This commit also updates `stdarch` git submodule.

At first, you might think, "Why not just click through to the aliased
type?" But, if a type alias instantiates all of the generic parameters
of the aliased type, then it can show layout info even though the
aliased type cannot (because we can't compute layout for generic types).
So, I think it's useful to show layout info for type aliases.

This is a followup of 78d4b453ad2e19d44011b26fc55c949bff5dba3d
(originally part of rust-lang#83501).

This commit does the following.
- Expands on some of the things already mentioned in comments.
- Describes the uniqueness assumption, which is critical but wasn't
  mentioned at all.
- Rewrites `empty()` into a clearer form, as provided by Daniel
  Henry-Mantilla on Zulip.
- Reorders things slightly so that more important things
  are higher up, and incidental things are lower down, which makes
  reading the code easier.

Make IdFunctor::try_map_id panic-safe

Addresses FIXME comment created in rust-lang#78313

r? `@lcnr`

…anieu

Add spin_loop hint for RISC-V architecture

This commit uses the PAUSE instruction (rust-lang/stdarch#1262) to implement RISC-V spin loop, and updates `stdarch` submodule to use the merged PAUSE instruction.

…y, r=lcnr

Improve the readability of `List<T>`.

This commit does the following.
- Expands on some of the things already mentioned in comments.
- Describes the uniqueness assumption, which is critical but wasn't
  mentioned at all.
- Rewrites `empty()` into a clearer form, as provided by Daniel
  Henry-Mantilla on Zulip.
- Reorders things slightly so that more important things
  are higher up, and incidental things are lower down, which makes
  reading the code easier.

r? `@lcnr`

Remove redundant [..]s

…al, r=jackh726

Add tests fixed by rust-lang#90023

The following issues were fixed by rust-lang#90023

Fixes rust-lang#79674
Fixes rust-lang#83765
Fixes rust-lang#86033
Fixes rust-lang#90318
Fixes rust-lang#88468

The following issues were duplicates of rust-lang#90654

Fixes rust-lang#86850
Fixes rust-lang#89022

r? `@jackh726`

rustdoc: Show type layout for type aliases

Fixes rust-lang#91265.

At first, you might think, "Why not just click through to the aliased
type?" But, if a type alias instantiates all of the generic parameters
of the aliased type, then it can show layout info even though the
aliased type cannot (because we can't compute layout for generic types).
So, I think it's useful to show layout info for type aliases.

This is a followup of 78d4b453ad2e19d44011b26fc55c949bff5dba3d
(originally part of rust-lang#83501).