Auto merge of #45039 - QuietMisdreavus:doc-spotlight, r=GuillaumeGomez,QuietMisdreavus

show in docs whether the return type of a function impls Iterator/Read/Write

Closes #25928

This PR makes it so that when rustdoc documents a function, it checks the return type to see whether it implements a handful of specific traits. If so, it will print the impl and any associated types. Rather than doing this via a whitelist within rustdoc, i chose to do this by a new `#[doc]` attribute parameter, so things like `Future` could tap into this if desired.

### Known shortcomings

~~The printing of impls currently uses the `where` class over the whole thing to shrink the font size relative to the function definition itself. Naturally, when the impl has a where clause of its own, it gets shrunken even further:~~ (This is no longer a problem because the design changed and rendered this concern moot.)

The lookup currently just looks at the top-level type, not looking inside things like Result or Option, which renders the spotlights on Read/Write a little less useful:

<details><summary>`File::{open, create}` don't have spotlight info (pic of old design)</summary>

![image](https://user-images.githubusercontent.com/5217170/31209495-e59d027e-a950-11e7-9998-ceefceb71c07.png)

</details>

All three of the initially spotlighted traits are generically implemented on `&mut` references. Rustdoc currently treats a `&mut T` reference-to-a-generic as an impl on the reference primitive itself. `&mut Self` counts as a generic in the eyes of rustdoc. All this combines to create this lovely scene on `Iterator::by_ref`:

<details><summary>`Iterator::by_ref` spotlights Iterator, Read, and Write (pic of old design)</summary>

![image](https://user-images.githubusercontent.com/5217170/31209554-50b271ca-a951-11e7-928b-4f83416c8681.png)

</details>

1 files changed, 46 insertions, 0 deletions

diff --git a/src/test/rustdoc/doc-spotlight.rs b/src/test/rustdoc/doc-spotlight.rs
new file mode 100644
index 00000000000..a570aa2d398
--- /dev/null
+++ b/src/test/rustdoc/doc-spotlight.rs
@@ -0,0 +1,46 @@
+// 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.
+
+#![feature(doc_spotlight)]
+
+pub struct Wrapper<T> {
+    inner: T,
+}
+
+impl<T: SomeTrait> SomeTrait for Wrapper<T> {}
+
+#[doc(spotlight)]
+pub trait SomeTrait {
+    // @has doc_spotlight/trait.SomeTrait.html
+    // @has - '//code[@class="content"]' 'impl<T: SomeTrait> SomeTrait for Wrapper<T>'
+    fn wrap_me(self) -> Wrapper<Self> where Self: Sized {
+        Wrapper {
+            inner: self,
+        }
+    }
+}
+
+pub struct SomeStruct;
+impl SomeTrait for SomeStruct {}
+
+impl SomeStruct {
+    // @has doc_spotlight/struct.SomeStruct.html
+    // @has - '//code[@class="content"]' 'impl SomeTrait for SomeStruct'
+    // @has - '//code[@class="content"]' 'impl<T: SomeTrait> SomeTrait for Wrapper<T>'
+    pub fn new() -> SomeStruct {
+        SomeStruct
+    }
+}
+
+// @has doc_spotlight/fn.bare_fn.html
+// @has - '//code[@class="content"]' 'impl SomeTrait for SomeStruct'
+pub fn bare_fn() -> SomeStruct {
+    SomeStruct
+}