auto merge of #13316 : eddyb/rust/ast-ptr, r=brson

Replaces Gc<T> in the AST with a custom owned smart pointer, P<T>. Fixes #7929.

## Benefits
* **Identity** (affinity?): sharing AST nodes is bad for the various analysis passes (e.g. one could bypass borrowck with a shared `ExprAddrOf` node taking a mutable borrow), the only reason we haven't hit any serious issues with it is because of inefficient folding passes which will always deduplicate any such shared nodes. Even if we were to switch to an arena, this would still hold, i.e. we wouldn't just use `&'a T` in the AST, but rather an wrapper (`P<'a, T>`?).

* **Immutability**: `P<T>` disallows mutating its inner `T` (unless that contains an `Unsafe` interior, which won't happen in the AST), unlike `~T`.

* **Efficiency**: folding can reuse allocation space for `P<T>` and `Vec<T>`, the latter even when the input and output types differ (as it would be the case with arenas or an AST with type parameters to toggle macro support). Also, various algorithms have been changed from copying `Gc<T>` to using `&T` and iterators.

* **Maintainability**: there is another reason I didn't just replace `Gc<T>` with `~T`: `P<T>` provides a fixed interface (`Deref`, `and_then` and `map`) which can remain fully functional even if the implementation changes (using a special thread-local heap, for example). Moreover, switching to, e.g. `P<'a, T>` (for a contextual arena) is easy and mostly automated.

Author: bors

mk/crates.mk

@@ -70,7 +70,7 @@ DEPS_graphviz := std
 DEPS_green := std native:context_switch
 DEPS_rustuv := std native:uv native:uv_support
 DEPS_native := std
-DEPS_syntax := std term serialize log fmt_macros debug
+DEPS_syntax := std term serialize log fmt_macros debug arena
 DEPS_rustc := syntax flate arena serialize getopts rbml \
               time log graphviz debug rustc_llvm rustc_back
 DEPS_rustc_llvm := native:rustllvm libc std

src/libfourcc/lib.rs

@@ -63,10 +63,9 @@ use syntax::ext::base::{ExtCtxt, MacExpr};
 use syntax::ext::build::AstBuilder;
 use syntax::parse::token;
 use syntax::parse::token::InternedString;
+use syntax::ptr::P;
 use rustc::plugin::Registry;
 
-use std::gc::Gc;
-
 #[plugin_registrar]
 pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_macro("fourcc", expand_syntax_ext);
@@ -135,7 +134,7 @@ struct Ident {
 }
 
 fn parse_tts(cx: &ExtCtxt,
-             tts: &[ast::TokenTree]) -> (Gc<ast::Expr>, Option<Ident>) {
+             tts: &[ast::TokenTree]) -> (P<ast::Expr>, Option<Ident>) {
     let p = &mut cx.new_parser_from_tts(tts);
     let ex = p.parse_expr();
     let id = if p.token == token::EOF {
@@ -156,7 +155,7 @@ fn parse_tts(cx: &ExtCtxt,
 fn target_endian_little(cx: &ExtCtxt, sp: Span) -> bool {
     let meta = cx.meta_name_value(sp, InternedString::new("target_endian"),
         ast::LitStr(InternedString::new("little"), ast::CookedStr));
-    contains(cx.cfg().as_slice(), meta)
+    contains(cx.cfg().as_slice(), &*meta)
 }
 
 // FIXME (10872): This is required to prevent an LLVM assert on Windows