Rollup merge of #102439 - fmease:rustdoc-simplify-cross-crate-trait-bounds, r=GuillaumeGomez

rustdoc: re-sugar more cross-crate trait bounds

Previously, we would only ever re-sugar cross-crate predicates like `Type: Trait, <Type as Trait>::Name == Rhs` to `Type: Trait<Name = Rhs>` if the `Type` was a generic parameter like `Self` or `T`. With this PR, `Type` can be any type.

Most notably, this means that we now re-sugar predicates involving associated types (where `Type` is of the form `Self::Name`) which are then picked up by the pre-existing logic that re-sugars them into bounds. As a result of that, the associated type `IntoIter` of `std`'s `IntoIterator` trait (re-exported from `core`) is no longer rendered as:

```rust
type IntoIter: Iterator
where
    <Self::IntoIter as Iterator>::Item == Self::Item;
```

but as one would expect: `type IntoIter: Iterator<Item = Self::Item>;`.

Cross-crate closure bounds like `F: Fn(i32) -> bool` are now also rendered properly (previously, the return type (`Self::Output`) would not be rendered and we would show the underlying equality predicate).

Fixes #77763.
Fixes #84579.
Fixes #102142.

`@rustbot` label T-rustdoc A-cross-crate-reexports
r? rustdoc

src/test/rustdoc/inline_cross/assoc_item_trait_bounds_with_bindings.out0.html

@@ -0,0 +1 @@
+<h4 class="code-header">type <a href="#associatedtype.Out0" class="associatedtype">Out0</a>: <a class="trait" href="../assoc_item_trait_bounds_with_bindings/trait.Support.html" title="trait assoc_item_trait_bounds_with_bindings::Support">Support</a>&lt;Item = <a class="primitive" href="{{channel}}/std/primitive.unit.html">()</a>&gt;</h4>

src/test/rustdoc/inline_cross/assoc_item_trait_bounds_with_bindings.out9.html

@@ -0,0 +1 @@
+<h4 class="code-header">type <a href="#associatedtype.Out9" class="associatedtype">Out9</a>: <a class="trait" href="{{channel}}/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(<a class="primitive" href="{{channel}}/std/primitive.i32.html">i32</a>) -&gt; <a class="primitive" href="{{channel}}/std/primitive.bool.html">bool</a> + <a class="trait" href="{{channel}}/core/clone/trait.Clone.html" title="trait core::clone::Clone">Clone</a></h4>

src/test/rustdoc/inline_cross/assoc_item_trait_bounds_with_bindings.rs

@@ -0,0 +1,40 @@
+// Regression test for issues #77763, #84579 and #102142.
+#![crate_name = "main"]
+
+// aux-build:assoc_item_trait_bounds_with_bindings.rs
+// build-aux-docs
+// ignore-cross-compile
+extern crate assoc_item_trait_bounds_with_bindings as aux;
+
+// FIXME(fmease): Don't render an incorrect `T: ?Sized` where-clause for parameters
+//                of GATs like `Main::Out{2,4}`. Add a snapshot test once it's fixed.
+// FIXME(fmease): Print the `for<>` parameter list in the bounds of
+//                `Main::Out{6,11,12}`.
+
+// @has main/trait.Main.html
+// @has - '//*[@id="associatedtype.Out0"]' 'type Out0: Support<Item = ()>'
+// @has - '//*[@id="associatedtype.Out1"]' 'type Out1: Support<Item = Self::Item>'
+// @has - '//*[@id="associatedtype.Out2"]' 'type Out2<T>: Support<Item = T>'
+// @has - '//*[@id="associatedtype.Out3"]' 'type Out3: Support<Produce<()> = bool>'
+// @has - '//*[@id="associatedtype.Out4"]' 'type Out4<T>: Support<Produce<T> = T>'
+// @has - '//*[@id="associatedtype.Out5"]' "type Out5: Support<Output<'static> = &'static ()>"
+// @has - '//*[@id="associatedtype.Out6"]' "type Out6: Support<Output<'a> = &'a ()>"
+// @has - '//*[@id="associatedtype.Out7"]' "type Out7: Support<Item = String, Produce<i32> = u32> + Unrelated"
+// @has - '//*[@id="associatedtype.Out8"]' "type Out8: Unrelated + Protocol<i16, Q1 = u128, Q0 = ()>"
+// @has - '//*[@id="associatedtype.Out9"]' "type Out9: FnMut(i32) -> bool + Clone"
+// @has - '//*[@id="associatedtype.Out10"]' "type Out10<'q>: Support<Output<'q> = ()>"
+// @has - '//*[@id="associatedtype.Out11"]' "type Out11: Helper<A<'s> = &'s (), B<'r> = ()>"
+// @has - '//*[@id="associatedtype.Out12"]' "type Out12: Helper<B<'w> = Cow<'w, str>, A<'w> = bool>"
+//
+// Snapshots: Check that we do not render any where-clauses for those associated types since all of
+// the trait bounds contained within were moved to the bounds of the respective item.
+//
+// @snapshot out0 - '//*[@id="associatedtype.Out0"]/*[@class="code-header"]'
+// @snapshot out9 - '//*[@id="associatedtype.Out9"]/*[@class="code-header"]'
+//
+// @has - '//*[@id="tymethod.make"]' \
+// "fn make<F>(F, impl FnMut(&str) -> bool)\
+// where \
+//     F: FnOnce(u32) -> String, \
+//     Self::Out2<()>: Protocol<u8, Q0 = Self::Item, Q1 = ()>"
+pub use aux::Main;

src/test/rustdoc/inline_cross/auxiliary/assoc_item_trait_bounds_with_bindings.rs

@@ -0,0 +1,40 @@
+pub trait Main {
+    type Item;
+
+    type Out0: Support<Item = ()>;
+    type Out1: Support<Item = Self::Item>;
+    type Out2<T>: Support<Item = T>;
+    type Out3: Support<Produce<()> = bool>;
+    type Out4<T>: Support<Produce<T> = T>;
+    type Out5: Support<Output<'static> = &'static ()>;
+    type Out6: for<'a> Support<Output<'a> = &'a ()>;
+    type Out7: Support<Item = String, Produce<i32> = u32> + Unrelated;
+    type Out8: Unrelated + Protocol<i16, Q1 = u128, Q0 = ()>;
+    type Out9: FnMut(i32) -> bool + Clone;
+    type Out10<'q>: Support<Output<'q> = ()>;
+    type Out11: for<'r, 's> Helper<A<'s> = &'s (), B<'r> = ()>;
+    type Out12: for<'w> Helper<B<'w> = std::borrow::Cow<'w, str>, A<'w> = bool>;
+
+    fn make<F>(_: F, _: impl FnMut(&str) -> bool)
+    where
+        F: FnOnce(u32) -> String,
+        Self::Out2<()>: Protocol<u8, Q0 = Self::Item, Q1 = ()>;
+}
+
+pub trait Support {
+    type Item;
+    type Output<'a>;
+    type Produce<T>;
+}
+
+pub trait Protocol<K> {
+    type Q0;
+    type Q1;
+}
+
+pub trait Unrelated {}
+
+pub trait Helper {
+    type A<'q>;
+    type B<'q>;
+}