Rollup merge of #25993 - nham:fix_23969, r=nikomatsakis

Adds two error codes, one for duplicate associated constants and one for types. I'm not certain these should each have their own code, but E0201 is already solely for duplicate associated functions so at least it kinda matches. This will lead to somewhat redundant error explanations, but that's nothing new!

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;
+}