Replace the `String` in `ParseResult::Failure` with `Token`.

This lets us delay creation of failure messages until they are needed,
which avoids ~1.6M allocations in html5ever.

src/libsyntax/ext/tt/macro_parser.rs

@@ -251,14 +251,22 @@ pub fn nameize(p_s: &ParseSess, ms: &[TokenTree], res: &[Rc<NamedMatch>])
 
 pub enum ParseResult<T> {
     Success(T),
-    /// Arm failed to match
-    Failure(syntax_pos::Span, String),
+    /// Arm failed to match. If the second parameter is `token::Eof`, it
+    /// indicates an unexpected end of macro invocation. Otherwise, it
+    /// indicates that no rules expected the given token.
+    Failure(syntax_pos::Span, Token),
     /// Fatal error (malformed macro?). Abort compilation.
     Error(syntax_pos::Span, String)
 }
 
+pub fn parse_failure_msg(tok: Token) -> String {
+    match tok {
+        token::Eof => "unexpected end of macro invocation".to_string(),
+        _ => format!("no rules expected the token `{}`", pprust::token_to_string(&tok)),
+    }
+}
+
 pub type NamedParseResult = ParseResult<HashMap<Ident, Rc<NamedMatch>>>;
-pub type PositionalParseResult = ParseResult<Vec<Rc<NamedMatch>>>;
 
 /// Perform a token equality check, ignoring syntax context (that is, an
 /// unhygienic comparison)
@@ -446,7 +454,7 @@ pub fn parse(sess: &ParseSess,
             } else if eof_eis.len() > 1 {
                 return Error(sp, "ambiguity: multiple successful parses".to_string());
             } else {
-                return Failure(sp, "unexpected end of macro invocation".to_string());
+                return Failure(sp, token::Eof);
             }
         } else {
             if (!bb_eis.is_empty() && !next_eis.is_empty())
@@ -467,8 +475,7 @@ pub fn parse(sess: &ParseSess,
                     }
                 ))
             } else if bb_eis.is_empty() && next_eis.is_empty() {
-                return Failure(sp, format!("no rules expected the token `{}`",
-                            pprust::token_to_string(&tok)));
+                return Failure(sp, tok);
             } else if !next_eis.is_empty() {
                 /* Now process the next token */
                 while !next_eis.is_empty() {

src/libsyntax/ext/tt/macro_rules.rs

@@ -16,7 +16,7 @@ use ext::expand::{Expansion, ExpansionKind};
 use ext::placeholders;
 use ext::tt::macro_parser::{Success, Error, Failure};
 use ext::tt::macro_parser::{MatchedSeq, MatchedNonterminal};
-use ext::tt::macro_parser::parse;
+use ext::tt::macro_parser::{parse, parse_failure_msg};
 use parse::ParseSess;
 use parse::lexer::new_tt_reader;
 use parse::parser::{Parser, Restrictions};
@@ -100,7 +100,7 @@ fn generic_extension<'cx>(cx: &'cx ExtCtxt,
 
     // Which arm's failure should we report? (the one furthest along)
     let mut best_fail_spot = DUMMY_SP;
-    let mut best_fail_msg = "internal error: ran no matchers".to_string();
+    let mut best_fail_tok = None;
 
     for (i, lhs) in lhses.iter().enumerate() { // try each arm's matchers
         let lhs_tt = match *lhs {
@@ -139,9 +139,9 @@ fn generic_extension<'cx>(cx: &'cx ExtCtxt,
                     macro_ident: name
                 })
             }
-            Failure(sp, msg) => if sp.lo >= best_fail_spot.lo {
+            Failure(sp, tok) => if sp.lo >= best_fail_spot.lo {
                 best_fail_spot = sp;
-                best_fail_msg = msg;
+                best_fail_tok = Some(tok);
             },
             Error(err_sp, ref msg) => {
                 cx.span_fatal(err_sp.substitute_dummy(sp), &msg[..])
@@ -149,7 +149,8 @@ fn generic_extension<'cx>(cx: &'cx ExtCtxt,
         }
     }
 
-     cx.span_fatal(best_fail_spot.substitute_dummy(sp), &best_fail_msg[..]);
+    let best_fail_msg = parse_failure_msg(best_fail_tok.expect("ran no matchers"));
+    cx.span_fatal(best_fail_spot.substitute_dummy(sp), &best_fail_msg);
 }
 
 pub struct MacroRulesExpander;
@@ -227,8 +228,12 @@ pub fn compile(sess: &ParseSess, def: &ast::MacroDef) -> SyntaxExtension {
 
     let argument_map = match parse(sess, &Vec::new(), arg_reader, &argument_gram) {
         Success(m) => m,
-        Failure(sp, str) | Error(sp, str) => {
-            panic!(sess.span_diagnostic.span_fatal(sp.substitute_dummy(def.span), &str));
+        Failure(sp, tok) => {
+            let s = parse_failure_msg(tok);
+            panic!(sess.span_diagnostic.span_fatal(sp.substitute_dummy(def.span), &s));
+        }
+        Error(sp, s) => {
+            panic!(sess.span_diagnostic.span_fatal(sp.substitute_dummy(def.span), &s));
         }
     };
 

src/test/run-pass-fulldeps/auxiliary/procedural_mbe_matching.rs

@@ -25,6 +25,7 @@ use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
 use syntax::ext::build::AstBuilder;
 use syntax::ext::tt::macro_parser::{MatchedSeq, MatchedNonterminal};
 use syntax::ext::tt::macro_parser::{Success, Failure, Error};
+use syntax::ext::tt::macro_parser::parse_failure_msg;
 use syntax::ptr::P;
 use syntax_pos::Span;
 use rustc_plugin::Registry;
@@ -58,8 +59,11 @@ fn expand_mbe_matches(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
                 _ => unreachable!()
             }
         }
-        Failure(_, s) | Error(_, s) => {
-            panic!("expected Success, but got Error/Failure: {}", s);
+        Failure(_, tok) => {
+            panic!("expected Success, but got Failure: {}", parse_failure_msg(tok));
+        }
+        Error(_, s) => {
+            panic!("expected Success, but got Error: {}", s);
         }
     };