Used Self for uniform style

merge

This officializes what was only shown as a code example in [the unsafe code guidelines](https://rust-lang.github.io/unsafe-code-guidelines/layout/function-pointers.html?highlight=box#use) and follows [the discussion](rust-lang/unsafe-code-guidelines#157) in the corresponding repository.

It is also related to [the issue](#52976) regarding marking `Box<T>` `#[repr(transparent)]`.

- Use meaningful names
- Clarify comments
- Fix C function declaration

Casting the booleans to `i8`s and converting their difference
into `Ordering` generates better assembly than casting them to
`u8`s and comparing them.

Co-Authored-By: lzutao <taolzu@gmail.com>

We now only propagate a scalar pair if the Rvalue is a tuple with two
scalars. This for example avoids propagating a (u8, u8) value when
Rvalue has type `((), u8, u8)` (see the regression test). While this is
a correct thing to do, implementation is tricky and will be done later.

Fixes #66971
Fixes #66339
Fixes #67019

Signed-off-by: David Wood <david@davidtw.co>

This commit corrects the diagnostic note for `async move {}` so that
`await` is mentioned, rather than `yield`.

Signed-off-by: David Wood <david@davidtw.co>

* `min_length` is now exact for const index elements.
* const index elements are always from the start.
* make array `Subslice` `PlaceElems` count both `from` and `to` from the
  start.

These passes were buggy, MIR building is now responsible for
canonicalizing `ConstantIndex` projections and `MoveData` is responsible
for splitting `Subslice` projections.

It can be made even more specialized.

From a `Vec<Ty>` to a `Vec<InferTy>`, because that's a more restrictive
type. This is a perf win because the ultra-hot function
`shallow_resolve_changed` has less pattern-matching to do.

Currently, the type `struct S { x: u32, y: u32, tag: () }` is
incorrectly described like this:
```
print-type-size type: `S`: 8 bytes, alignment: 4 bytes
print-type-size     field `.x`: 4 bytes
print-type-size     field `.tag`: 0 bytes, offset: 0 bytes, alignment: 1 bytes
print-type-size     padding: 4 bytes
print-type-size     field `.y`: 4 bytes, alignment: 4 bytes
```
Specifically:
- The `padding` line is wrong. (There is no padding.)
- The `offset` and `alignment` on the `.tag` line shouldn't be printed.

The problem is that multiple fields can end up with the same offset, and
the printing code doesn't handle this correctly.

This commit fixes it by adjusting the field sorting so that zero-sized fields
are dealt with before non-zero-sized fields. With that in place, the
printing code works correctly.

The commit also corrects the "something is very wrong" comment.

The new output looks like this:
```
print-type-size type: `S`: 8 bytes, alignment: 4 bytes
print-type-size     field `.tag`: 0 bytes
print-type-size     field `.x`: 4 bytes
print-type-size     field `.y`: 4 bytes
```

Used Self for uniform style

The reason we were invoking `builtin_deref` was to enable comparisons
when the type was `&T`. For the reasons outlined in the comment, those
comparisons failed because the regions disagreed.