Rollup merge of rust-lang#74370 - Manishearth:re-spotlight, r=GuillaumeGomez

Reintroduce spotlight / "important traits" feature

(Reopened version of rust-lang#74111 because Github is broken, see discussion there)

Fixes rust-lang#73785

This PR reintroduces the "spotlight" ("important traits") feature.

A couple changes have been made:

As there were concerns about its visibility, it has been moved to be next to the return type, as opposed to being on the side.

It also no longer produces a modal, it shows the traits on hover, and it can be clicked on to pin the hover bubble.

![image](https://user-images.githubusercontent.com/1617736/86674555-a82d2600-bfad-11ea-9a4a-a1a9ffd66ae5.png)

![image](https://user-images.githubusercontent.com/1617736/86674533-a1061800-bfad-11ea-9e8a-c62ad86ed0d7.png)

It also works fine on mobile:

![image](https://user-images.githubusercontent.com/1617736/86674638-bda25000-bfad-11ea-8d8d-1798b608923e.png)

Author: Manishearth

src/doc/rustdoc/src/unstable-features.md

@@ -150,6 +150,27 @@ Book][unstable-doc-cfg] and [its tracking issue][issue-doc-cfg].
 [unstable-doc-cfg]: ../unstable-book/language-features/doc-cfg.html
 [issue-doc-cfg]: https://github.com/rust-lang/rust/issues/43781
 
+### Adding your trait to the "Important Traits" dialog
+
+Rustdoc keeps a list of a few traits that are believed to be "fundamental" to a given type when
+implemented on it. These traits are intended to be the primary interface for their types, and are
+often the only thing available to be documented on their types. For this reason, Rustdoc will track
+when a given type implements one of these traits and call special attention to it when a function
+returns one of these types. This is the "Important Traits" dialog, visible as a circle-i button next
+to the function, which, when clicked, shows the dialog.
+
+In the standard library, the traits that qualify for inclusion are `Iterator`, `io::Read`, and
+`io::Write`. However, rather than being implemented as a hard-coded list, these traits have a
+special marker attribute on them: `#[doc(spotlight)]`. This means that you could apply this
+attribute to your own trait to include it in the "Important Traits" dialog in documentation.
+
+The `#[doc(spotlight)]` attribute currently requires the `#![feature(doc_spotlight)]` feature gate.
+For more information, see [its chapter in the Unstable Book][unstable-spotlight] and [its tracking
+issue][issue-spotlight].
+
+[unstable-spotlight]: ../unstable-book/language-features/doc-spotlight.html
+[issue-spotlight]: https://github.com/rust-lang/rust/issues/45040
+
 ### Exclude certain dependencies from documentation
 
 The standard library uses several dependencies which, in turn, use several types and traits from the

src/doc/unstable-book/src/language-features/doc-spotlight.md

@@ -0,0 +1,30 @@
+# `doc_spotlight`
+
+The tracking issue for this feature is: [#45040]
+
+The `doc_spotlight` feature allows the use of the `spotlight` parameter to the `#[doc]` attribute,
+to "spotlight" a specific trait on the return values of functions. Adding a `#[doc(spotlight)]`
+attribute to a trait definition will make rustdoc print extra information for functions which return
+a type that implements that trait. This attribute is applied to the `Iterator`, `io::Read`, and
+`io::Write` traits in the standard library.
+
+You can do this on your own traits, like this:
+
+```
+#![feature(doc_spotlight)]
+
+#[doc(spotlight)]
+pub trait MyTrait {}
+
+pub struct MyStruct;
+impl MyTrait for MyStruct {}
+
+/// The docs for this function will have an extra line about `MyStruct` implementing `MyTrait`,
+/// without having to write that yourself!
+pub fn my_fn() -> MyStruct { MyStruct }
+```
+
+This feature was originally implemented in PR [#45039].
+
+[#45040]: https://github.com/rust-lang/rust/issues/45040
+[#45039]: https://github.com/rust-lang/rust/pull/45039

src/libcore/future/future.rs

@@ -24,6 +24,7 @@ use crate::task::{Context, Poll};
 /// `.await` the value.
 ///
 /// [`Waker`]: ../task/struct.Waker.html
+#[doc(spotlight)]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[lang = "future_trait"]

src/libcore/iter/traits/iterator.rs

@@ -92,6 +92,7 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
     label = "`{Self}` is not an iterator",
     message = "`{Self}` is not an iterator"
 )]
+#[doc(spotlight)]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub trait Iterator {
     /// The type of the elements being iterated over.

src/libcore/lib.rs

@@ -96,6 +96,7 @@
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]
+#![cfg_attr(not(bootstrap), feature(doc_spotlight))]
 #![feature(duration_consts_2)]
 #![feature(extern_types)]
 #![feature(fundamental)]

src/librustc_ast_passes/feature_gate.rs

@@ -253,6 +253,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                     include => external_doc
                     cfg => doc_cfg
                     masked => doc_masked
+                    spotlight => doc_spotlight
                     alias => doc_alias
                     keyword => doc_keyword
                 );

src/librustc_feature/active.rs

@@ -368,6 +368,9 @@ declare_features! (
     /// Allows `#[doc(masked)]`.
     (active, doc_masked, "1.21.0", Some(44027), None),
 
+    /// Allows `#[doc(spotlight)]`.
+    (active, doc_spotlight, "1.22.0", Some(45040), None),
+
     /// Allows `#[doc(include = "some-file")]`.
     (active, external_doc, "1.22.0", Some(44732), None),
 

src/librustc_span/symbol.rs

@@ -400,6 +400,7 @@ symbols! {
         doc_cfg,
         doc_keyword,
         doc_masked,
+        doc_spotlight,
         doctest,
         document_private_items,
         dotdot_in_tuple_patterns,
@@ -968,6 +969,7 @@ symbols! {
         soft,
         specialization,
         speed,
+        spotlight,
         sqrtf32,
         sqrtf64,
         sse4a_target_feature,

src/librustdoc/clean/inline.rs

@@ -12,7 +12,7 @@ use rustc_metadata::creader::LoadedMacro;
 use rustc_middle::ty;
 use rustc_mir::const_eval::is_min_const_fn;
 use rustc_span::hygiene::MacroKind;
-use rustc_span::symbol::Symbol;
+use rustc_span::symbol::{sym, Symbol};
 use rustc_span::Span;
 
 use crate::clean::{self, GetDefId, ToSource, TypeKind};
@@ -194,13 +194,15 @@ pub fn build_external_trait(cx: &DocContext<'_>, did: DefId) -> clean::Trait {
     let generics = (cx.tcx.generics_of(did), predicates).clean(cx);
     let generics = filter_non_trait_generics(did, generics);
     let (generics, supertrait_bounds) = separate_supertrait_bounds(generics);
+    let is_spotlight = load_attrs(cx, did).clean(cx).has_doc_flag(sym::spotlight);
     let is_auto = cx.tcx.trait_is_auto(did);
     clean::Trait {
         auto: auto_trait,
         unsafety: cx.tcx.trait_def(did).unsafety,
         generics,
         items: trait_items,
         bounds: supertrait_bounds,
+        is_spotlight,
         is_auto,
     }
 }

src/librustdoc/clean/mod.rs

@@ -1007,6 +1007,7 @@ impl Clean<FnRetTy> for hir::FnRetTy<'_> {
 impl Clean<Item> for doctree::Trait<'_> {
     fn clean(&self, cx: &DocContext<'_>) -> Item {
         let attrs = self.attrs.clean(cx);
+        let is_spotlight = attrs.has_doc_flag(sym::spotlight);
         Item {
             name: Some(self.name.clean(cx)),
             attrs,
@@ -1021,6 +1022,7 @@ impl Clean<Item> for doctree::Trait<'_> {
                 items: self.items.iter().map(|ti| ti.clean(cx)).collect(),
                 generics: self.generics.clean(cx),
                 bounds: self.bounds.clean(cx),
+                is_spotlight,
                 is_auto: self.is_auto.clean(cx),
             }),
         }