Auto merge of #44920 - vi:rustdoc_implementors_src_link, r=GuillaumeGomez

rustdoc: Render [src] links for trait implementors

Should close #43893.

<s>No tests [yet].</s>

r? @QuietMisdreavus

src/test/rustdoc/issue-43893.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 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.
+
+// ignore-cross-compile
+
+#![crate_name = "foo"]
+
+pub trait SomeTrait {}
+pub struct SomeStruct;
+
+// @has foo/trait.SomeTrait.html '//a/@href' '../src/foo/issue-43893.rs.html#19'
+impl SomeTrait for usize {}
+
+// @has foo/trait.SomeTrait.html '//a/@href' '../src/foo/issue-43893.rs.html#22-24'
+impl SomeTrait for SomeStruct {
+    // deliberately multi-line impl
+}