Use SmallVec for the inner vectors in MatcherPos::matches.

This avoids some allocations.

src/libsyntax/ext/tt/macro_parser.rs

@@ -143,6 +143,8 @@ struct MatcherTtFrame<'a> {
     idx: usize,
 }
 
+type NamedMatchVec = SmallVec<[NamedMatch; 4]>;
+
 /// Represents a single "position" (aka "matcher position", aka "item"), as described in the module
 /// documentation.
 #[derive(Clone)]
@@ -168,7 +170,7 @@ struct MatcherPos<'a> {
     /// all bound matches from the submatcher into the shared top-level `matches` vector. If `sep`
     /// and `up` are `Some`, then `matches` is _not_ the shared top-level list. Instead, if one
     /// wants the shared `matches`, one should use `up.matches`.
-    matches: Box<[Rc<Vec<NamedMatch>>]>,
+    matches: Box<[Rc<NamedMatchVec>]>,
     /// The position in `matches` corresponding to the first metavar in this matcher's sequence of
     /// token trees. In other words, the first metavar in the first token of `top_elts` corresponds
     /// to `matches[match_lo]`.
@@ -279,11 +281,11 @@ pub fn count_names(ms: &[TokenTree]) -> usize {
 }
 
 /// `len` `Vec`s (initially shared and empty) that will store matches of metavars.
-fn create_matches(len: usize) -> Box<[Rc<Vec<NamedMatch>>]> {
+fn create_matches(len: usize) -> Box<[Rc<NamedMatchVec>]> {
     if len == 0 {
         vec![]
     } else {
-        let empty_matches = Rc::new(Vec::new());
+        let empty_matches = Rc::new(SmallVec::new());
         vec![empty_matches.clone(); len]
     }.into_boxed_slice()
 }
@@ -337,7 +339,7 @@ fn initial_matcher_pos(ms: &[TokenTree], open: Span) -> MatcherPos {
 /// token tree it was derived from.
 #[derive(Debug, Clone)]
 pub enum NamedMatch {
-    MatchedSeq(Rc<Vec<NamedMatch>>, DelimSpan),
+    MatchedSeq(Rc<NamedMatchVec>, DelimSpan),
     MatchedNonterminal(Rc<Nonterminal>),
 }
 
@@ -545,7 +547,7 @@ fn inner_parse_loop<'a>(
                         new_item.match_cur += seq.num_captures;
                         new_item.idx += 1;
                         for idx in item.match_cur..item.match_cur + seq.num_captures {
-                            new_item.push_match(idx, MatchedSeq(Rc::new(vec![]), sp));
+                            new_item.push_match(idx, MatchedSeq(Rc::new(smallvec![]), sp));
                         }
                         cur_items.push(new_item);
                     }