rustc: Add error about obsolete struct deref

Author: brson

src/librustc/middle/typeck/check/mod.rs

@@ -2765,6 +2765,16 @@ pub fn check_expr_with_unifier(fcx: @FnCtxt,
                         }
                         None => {
                             match *sty {
+                                ty::ty_struct(did, ref substs) if {
+                                    let fields = ty::struct_fields(fcx.tcx(), did, substs);
+                                    fields.len() == 1
+                                      && fields[0].ident == token::special_idents::unnamed_field
+                                } => {
+                                    // This is an obsolete struct deref
+                                    tcx.sess.span_err(
+                                        expr.span,
+                                        "single-field tuple-structs can no longer be dereferenced");
+                                }
                                 _ => {
                                     fcx.type_error_message(expr.span,
                                         |actual| {

src/test/compile-fail/obsolete-tuple-struct-deref.rs

@@ -0,0 +1,15 @@
+// Copyright 2013 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.
+
+fn main() {
+    struct S(int);
+    let s = S(0);
+    let x = *s; //~ ERROR single-field tuple-structs can no longer be dereferenced
+}