Better detection of duplicate associated items.

Expands E0201 to be used for any duplicate associated items, not just duplicate
methods/functions. It also correctly detects when two different kinds of items
(like a constant and a method) have the same name.

Fixes #23969.

src/test/compile-fail/associated-item-duplicate-names-2.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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.
+
+#![feature(associated_consts)]
+
+struct Foo;
+
+impl Foo {
+    const bar: bool = true;
+    fn bar() {} //~ ERROR duplicate associated function
+}
+
+fn main() {}

src/test/compile-fail/associated-item-duplicate-names-3.rs

@@ -0,0 +1,28 @@
+// Copyright 2015 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.
+//
+// Before the introduction of the "duplicate associated type" error, the
+// program below used to result in the "ambiguous associated type" error E0223,
+// which is unexpected.
+
+trait Foo {
+    type Bar;
+}
+
+struct Baz;
+
+impl Foo for Baz {
+    type Bar = i16;
+    type Bar = u16; //~ ERROR duplicate associated type
+}
+
+fn main() {
+    let x: Baz::Bar = 5;
+}

src/test/compile-fail/associated-item-duplicate-names.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 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.
+
+// Test for issue #23969
+
+#![feature(associated_consts)]
+
+trait Foo {
+    type Ty;
+    const BAR: u32;
+}
+
+impl Foo for () {
+    type Ty = ();
+    type Ty = usize; //~ ERROR duplicate associated type
+    const BAR: u32 = 7;
+    const BAR: u32 = 8; //~ ERROR duplicate associated constant
+}
+
+fn main() {
+    let _: <() as Foo>::Ty = ();
+    let _: u32 = <() as Foo>::BAR;
+}