auto merge of #16866 : P1start/rust/tuple-indexing, r=brson

This allows code to access the fields of tuples and tuple structs behind the feature gate `tuple_indexing`:

```rust
#![feature(tuple_indexing)]

let x = (1i, 2i);
assert_eq!(x.1, 2);

struct Point(int, int);
let origin = Point(0, 0);
assert_eq!(origin.0, 0);
assert_eq!(origin.1, 0);
```

Implements [RFC 53](https://github.com/rust-lang/rfcs/blob/master/active/0053-tuple-accessors.md). Closes #16950.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 64ab0e5cb19..6bd1fba4b58 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -120,6 +120,8 @@ pub trait AstBuilder {
     fn expr_mut_addr_of(&self, sp: Span, e: Gc<ast::Expr>) -> Gc<ast::Expr>;
     fn expr_field_access(&self, span: Span, expr: Gc<ast::Expr>,
                          ident: ast::Ident) -> Gc<ast::Expr>;
+    fn expr_tup_field_access(&self, sp: Span, expr: Gc<ast::Expr>,
+                             idx: uint) -> Gc<ast::Expr>;
     fn expr_call(&self, span: Span, expr: Gc<ast::Expr>,
                  args: Vec<Gc<ast::Expr>>) -> Gc<ast::Expr>;
     fn expr_call_ident(&self, span: Span, id: ast::Ident,
@@ -605,6 +607,16 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         let id = Spanned { node: ident, span: field_span };
         self.expr(sp, ast::ExprField(expr, id, Vec::new()))
     }
+    fn expr_tup_field_access(&self, sp: Span, expr: Gc<ast::Expr>, idx: uint) -> Gc<ast::Expr> {
+        let field_span = Span {
+            lo: sp.lo - Pos::from_uint(idx.to_string().len()),
+            hi: sp.hi,
+            expn_info: sp.expn_info,
+        };
+
+        let id = Spanned { node: idx, span: field_span };
+        self.expr(sp, ast::ExprTupField(expr, id, Vec::new()))
+    }
     fn expr_addr_of(&self, sp: Span, e: Gc<ast::Expr>) -> Gc<ast::Expr> {
         self.expr(sp, ast::ExprAddrOf(ast::MutImmutable, e))
     }