auto merge of #6427 : catamorphism/rust/issue-6319, r=nikomatsakis

r? @nikomatsakis In #6319, several people mentioned they ran into a "computing
fictitious type" ICE in trans. This turns out to be because some
of my recent changes to typeck::check::_match resulted in type errors
getting reported with ty_err as the expected type, which meant the errors
were suppressed, and typechecking incorrectly succeeded (since the errors
weren't recorded).

Changed the error messages in these cases not to use an expected type at all,
rather, printing out a string describing the type that was expected (which is
what the code originally did). The result is a bit repetitive and the
proliferation of error-reporting functions in typeck::infer is a bit annoying,
but I thought it was important to fix this now; more cleanup can happen later.

src/test/compile-fail/issue-5100.rs

@@ -17,7 +17,7 @@ fn main() {
     }
 
     match (true, false) {
-        (true, false, false) => () //~ ERROR mismatched types: expected `(bool,bool)` but found `(bool,bool,bool)` (expected a tuple with 2 elements but found one with 3 elements)
+        (true, false, false) => () //~ ERROR mismatched types: expected `(bool,bool)` but found tuple (expected a tuple with 2 elements but found one with 3 elements)
     }
 
     match (true, false) {

src/test/compile-fail/suppressed-error.rs

@@ -0,0 +1,14 @@
+// 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() {
+    let (x, y) = (); //~ ERROR expected `()` but found tuple (types differ)
+    return x;
+}