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.

src/librustdoc/html/render.rs

@@ -1325,7 +1325,7 @@ impl DocFolder for Cache {
                 // Figure out the id of this impl. This may map to a
                 // primitive rather than always to a struct/enum.
                 // Note: matching twice to restrict the lifetime of the `i` borrow.
-                let mut dids = vec![];
+                let mut dids = FxHashSet();
                 if let clean::Item { inner: clean::ImplItem(ref i), .. } = item {
                     let masked_trait = i.trait_.def_id().map_or(false,
                         |d| self.masked_crates.contains(&d.krate));
@@ -1335,15 +1335,15 @@ impl DocFolder for Cache {
                             clean::BorrowedRef {
                                 type_: box clean::ResolvedPath { did, .. }, ..
                             } => {
-                                dids.push(did);
+                                dids.insert(did);
                             }
                             ref t => {
                                 let did = t.primitive_type().and_then(|t| {
                                     self.primitive_locations.get(&t).cloned()
                                 });
 
                                 if let Some(did) = did {
-                                    dids.push(did);
+                                    dids.insert(did);
                                 }
                             }
                         }
@@ -1352,7 +1352,7 @@ impl DocFolder for Cache {
                     if let Some(generics) = i.trait_.as_ref().and_then(|t| t.generics()) {
                         for bound in generics {
                             if let Some(did) = bound.def_id() {
-                                dids.push(did);
+                                dids.insert(did);
                             }
                         }
                     }

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 {}