Improve tt-heavy expansion performance.

src/libsyntax/ext/base.rs

@@ -615,7 +615,9 @@ impl<'a> ExtCtxt<'a> {
 
     pub fn new_parser_from_tts(&self, tts: &[tokenstream::TokenTree])
         -> parser::Parser<'a> {
-        parse::tts_to_parser(self.parse_sess, tts.to_vec())
+        let mut parser = parse::tts_to_parser(self.parse_sess, tts.to_vec());
+        parser.allow_interpolated_tts = false; // FIXME(jseyfried) `quote!` can't handle these yet
+        parser
     }
     pub fn codemap(&self) -> &'a CodeMap { self.parse_sess.codemap() }
     pub fn parse_sess(&self) -> &'a parse::ParseSess { self.parse_sess }

src/libsyntax/ext/tt/macro_parser.rs

@@ -496,10 +496,19 @@ pub fn parse_nt<'a>(p: &mut Parser<'a>, sp: Span, name: &str) -> Nonterminal {
     match name {
         "tt" => {
             p.quote_depth += 1; //but in theory, non-quoted tts might be useful
-            let res: ::parse::PResult<'a, _> = p.parse_token_tree();
-            let res = token::NtTT(panictry!(res));
+            let mut tt = panictry!(p.parse_token_tree());
             p.quote_depth -= 1;
-            return res;
+            loop {
+                let nt = match tt {
+                    TokenTree::Token(_, token::Interpolated(ref nt)) => nt.clone(),
+                    _ => break,
+                };
+                match *nt {
+                    token::NtTT(ref sub_tt) => tt = sub_tt.clone(),
+                    _ => break,
+                }
+            }
+            return token::NtTT(tt);
         }
         _ => {}
     }

src/libsyntax/parse/parser.rs

@@ -212,6 +212,7 @@ pub struct Parser<'a> {
     pub expected_tokens: Vec<TokenType>,
     pub tts: Vec<(TokenTree, usize)>,
     pub desugar_doc_comments: bool,
+    pub allow_interpolated_tts: bool,
 }
 
 #[derive(PartialEq, Eq, Clone)]
@@ -301,6 +302,7 @@ impl<'a> Parser<'a> {
             expected_tokens: Vec::new(),
             tts: Vec::new(),
             desugar_doc_comments: desugar_doc_comments,
+            allow_interpolated_tts: true,
         };
 
         let tok = parser.next_tok();
@@ -2718,7 +2720,12 @@ impl<'a> Parser<'a> {
                 if self.tts.last().map(|&(_, i)| i == 1).unwrap_or(false) {
                     let tt = self.tts.pop().unwrap().0;
                     self.bump();
-                    return Ok(tt);
+                    return Ok(if self.allow_interpolated_tts {
+                        // avoid needlessly reparsing token trees in recursive macro expansions
+                        TokenTree::Token(tt.span(), token::Interpolated(Rc::new(token::NtTT(tt))))
+                    } else {
+                        tt
+                    });
                 }
 
                 let parsing_token_tree = ::std::mem::replace(&mut self.parsing_token_tree, true);