librustc: Implement simple where clauses.

These `where` clauses are accepted everywhere generics are currently
accepted and desugar during type collection to the type parameter bounds
we have today.

A new keyword, `where`, has been added. Therefore, this is a breaking
change. Change uses of `where` to other identifiers.

[breaking-change]

src/test/compile-fail/where-clauses-no-bounds-or-predicates.rs

@@ -0,0 +1,23 @@
+// Copyright 2014 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 equal1<T>(_: &T, _: &T) -> bool where {
+//~^ ERROR a `where` clause must have at least one predicate in it
+    true
+}
+
+fn equal2<T>(_: &T, _: &T) -> bool where T: {
+//~^ ERROR each predicate in a `where` clause must have at least one bound
+    true
+}
+
+fn main() {
+}
+

src/test/compile-fail/where-clauses-not-parameter.rs

@@ -0,0 +1,17 @@
+// Copyright 2014 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 equal<T>(_: &T, _: &T) -> bool where int : Eq {
+    //~^ ERROR undeclared type parameter
+}
+
+fn main() {
+}
+

src/test/compile-fail/where-clauses-unsatisfied.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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 equal<T>(_: &T, _: &T) -> bool where T : Eq {
+}
+
+struct Struct;
+
+fn main() {
+    equal(&Struct, &Struct)
+    //~^ ERROR failed to find an implementation of trait
+}
+