rustc: Obsolete the @ syntax entirely

This removes all remnants of `@` pointers from rustc. Additionally, this removes
the `GC` structure from the prelude as it seems odd exporting an experimental
type in the prelude by default.

Closes #14193
[breaking-change]

src/libsyntax/util/parser_testing.rs

@@ -15,6 +15,8 @@ use parse::{new_parser_from_source_str};
 use parse::parser::Parser;
 use parse::token;
 
+use std::gc::Gc;
+
 // map a string to tts, using a made-up filename:
 pub fn string_to_tts(source_str: String) -> Vec<ast::TokenTree> {
     let ps = new_parse_sess();
@@ -46,21 +48,21 @@ pub fn string_to_crate (source_str : String) -> ast::Crate {
 }
 
 // parse a string, return an expr
-pub fn string_to_expr (source_str : String) -> @ast::Expr {
+pub fn string_to_expr (source_str : String) -> Gc<ast::Expr> {
     with_error_checking_parse(source_str, |p| {
         p.parse_expr()
     })
 }
 
 // parse a string, return an item
-pub fn string_to_item (source_str : String) -> Option<@ast::Item> {
+pub fn string_to_item (source_str : String) -> Option<Gc<ast::Item>> {
     with_error_checking_parse(source_str, |p| {
         p.parse_item(Vec::new())
     })
 }
 
 // parse a string, return a stmt
-pub fn string_to_stmt(source_str : String) -> @ast::Stmt {
+pub fn string_to_stmt(source_str : String) -> Gc<ast::Stmt> {
     with_error_checking_parse(source_str, |p| {
         p.parse_stmt(Vec::new())
     })
@@ -68,7 +70,7 @@ pub fn string_to_stmt(source_str : String) -> @ast::Stmt {
 
 // parse a string, return a pat. Uses "irrefutable"... which doesn't
 // (currently) affect parsing.
-pub fn string_to_pat(source_str: String) -> @ast::Pat {
+pub fn string_to_pat(source_str: String) -> Gc<ast::Pat> {
     string_to_parser(&new_parse_sess(), source_str).parse_pat()
 }