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.

Author: bors

src/doc/rust.md

@@ -2555,6 +2555,8 @@ The currently implemented features of the reference compiler are:
                         which is considered wildly unsafe and will be
                         obsoleted by language improvements.
 
+* `tuple_indexing` - Allows use of tuple indexing (expressions like `expr.0`)
+
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about #[feature] directives which enabled
 the new feature (because the directive is no longer necessary). However, if

src/librustc/front/feature_gate.rs

@@ -70,6 +70,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("unboxed_closures", Active),
     ("import_shadowing", Active),
     ("advanced_slice_patterns", Active),
+    ("tuple_indexing", Active),
 
     // if you change this list without updating src/doc/rust.md, cmr will be sad
 
@@ -338,6 +339,11 @@ impl<'a> Visitor<()> for Context<'a> {
                                   "unboxed closures are a work-in-progress \
                                    feature with known bugs");
             }
+            ast::ExprTupField(..) => {
+                self.gate_feature("tuple_indexing",
+                                  e.span,
+                                  "tuple indexing is experimental");
+            }
             _ => {}
         }
         visit::walk_expr(self, e, ());

src/librustc/lint/builtin.rs

@@ -1056,6 +1056,7 @@ impl UnnecessaryParens {
                 ast::ExprUnary(_, ref x) |
                 ast::ExprCast(ref x, _) |
                 ast::ExprField(ref x, _, _) |
+                ast::ExprTupField(ref x, _, _) |
                 ast::ExprIndex(ref x, _) => {
                     // &X { y: 1 }, X { y: 1 }.y
                     contains_exterior_struct_lit(&**x)

src/librustc/middle/borrowck/mod.rs

@@ -807,7 +807,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                         out.push_str(token::get_name(fname).get());
                     }
                     mc::PositionalField(idx) => {
-                        out.push_char('#'); // invent a notation here
+                        out.push_char('.');
                         out.push_str(idx.to_string().as_slice());
                     }
                 }

src/librustc/middle/cfg/construct.rs

@@ -467,7 +467,8 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             ast::ExprCast(e, _) |
             ast::ExprUnary(_, e) |
             ast::ExprParen(e) |
-            ast::ExprField(e, _, _) => {
+            ast::ExprField(e, _, _) |
+            ast::ExprTupField(e, _, _) => {
                 self.straightline(expr, pred, [e])
             }
 

src/librustc/middle/check_const.rs

@@ -173,6 +173,7 @@ fn check_expr(v: &mut CheckCrateVisitor, e: &Expr, is_const: bool) {
           ExprAddrOf(MutImmutable, _) |
           ExprParen(..) |
           ExprField(..) |
+          ExprTupField(..) |
           ExprIndex(..) |
           ExprTup(..) |
           ExprRepeat(..) |

src/librustc/middle/check_static.rs

@@ -106,7 +106,7 @@ impl<'a, 'tcx> Visitor<bool> for CheckStaticVisitor<'a, 'tcx> {
         }
 
         match e.node {
-            ast::ExprField(..) | ast::ExprVec(..) |
+            ast::ExprField(..) | ast::ExprTupField(..) | ast::ExprVec(..) |
             ast::ExprBlock(..) | ast::ExprTup(..)  => {
                 visit::walk_expr(self, e, is_const);
             }

src/librustc/middle/const_eval.rs

@@ -225,6 +225,8 @@ impl<'a, 'tcx> ConstEvalVisitor<'a, 'tcx> {
 
             ast::ExprField(ref base, _, _) => self.classify(&**base),
 
+            ast::ExprTupField(ref base, _, _) => self.classify(&**base),
+
             ast::ExprIndex(ref base, ref idx) =>
                 join(self.classify(&**base), self.classify(&**idx)),
 

src/librustc/middle/dead.rs

@@ -145,6 +145,17 @@ impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
         }
     }
 
+    fn handle_tup_field_access(&mut self, lhs: &ast::Expr, idx: uint) {
+        match ty::get(ty::expr_ty_adjusted(self.tcx, lhs)).sty {
+            ty::ty_struct(id, _) => {
+                let fields = ty::lookup_struct_fields(self.tcx, id);
+                let field_id = fields[idx].id;
+                self.live_symbols.insert(field_id.node);
+            },
+            _ => ()
+        }
+    }
+
     fn handle_field_pattern_match(&mut self, lhs: &ast::Pat, pats: &[ast::FieldPat]) {
         let id = match self.tcx.def_map.borrow().get(&lhs.id) {
             &def::DefVariant(_, id, _) => id,
@@ -255,6 +266,9 @@ impl<'a, 'tcx> Visitor<MarkSymbolVisitorContext> for MarkSymbolVisitor<'a, 'tcx>
             ast::ExprField(ref lhs, ref ident, _) => {
                 self.handle_field_access(&**lhs, &ident.node);
             }
+            ast::ExprTupField(ref lhs, idx, _) => {
+                self.handle_tup_field_access(&**lhs, idx.node);
+            }
             _ => ()
         }
 

src/librustc/middle/expr_use_visitor.rs

@@ -324,6 +324,10 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,TYPER> {
                 self.select_from_expr(&**base);
             }
 
+            ast::ExprTupField(ref base, _, _) => {         // base.<n>
+                self.select_from_expr(&**base);
+            }
+
             ast::ExprIndex(ref lhs, ref rhs) => {           // lhs[rhs]
                 if !self.walk_overloaded_operator(expr, &**lhs, [rhs.clone()]) {
                     self.select_from_expr(&**lhs);