Adds a ProcMacro form of syntax extension

This commit adds syntax extension forms matching the types for procedural macros 2.0 (RFC #1566), these still require the usual syntax extension boiler plate, but this is a first step towards proper implementation and should be useful for macros 1.1 stuff too.

Supports both attribute-like and function-like macros.

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

@@ -0,0 +1,56 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(plugin, plugin_registrar, rustc_private)]
+
+extern crate proc_macro;
+extern crate rustc_plugin;
+extern crate syntax;
+
+use proc_macro::prelude::*;
+use rustc_plugin::Registry;
+use syntax::ext::base::SyntaxExtension;
+use syntax::ext::proc_macro_shim::prelude::*;
+
+#[plugin_registrar]
+pub fn plugin_registrar(reg: &mut Registry) {
+    reg.register_syntax_extension(token::intern("attr_tru"),
+                                  SyntaxExtension::AttrProcMacro(Box::new(attr_tru)));
+    reg.register_syntax_extension(token::intern("attr_identity"),
+                                  SyntaxExtension::AttrProcMacro(Box::new(attr_identity)));
+    reg.register_syntax_extension(token::intern("tru"),
+                                  SyntaxExtension::ProcMacro(Box::new(tru)));
+    reg.register_syntax_extension(token::intern("ret_tru"),
+                                  SyntaxExtension::ProcMacro(Box::new(ret_tru)));
+    reg.register_syntax_extension(token::intern("identity"),
+                                  SyntaxExtension::ProcMacro(Box::new(identity)));
+}
+
+fn attr_tru(_attr: TokenStream, _item: TokenStream) -> TokenStream {
+    lex("fn f1() -> bool { true }")
+}
+
+fn attr_identity(_attr: TokenStream, item: TokenStream) -> TokenStream {
+    let source = item.to_string();
+    lex(&source)
+}
+
+fn tru(_ts: TokenStream) -> TokenStream {
+    lex("true")
+}
+
+fn ret_tru(_ts: TokenStream) -> TokenStream {
+    lex("return true;")
+}
+
+fn identity(ts: TokenStream) -> TokenStream {
+    let source = ts.to_string();
+    lex(&source)
+}

src/test/run-pass-fulldeps/proc_macro.rs

@@ -0,0 +1,48 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:proc_macro_def.rs
+// ignore-stage1
+// ignore-cross-compile
+
+#![feature(plugin, custom_attribute)]
+#![feature(type_macros)]
+
+#![plugin(proc_macro_def)]
+
+#[attr_tru]
+fn f1() -> bool {
+    return false;
+}
+
+#[attr_identity]
+fn f2() -> bool {
+    return identity!(true);
+}
+
+fn f3() -> identity!(bool) {
+    ret_tru!();
+}
+
+fn f4(x: bool) -> bool {
+    match x {
+        identity!(true) => false,
+        identity!(false) => true,
+    }
+}
+
+fn main() {
+    assert!(f1());
+    assert!(f2());
+    assert!(tru!());
+    assert!(f3());
+    assert!(identity!(5 == 5));
+    assert!(f4(false));
+}