This allows writing the following function signatures:
```rust
fn f0() -> impl Fn() -> impl Trait;
fn f3() -> &'static dyn Fn() -> impl Trait;
```

These signatures were already allowed for common traits and associated
types, there is no reason why `Fn*` traits should be special in this
regard.

…me is in the same crate

There's nothing underneath it anyway. The conversation on
b615c0c never really spelled out why it
was added.

…errors

Allow `impl Fn() -> impl Trait` in return position

_This was originally proposed as part of rust-lang#93082 which was [closed](rust-lang#93082 (comment)) due to allowing `impl Fn() -> impl Trait` in argument position._

This allows writing the following function signatures:
```rust
fn f0() -> impl Fn() -> impl Trait;
fn f3() -> &'static dyn Fn() -> impl Trait;
```

These signatures were already allowed for common traits and associated types, there is no reason why `Fn*` traits should be special in this regard.

`impl Trait` in both `f0` and `f3` means "new existential type", just like with `-> impl Iterator<Item = impl Trait>` and such.

Arrow in `impl Fn() ->` is right-associative and binds from right to left, it's tested by [this test](https://github.com/WaffleLapkin/rust/blob/a819fecb8dea438fc70488ddec30a61e52942672/src/test/ui/impl-trait/impl_fn_associativity.rs).

There even is a test that `f0` compiles:
https://github.com/rust-lang/rust/blob/2f004d2d401682e553af3984ebd9a3976885e752/src/test/ui/impl-trait/nested_impl_trait.rs#L25-L28

But it was changed in [PR 48084 (lines)](https://github.com/rust-lang/rust/pull/48084/files#diff-ccecca938872d65ffe8cd1c3ef1956e309fac83bcda547d8b16b89257e53a437R37)  to test the opposite, probably unintentionally given [PR 48084 (lines)](https://github.com/rust-lang/rust/pull/48084/files#diff-5a02f1ed43debed1fd24f7aad72490064f795b9420f15d847bac822aa4621a1cR476-R477).

r? `@nikomatsakis`

----

This limitation is especially annoying with async code, since it forces one to write this:
```rust
trait AsyncFn3<A, B, C>: Fn(A, B, C) -> <Self as AsyncFn3<A, B, C>>::Future {
    type Future: Future<Output = Self::Out>;

    type Out;
}

impl<A, B, C, Fut, F> AsyncFn3<A, B, C> for F
where
    F: Fn(A, B, C) -> Fut,
    Fut: Future,
{
    type Future = Fut;

    type Out = Fut::Output;
}

fn async_closure() -> impl AsyncFn3<i32, i32, i32, Out = u32> {
    |a, b, c| async move { (a + b + c) as u32 }
}
```
Instead of:
```rust
fn async_closure() -> impl Fn(i32, i32, i32) -> impl Future<Output = u32> {
    |a, b, c| async move { (a + b + c) as u32 }
}
```

Point only to the identifiers in the typo suggestions of shadowed names instead of the entire struct

Fixes rust-lang#103358.

As discussed in the issue, the `Span` of the candidate `Ident` for a typo replacement is stored alongside its `Symbol` in `TypoSuggestion`. Then, the span of the identifier is what the "you might have meant to refer to" note is pointed at, rather than the entire struct definition.

Comments in rust-lang#103111 and the issue both suggest that it is desirable to:
1. include names defined in the same crate as the typo,
2. ignore names defined elsewhere such as in `std`, _and_
3. include names introduced indirectly via `use`.

Since a name from another crate but introduced via `use` has non-local `def_id`, to achieve this, a suggestion is displayed if either the `def_id` of the suggested name is local, or the `span` of the suggested name is in the same file as the typo itself.

Some UI tests have also been modified to reflect this change.

r? `@cjgillot`

…notriddle

rustdoc: add missing URL redirect

rust-lang#94753 missed some redirect settings, and one of the missing URL shows up in an error message. This PR adds those redirects.

Do fewer passes and generally be more efficient when filtering tests

Follow-on of the work I started with this PR: rust-lang#99939

Basically, the startup code for libtest is really inefficient, but that's not usually a problem because it is distributed in release and workloads are small. But under Miri which can be 100x slower than a debug build, these inefficiencies explode.

Most of the diff here is making test filtering single-pass. There are a few other small optimizations as well, but they are more straightforward.

With this PR, the startup time of the `iced` tests with `--features=code_asm,mvex` drops from 17 to 2 minutes (I think Miri has gotten slower under this workload since rust-lang#99939). The easiest way to try this out is to set `MIRI_LIB_SRC` to a checkout of this branch when running `cargo +nightly miri test --features=code_asm,mvex`.

r? `@thomcc`

…-padding-bottom, r=Dylan-DPC

rustdoc: remove unnecessary CSS `.search-results { padding-bottom }`

There's nothing underneath it anyway. The conversation on rust-lang#84462 never really spelled out why it was added.