rollup merge of #21421: huonw/one-suggestion-per-trait

This is clearly useless, the user doesn't need to know that they could
implement/import `foo::bar::Baz` 4 times.

Fixes #21405.

src/test/compile-fail/method-suggestion-no-duplication.rs

@@ -0,0 +1,22 @@
+// 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.
+
+// issue #21405
+
+fn foo<F>(f: F) where F: FnMut(usize) {}
+
+fn main() {
+    foo(|s| s.is_empty());
+    //~^ ERROR does not implement any method
+    //~^^ HELP #1: `core::slice::SliceExt`
+    //~^^^ HELP #2: `core::str::StrExt`
+    //~^^^^ HELP #3: `collections::slice::SliceExt`
+    //~^^^^^ HELP #4: `collections::str::StrExt`
+}