Auto merge of #50682 - F001:issue-50589, r=petrochenkov

Add lint for multiple associated types

Fix https://github.com/rust-lang/rust/issues/50589. cc @abonander

src/test/ui/lint/issue-50589-multiple-associated-types.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 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.
+
+// compile-pass
+
+use std::iter::Iterator;
+
+type Unit = ();
+
+fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
+    Box::new(None.into_iter())
+}
+
+fn main() {
+    test();
+}

src/test/ui/lint/issue-50589-multiple-associated-types.stderr

@@ -0,0 +1,23 @@
+warning: associated type binding `Item` specified more than once
+  --> $DIR/issue-50589-multiple-associated-types.rs:17:39
+   |
+LL | fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
+   |                            ---------  ^^^^^^^^^^^ used more than once
+   |                            |
+   |                            first use of `Item`
+   |
+   = note: #[warn(duplicate_associated_type_bindings)] on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #50589 <https://github.com/rust-lang/rust/issues/50589>
+
+warning: associated type binding `Item` specified more than once
+  --> $DIR/issue-50589-multiple-associated-types.rs:17:39
+   |
+LL | fn test() ->  Box<Iterator<Item = (), Item = Unit>> {
+   |                            ---------  ^^^^^^^^^^^ used more than once
+   |                            |
+   |                            first use of `Item`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #50589 <https://github.com/rust-lang/rust/issues/50589>
+