Auto merge of #123340 - fmease:rustdoc-simplify-auto-trait-impl-synth, r=GuillaumeGomez

rustdoc: heavily simplify the synthesis of auto trait impls

`gd --numstat HEAD~2 HEAD src/librustdoc/clean/auto_trait.rs`
**+315 -705** 🟩🟥🟥🟥⬛

---

As outlined in issue #113015, there are currently 3[^1] large separate routines that “clean” `rustc_middle::ty` data types related to generics & predicates to rustdoc data types. Every single one has their own kinds of bugs. While I've patched a lot of bugs in each of the routines in the past, it's about time to unify them. This PR is only the first in a series. It completely **yanks** the custom “bounds cleaning” of mod `auto_trait` and reuses the routines found in mod `simplify`. As alluded to, `simplify` is also flawed but it's still more complete than `auto_trait`'s routines. [See also my review comment over at `tests/rustdoc/synthetic_auto/bounds.rs`](https://github.com/rust-lang/rust/pull/123340#discussion_r1546900539).

This is preparatory work for rewriting “bounds cleaning” from scratch in follow-up PRs in order to finally [fix] #113015.

Apart from that, I've eliminated all potential sources of *instability* in the rendered output.
See also #119597. I'm pretty sure this fixes #119597.

This PR does not attempt to fix [any other issues related to synthetic auto trait impls](https://github.com/rust-lang/rust/issues?q=is%3Aissue+is%3Aopen+label%3AA-synthetic-impls%20label%3AA-auto-traits).
However, it's definitely meant to be a *stepping stone* by making `auto_trait` more contributor-friendly.

---

* Replace `FxHash{Map,Set}` with `FxIndex{Map,Set}` to guarantee a stable iteration order
  * Or as a perf opt, `UnordSet` (a thin wrapper around `FxHashSet`) in cases where we never iterate over the set.
  * Yes, we do make use of `swap_remove` but that shouldn't matter since all the callers are deterministic. It does make the output less “predictable” but it's still better than before. Ofc, I rely on `rustc_infer` being deterministic. I hope that holds.
* Utilizing `clean::simplify` over the custom “bounds cleaning” routines wipes out the last reference to `collect_referenced_late_bound_regions` in rustdoc (`simplify` uses `bound_vars`) which was a source of instability / unpredictability (cc #116388)
* Remove the types `RegionTarget` and `RegionDeps` from `librustdoc`. They were duplicates of the identical types found in `rustc`. Just import them from `rustc`. For some reason, they were duplicated when splitting `auto_trait` in two in #49711.
* Get rid of the useless “type namespace” `AutoTraitFinder` in `librustdoc`
  * The struct only held a `DocContext`, it was over-engineered
  * Turn the associated functions into free ones
    * Eliminates rightward drift; increases legibility
  * `rustc` also contains a useless `AutoTraitFinder` struct but I plan on removing that in a follow-up PR
* Rename a bunch of methods to be way more descriptive
* Eliminate `use super::*;`
  * Lead to `clean/mod.rs` accumulating a lot of unnecessary imports
  * Made `auto_traits` less modular
* Eliminate a custom `TypeFolder`: We can just use the rustc helper `fold_regions` which does that for us

I plan on adding extensive documentation to `librustdoc`'s `auto_trait` in follow-up PRs.
I don't want to do that in this PR because further refactoring & bug fix PRs may alter the overall structure of `librustdoc`'s & `rustc`'s `auto_trait` modules to a great degree. I'm slowly digging into the dark details of `rustc`'s `auto_trait` module again and once I have the full picture I will be able to provide proper docs.

---

While this PR does indeed touch `rustc`'s `auto_trait` — mostly tiny refactorings — I argue this PR doesn't need any compiler reviewers next to rustdoc ones since that module falls under the purview of rustdoc — it used to be part of `librustdoc` after all (#49711).

Sorry for not having split this into more commits. If you'd like me to I can try to split it into more atomic commits retroactively. However, I don't know if that would actually make reviewing easier. I think the best way to review this might just be to place the master version of `auto_trait` on the left of your screen and the patched one on the right, not joking.

r? `@GuillaumeGomez`

[^1]: Or even 4 depending on the way you're counting.

5 files changed, 28 insertions, 10 deletions

diff --git a/tests/rustdoc/synthetic_auto/bounds.rs b/tests/rustdoc/synthetic_auto/bounds.rs
new file mode 100644
index 00000000000..17528d01c8d
--- /dev/null
+++ b/tests/rustdoc/synthetic_auto/bounds.rs
@@ -0,0 +1,21 @@
+pub struct Outer<T>(Inner<T>);
+pub struct Inner<T>(T);
+
+// @has bounds/struct.Outer.html
+// @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
+// "impl<T> Unpin for Outer<T>where \
+//     T: for<'any> Trait<A = (), B<'any> = (), X = ()>,"
+
+impl<T> std::marker::Unpin for Inner<T>
+where
+    T: for<'any> Trait<A = (), B<'any> = (), X = ()>,
+{}
+
+pub trait Trait: SuperTrait {
+    type A;
+    type B<'a>;
+}
+
+pub trait SuperTrait {
+    type X;
+}
diff --git a/tests/rustdoc/synthetic_auto/complex.rs b/tests/rustdoc/synthetic_auto/complex.rs
index 4c39f0bf1e0..2722f6d338f 100644
--- a/tests/rustdoc/synthetic_auto/complex.rs
+++ b/tests/rustdoc/synthetic_auto/complex.rs
@@ -21,8 +21,8 @@ mod foo {
 
 // @has complex/struct.NotOuter.html
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
-// "impl<'a, T, K: ?Sized> Send for Outer<'a, T, K>where K: for<'b> Fn((&'b bool, &'a u8)) \
-// -> &'b i8, T: MyTrait<'a>, <T as MyTrait<'a>>::MyItem: Copy, 'a: 'static"
+// "impl<'a, T, K> Send for Outer<'a, T, K>where 'a: 'static, T: MyTrait<'a>, \
+// K: for<'b> Fn((&'b bool, &'a u8)) -> &'b i8 + ?Sized, <T as MyTrait<'a>>::MyItem: Copy,"
 
 pub use foo::{Foo, Inner as NotInner, MyTrait as NotMyTrait, Outer as NotOuter};
 
diff --git a/tests/rustdoc/synthetic_auto/lifetimes.rs b/tests/rustdoc/synthetic_auto/lifetimes.rs
index 71265b3078a..23e1efdaeef 100644
--- a/tests/rustdoc/synthetic_auto/lifetimes.rs
+++ b/tests/rustdoc/synthetic_auto/lifetimes.rs
@@ -10,7 +10,7 @@ where
 
 // @has lifetimes/struct.Foo.html
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
-// "impl<'c, K> Send for Foo<'c, K>where K: for<'b> Fn(&'b bool) -> &'c u8, 'c: 'static"
+// "impl<'c, K> Send for Foo<'c, K>where 'c: 'static, K: for<'b> Fn(&'b bool) -> &'c u8,"
 //
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
 // "impl<'c, K> Sync for Foo<'c, K>where K: Sync"
diff --git a/tests/rustdoc/synthetic_auto/no-redundancy.rs b/tests/rustdoc/synthetic_auto/no-redundancy.rs
index d30b38dd4dc..64dab429647 100644
--- a/tests/rustdoc/synthetic_auto/no-redundancy.rs
+++ b/tests/rustdoc/synthetic_auto/no-redundancy.rs
@@ -1,6 +1,3 @@
-// FIXME(fmease, #119216): Reenable this test!
-//@ ignore-test
-
 pub struct Inner<T> {
     field: T,
 }
@@ -13,7 +10,7 @@ where
 
 // @has no_redundancy/struct.Outer.html
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
-// "impl<T> Send for Outer<T>where T: Send + Copy"
+// "impl<T> Send for Outer<T>where T: Copy + Send"
 pub struct Outer<T> {
     inner_field: Inner<T>,
 }
diff --git a/tests/rustdoc/synthetic_auto/project.rs b/tests/rustdoc/synthetic_auto/project.rs
index 7c9412ae962..f4ede76e6de 100644
--- a/tests/rustdoc/synthetic_auto/project.rs
+++ b/tests/rustdoc/synthetic_auto/project.rs
@@ -24,11 +24,11 @@ where
 
 // @has project/struct.Foo.html
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
-// "impl<'c, K> Send for Foo<'c, K>where K: MyTrait<MyItem = bool>, 'c: 'static"
+// "impl<'c, K> Send for Foo<'c, K>where 'c: 'static, K: MyTrait<MyItem = bool>,"
 //
 // @has - '//*[@id="synthetic-implementations-list"]//*[@class="impl"]//h3[@class="code-header"]' \
-// "impl<'c, K> Sync for Foo<'c, K>where K: MyTrait, <K as MyTrait>::MyItem: OtherTrait, \
-// 'c: 'static,"
+// "impl<'c, K> Sync for Foo<'c, K>where 'c: 'static, K: MyTrait, \
+// <K as MyTrait>::MyItem: OtherTrait,"
 pub struct Foo<'c, K: 'c> {
     inner_field: Inner<'c, K>,
 }