this_tok = self.token.take();
         let (next_tok, is_next_tok_preceded_by_whitespace) = self.string_reader.next_token();
         let this_spacing = if is_next_tok_preceded_by_whitespace || !next_tok.is_op() {
             Spacing::Alone
         } else {
             Spacing::Joint
         };
-        self.token = next_tok;
+        let this_tok = std::mem::replace(&mut self.token, next_tok);
         TokenTree::Token(this_tok, this_spacing)
     }
 }