Co-authored-by: David Tolnay <dtolnay@gmail.com>

An odd feature of Rust is that `Foo` is invalid, but `Bar` is okay:

    type Foo<'a, 'b> = Box<dyn PartialEq<Foo<'a, 'b>>>;
    type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>>;

To get it right, track every time rustdoc descends into a type alias,
so if it shows up twice, it can be write the path instead of
infinitely expanding it.

…o_box, r=dtolnay

Loosen `From<&[T]> for Box<[T]>` bound to `T: Clone`

Also loosens `From<Cow<'_, [T]>> for Box<[T]>`'s bound.

[Discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/From.3C.26.5BT.5D.3E.20impls.20consistency)

…le, r=GuillaumeGomez

rustdoc: catch and don't blow up on impl Trait cycles

Fixes rust-lang#110629

An odd feature of Rust is that `Foo` is invalid, but `Bar` is okay:

    type Foo<'a, 'b> = Box<dyn PartialEq<Foo<'a, 'b>>>;
    type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>>;

To get it right, track every time rustdoc descends into a type alias, so if it shows up twice, it can be write the path instead of infinitely expanding it.

Make ConstProp some tests unit.

Part of the effort to tidy up mir-opt test suite.

…=compiler-errors

uplift `clippy::clone_double_ref` as `suspicious_double_ref_op`

Split from rust-lang#109842.

r? `@compiler-errors`

…nonur

bootstrap: Fix compile error: unused-mut

Compile errors:

```
   Compiling bootstrap v0.0.0 (/home/hev/rust/rust/src/bootstrap)
error: variable does not need to be mutable
    --> config.rs:1312:17
     |
1312 |             let mut build_target = config
     |                 ----^^^^^^^^^^^^
     |                 |
     |                 help: remove this `mut`
     |
     = note: `-D unused-mut` implied by `-D warnings`

error: could not compile `bootstrap` (lib) due to previous error
```

…unction, r=jyn514

Remove unneeded function call in `core::option`.

r? ``@jyn514``