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

Reintroduce spotlight / "important traits" feature

(Reopened version of https://github.com/rust-lang/rust/pull/74111 because Github is broken, see discussion there)

Fixes https://github.com/rust-lang/rust/issues/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)

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};
@@ -194,6 +194,7 @@ 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,
@@ -201,6 +202,7 @@ pub fn build_external_trait(cx: &DocContext<'_>, did: DefId) -> clean::Trait {
         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),
             }),
         }

src/librustdoc/clean/types.rs

@@ -967,6 +967,7 @@ pub struct Trait {
     pub items: Vec<Item>,
     pub generics: Generics,
     pub bounds: Vec<GenericBound>,
+    pub is_spotlight: bool,
     pub is_auto: bool,
 }
 

src/librustdoc/html/format.rs

@@ -63,10 +63,22 @@ impl Buffer {
         Buffer { for_html: false, buffer: String::new() }
     }
 
+    crate fn is_empty(&self) -> bool {
+        self.buffer.is_empty()
+    }
+
     crate fn into_inner(self) -> String {
         self.buffer
     }
 
+    crate fn insert_str(&mut self, idx: usize, s: &str) {
+        self.buffer.insert_str(idx, s);
+    }
+
+    crate fn push_str(&mut self, s: &str) {
+        self.buffer.push_str(s);
+    }
+
     // Intended for consumption by write! and writeln! (std::fmt) but without
     // the fmt::Result return type imposed by fmt::Write (and avoiding the trait
     // import).

src/librustdoc/html/render.rs

@@ -2415,7 +2415,7 @@ fn item_function(w: &mut Buffer, cx: &Context, it: &clean::Item, f: &clean::Func
     write!(
         w,
         "{vis}{constness}{asyncness}{unsafety}{abi}fn \
-           {name}{generics}{decl}{where_clause}</pre>",
+           {name}{generics}{decl}{spotlight}{where_clause}</pre>",
         vis = it.visibility.print_with_space(),
         constness = f.header.constness.print_with_space(),
         asyncness = f.header.asyncness.print_with_space(),
@@ -2425,7 +2425,8 @@ fn item_function(w: &mut Buffer, cx: &Context, it: &clean::Item, f: &clean::Func
         generics = f.generics.print(),
         where_clause = WhereClause { gens: &f.generics, indent: 0, end_newline: true },
         decl = Function { decl: &f.decl, header_len, indent: 0, asyncness: f.header.asyncness }
-            .print()
+            .print(),
+        spotlight = spotlight_decl(&f.decl),
     );
     document(w, cx, it)
 }
@@ -2612,7 +2613,7 @@ fn item_trait(w: &mut Buffer, cx: &Context, it: &clean::Item, t: &clean::Trait)
         let name = m.name.as_ref().unwrap();
         let item_type = m.type_();
         let id = cx.derive_id(format!("{}.{}", item_type, name));
-        write!(w, "<h3 id='{id}' class='method'><code>", id = id);
+        write!(w, "<h3 id='{id}' class='method'><code>", id = id,);
         render_assoc_item(w, m, AssocItemLink::Anchor(Some(&id)), ItemType::Impl);
         write!(w, "</code>");
         render_stability_since(w, m, t);
@@ -2926,7 +2927,7 @@ fn render_assoc_item(
         write!(
             w,
             "{}{}{}{}{}{}{}fn <a href='{href}' class='fnname'>{name}</a>\
-                   {generics}{decl}{where_clause}",
+                   {generics}{decl}{spotlight}{where_clause}",
             if parent == ItemType::Trait { "    " } else { "" },
             meth.visibility.print_with_space(),
             header.constness.print_with_space(),
@@ -2938,6 +2939,7 @@ fn render_assoc_item(
             name = name,
             generics = g.print(),
             decl = Function { decl: d, header_len, indent, asyncness: header.asyncness }.print(),
+            spotlight = spotlight_decl(&d),
             where_clause = WhereClause { gens: g, indent, end_newline }
         )
     }
@@ -3559,6 +3561,62 @@ fn should_render_item(item: &clean::Item, deref_mut_: bool) -> bool {
     }
 }
 
+fn spotlight_decl(decl: &clean::FnDecl) -> String {
+    let mut out = Buffer::html();
+    let mut trait_ = String::new();
+
+    if let Some(did) = decl.output.def_id() {
+        let c = cache();
+        if let Some(impls) = c.impls.get(&did) {
+            for i in impls {
+                let impl_ = i.inner_impl();
+                if impl_.trait_.def_id().map_or(false, |d| c.traits[&d].is_spotlight) {
+                    if out.is_empty() {
+                        out.push_str(&format!(
+                            "<h3 class=\"important\">Important traits for {}</h3>\
+                                      <code class=\"content\">",
+                            impl_.for_.print()
+                        ));
+                        trait_.push_str(&impl_.for_.print().to_string());
+                    }
+
+                    //use the "where" class here to make it small
+                    out.push_str(&format!(
+                        "<span class=\"where fmt-newline\">{}</span>",
+                        impl_.print()
+                    ));
+                    let t_did = impl_.trait_.def_id().unwrap();
+                    for it in &impl_.items {
+                        if let clean::TypedefItem(ref tydef, _) = it.inner {
+                            out.push_str("<span class=\"where fmt-newline\">    ");
+                            assoc_type(
+                                &mut out,
+                                it,
+                                &[],
+                                Some(&tydef.type_),
+                                AssocItemLink::GotoSource(t_did, &FxHashSet::default()),
+                                "",
+                            );
+                            out.push_str(";</span>");
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    if !out.is_empty() {
+        out.insert_str(
+            0,
+            "<span class=\"important-traits\"><span class=\"important-traits-tooltip\">ⓘ<div class='important-traits-tooltiptext'><span class=\"docblock\">"
+
+        );
+        out.push_str("</code></span></div></span></span>");
+    }
+
+    out.into_inner()
+}
+
 fn render_impl(
     w: &mut Buffer,
     cx: &Context,
@@ -3670,7 +3728,8 @@ fn render_impl(
                 // Only render when the method is not static or we allow static methods
                 if render_method_item {
                     let id = cx.derive_id(format!("{}.{}", item_type, name));
-                    write!(w, "<h4 id='{}' class=\"{}{}\"><code>", id, item_type, extra_class);
+                    write!(w, "<h4 id='{}' class=\"{}{}\">", id, item_type, extra_class);
+                    write!(w, "<code>");
                     render_assoc_item(w, item, link.anchor(&id), ItemType::Impl);
                     write!(w, "</code>");
                     render_stability_since_raw(w, item.stable_since(), outer_version);

src/librustdoc/html/static/main.js

@@ -2636,6 +2636,13 @@ function defocusSearchBar() {
         });
     }());
 
+    onEachLazy(document.getElementsByClassName("important-traits"), function(e) {
+        e.onclick = function() {
+            this.getElementsByClassName('important-traits-tooltiptext')[0]
+                .classList.toggle("force-tooltip");
+        };
+    });
+
     // In the search display, allows to switch between tabs.
     function printTab(nb) {
         if (nb === 0 || nb === 1 || nb === 2) {

src/librustdoc/html/static/rustdoc.css

@@ -146,9 +146,12 @@ code, pre, a.test-arrow {
 	border-radius: 3px;
 	padding: 0 0.1em;
 }
-.docblock pre code, .docblock-short pre code {
+.docblock pre code, .docblock-short pre code, .docblock code.spotlight {
 	padding: 0;
 }
+.docblock code.spotlight :last-child {
+	padding-bottom: 0.6em;
+}
 pre {
 	padding: 14px;
 }
@@ -523,7 +526,7 @@ h4 > code, h3 > code, .invisible > code {
 	font-size: 0.8em;
 }
 
-.content .methods > div {
+.content .methods > div:not(.important-traits) {
 	margin-left: 40px;
 	margin-bottom: 15px;
 }
@@ -1079,10 +1082,6 @@ h3 > .collapse-toggle, h4 > .collapse-toggle {
 	font-size: 16px;
 }
 
-.tooltip:hover .tooltiptext {
-	display: inline;
-}
-
 .tooltip .tooltiptext::after {
 	content: " ";
 	position: absolute;
@@ -1098,13 +1097,52 @@ h3 > .collapse-toggle, h4 > .collapse-toggle {
 	font-size: 20px;
 }
 
-.tooltip .tooltiptext {
+.important-traits-tooltip {
+	display: inline-block;
+	cursor: pointer;
+}
+
+.important-traits:hover .important-traits-tooltiptext,
+.important-traits .important-traits-tooltiptext.force-tooltip {
+	display: inline-block;
+}
+
+.important-traits .important-traits-tooltiptext {
+	display: none;
+	padding: 5px 3px 3px 3px;
+	border-radius: 6px;
+	margin-left: 5px;
+	z-index: 10;
+	font-size: 16px;
+	cursor: default;
+	position: absolute;
 	border: 1px solid;
-	font-weight: normal;
+}
+
+.important-traits-tooltip::after {
+	/* The margin on the tooltip does not capture hover events,
+	   this extends the area of hover enough so that mouse hover is not
+	   lost when moving the mouse to the tooltip */
+    content: "\00a0\00a0\00a0";
+}
+
+.important-traits .important, .important-traits .docblock {
+	margin: 0;
+}
+
+.important-traits .docblock code.content{
+    margin: 0;
+    padding: 0;
+    font-size: 20px;
+}
+
+/* Example code has the "Run" button that
+   needs to be positioned relative to the pre */
+pre.rust.rust-example-rendered {
+	position: relative;
 }
 
 pre.rust {
-	position: relative;
 	tab-size: 4;
 	-moz-tab-size: 4;
 }
@@ -1144,6 +1182,18 @@ pre.rust {
 	font-size: 16px;
 }
 
+.important-traits {
+	cursor: pointer;
+	z-index: 2;
+	margin-left: 5px;
+}
+
+h4 > .important-traits {
+	position: absolute;
+	left: -44px;
+	top: 2px;
+}
+
 #all-types {
 	text-align: center;
 	border: 1px solid;
@@ -1370,6 +1420,12 @@ pre.rust {
 		z-index: 1;
 	}
 
+	h4 > .important-traits {
+		position: absolute;
+		left: -22px;
+		top: 24px;
+	}
+
 	#titles > div > div.count {
 		float: left;
 		width: 100%;

src/librustdoc/html/static/themes/ayu.css

@@ -394,6 +394,11 @@ pre.ignore:hover, .information:hover + pre.ignore {
 	border-color: transparent #314559 transparent transparent;
 }
 
+.important-traits-tooltiptext {
+    background-color: #314559;
+    border-color: #5c6773;
+}
+
 #titles > div.selected {
     background-color: #141920 !important;
 	border-bottom: 1px solid #ffb44c !important;

src/librustdoc/html/static/themes/dark.css

@@ -337,6 +337,11 @@ pre.ignore:hover, .information:hover + pre.ignore {
 	border-color: transparent black transparent transparent;
 }
 
+.important-traits-tooltiptext {
+	background-color: #111;
+	border-color: #777;
+}
+
 #titles > div:not(.selected) {
 	background-color: #252525;
 	border-top-color: #252525;

src/librustdoc/html/static/themes/light.css

@@ -331,6 +331,11 @@ pre.ignore:hover, .information:hover + pre.ignore {
 	border-color: transparent black transparent transparent;
 }
 
+.important-traits-tooltiptext {
+	background-color: #eee;
+	border-color: #999;
+}
+
 #titles > div:not(.selected) {
 	background-color: #e6e6e6;
 	border-top-color: #e6e6e6;

src/libstd/io/mod.rs

@@ -499,6 +499,7 @@ where
 /// [`&str`]: ../../std/primitive.str.html
 /// [slice]: ../../std/primitive.slice.html
 #[stable(feature = "rust1", since = "1.0.0")]
+#[doc(spotlight)]
 pub trait Read {
     /// Pull some bytes from this source into the specified buffer, returning
     /// how many bytes were read.
@@ -1261,6 +1262,7 @@ impl Initializer {
 ///
 /// [`write_all`]: #method.write_all
 #[stable(feature = "rust1", since = "1.0.0")]
+#[doc(spotlight)]
 pub trait Write {
     /// Write a buffer into this writer, returning how many bytes were written.
     ///

src/libstd/lib.rs

@@ -261,6 +261,7 @@
 #![feature(doc_cfg)]
 #![feature(doc_keyword)]
 #![feature(doc_masked)]
+#![cfg_attr(not(bootstrap), feature(doc_spotlight))]
 #![feature(dropck_eyepatch)]
 #![feature(duration_constants)]
 #![feature(exact_size_is_empty)]

src/test/rustdoc/doc-spotlight.rs

@@ -0,0 +1,36 @@
+#![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
+}

src/test/ui/feature-gates/feature-gate-doc_spotlight.rs

@@ -0,0 +1,4 @@
+#[doc(spotlight)] //~ ERROR: `#[doc(spotlight)]` is experimental
+trait SomeTrait {}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-doc_spotlight.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `#[doc(spotlight)]` is experimental
+  --> $DIR/feature-gate-doc_spotlight.rs:1:1
+   |
+LL | #[doc(spotlight)]
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #45040 <https://github.com/rust-lang/rust/issues/45040> for more information
+   = help: add `#![feature(doc_spotlight)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.