Use SmallVector for TtReader::stack.

This avoids 800,000 heap allocations when compiling html5ever. It
requires tweaking `SmallVector` a little.

src/libsyntax/ext/tt/transcribe.rs

@@ -10,14 +10,15 @@
 use self::LockstepIterSize::*;
 
 use ast::Ident;
-use syntax_pos::{Span, DUMMY_SP};
 use errors::{Handler, DiagnosticBuilder};
 use ext::tt::macro_parser::{NamedMatch, MatchedSeq, MatchedNonterminal};
 use parse::token::{DocComment, MatchNt, SubstNt};
 use parse::token::{Token, Interpolated, NtIdent, NtTT};
 use parse::token;
 use parse::lexer::TokenAndSpan;
+use syntax_pos::{Span, DUMMY_SP};
 use tokenstream::{self, TokenTree};
+use util::small_vector::SmallVector;
 
 use std::rc::Rc;
 use std::ops::Add;
@@ -36,7 +37,7 @@ struct TtFrame {
 pub struct TtReader<'a> {
     pub sp_diag: &'a Handler,
     /// the unzipped tree:
-    stack: Vec<TtFrame>,
+    stack: SmallVector<TtFrame>,
     /* for MBE-style macro transcription */
     interpolations: HashMap<Ident, Rc<NamedMatch>>,
 
@@ -74,7 +75,7 @@ pub fn new_tt_reader_with_doc_flag(sp_diag: &Handler,
                                    -> TtReader {
     let mut r = TtReader {
         sp_diag: sp_diag,
-        stack: vec!(TtFrame {
+        stack: SmallVector::one(TtFrame {
             forest: TokenTree::Sequence(DUMMY_SP, Rc::new(tokenstream::SequenceRepetition {
                 tts: src,
                 // doesn't matter. This merely holds the root unzipping.

src/libsyntax/lib.rs

@@ -35,6 +35,7 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(specialization)]
 
+extern crate core;
 extern crate serialize;
 extern crate term;
 extern crate libc;

src/libsyntax/util/small_vector.rs

@@ -11,6 +11,7 @@
 use self::SmallVectorRepr::*;
 use self::IntoIterRepr::*;
 
+use core::ops;
 use std::iter::{IntoIterator, FromIterator};
 use std::mem;
 use std::slice;
@@ -19,10 +20,12 @@ use std::vec;
 use util::move_map::MoveMap;
 
 /// A vector type optimized for cases where the size is almost always 0 or 1
+#[derive(Clone)]
 pub struct SmallVector<T> {
     repr: SmallVectorRepr<T>,
 }
 
+#[derive(Clone)]
 enum SmallVectorRepr<T> {
     Zero,
     One(T),
@@ -75,16 +78,11 @@ impl<T> SmallVector<T> {
     }
 
     pub fn as_slice(&self) -> &[T] {
-        match self.repr {
-            Zero => {
-                let result: &[T] = &[];
-                result
-            }
-            One(ref v) => {
-                unsafe { slice::from_raw_parts(v, 1) }
-            }
-            Many(ref vs) => vs
-        }
+        self
+    }
+
+    pub fn as_mut_slice(&mut self) -> &mut [T] {
+        self
     }
 
     pub fn pop(&mut self) -> Option<T> {
@@ -163,6 +161,38 @@ impl<T> SmallVector<T> {
     }
 }
 
+impl<T> ops::Deref for SmallVector<T> {
+    type Target = [T];
+
+    fn deref(&self) -> &[T] {
+        match self.repr {
+            Zero => {
+                let result: &[T] = &[];
+                result
+            }
+            One(ref v) => {
+                unsafe { slice::from_raw_parts(v, 1) }
+            }
+            Many(ref vs) => vs
+        }
+    }
+}
+
+impl<T> ops::DerefMut for SmallVector<T> {
+    fn deref_mut(&mut self) -> &mut [T] {
+        match self.repr {
+            Zero => {
+                let result: &mut [T] = &mut [];
+                result
+            }
+            One(ref mut v) => {
+                unsafe { slice::from_raw_parts_mut(v, 1) }
+            }
+            Many(ref mut vs) => vs
+        }
+    }
+}
+
 impl<T> IntoIterator for SmallVector<T> {
     type Item = T;
     type IntoIter = IntoIter<T>;