Auto merge of #45620 - ollie27:rustdoc_impl_generic_dupe, r=QuietMisdreavus

rustdoc: Fix duplicated impls with generics

The same type can appear multiple times in impls so we need to use a set
to avoid adding it multiple times.

Fixes: #45584

src/test/rustdoc/issue-45584.rs

@@ -0,0 +1,25 @@
+// Copyright 2017 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.
+
+#![crate_name = "foo"]
+
+pub trait Bar<T, U> {}
+
+// @has 'foo/struct.Foo1.html'
+pub struct Foo1;
+// @count - '//*[@class="impl"]' 1
+// @has - '//*[@class="impl"]' "impl Bar<Foo1, &'static Foo1> for Foo1"
+impl Bar<Foo1, &'static Foo1> for Foo1 {}
+
+// @has 'foo/struct.Foo2.html'
+pub struct Foo2;
+// @count - '//*[@class="impl"]' 1
+// @has - '//*[@class="impl"]' "impl Bar<&'static Foo2, Foo2> for u8"
+impl Bar<&'static Foo2, Foo2> for u8 {}