Add as_ref and as_mut methods for Bound

Add as_ref and as_mut method for std::ops::range::Bound, patterned off
of the methods of the same name on Option.

I'm not quite sure what the process is for introducing new feature gates (this is my first contribution) so I've left these ungated, but happy to do whatever is necessary to gate them.

Add a `std::io::read_to_string` function

I recognize that you're usually supposed to open an issue first, but the
implementation is very small so it's okay if this is closed and it was 'wasted
work' :)

-----

The equivalent of `std::fs::read_to_string`, but generalized to all
`Read` impls.

As the documentation on `std::io::read_to_string` says, the advantage of
this function is that it means you don't have to create a variable first
and it provides more type safety since you can only get the buffer out
if there were no errors. If you use `Read::read_to_string`, you have to
remember to check whether the read succeeded because otherwise your
buffer will be empty.

It's friendlier to newcomers and better in most cases to use an explicit
return value instead of an out parameter.

Recommend panic::resume_unwind instead of panicking.

Fixes https://github.com/rust-lang/rust/issues/79950.

Suggest `_` and `..` if a pattern has too few fields

Fixes #80010.

Add missing methods to unix ExitStatusExt

These are the methods corresponding to the remaining exit status examination macros from `wait.h`.  `WCOREDUMP` isn't in SuS but is it is very standard.  I have not done portability testing to see if this builds everywhere, so I may need to Do Something if it doesn't.

There is also a bugfix and doc improvement to `.signal()`, and an `.into_raw()` accessor.

This would fix #73128 and fix #73129.  Please let me know if you like this direction, and if so I will open the tracking issue and so on.

If this MR goes well, I may tackle #73125 next - I have an idea for how to do it.

Reintroduce hir::ExprKind::If

Basically copied and paste #59288/https://github.com/rust-lang/rust-clippy/pull/4080 with some modifications.

The vast majority of tests were fixed and now there are only a few remaining. Since I am still unable to figure out the missing pieces, any help with the following list is welcome.

- [ ] **Unnecessary `typeck` exception**: [Cheated on this one to make CI green.](https://github.com/rust-lang/rust/pull/79328/files#diff-3faee9ba23fc54a12b7c43364ba81f8c5660045c7e1d7989a02a0cee1c5b2051)
- [x] **Incorrect span**: [Span should reference `then` and `else` separately.](https://github.com/rust-lang/rust/pull/79328/files#diff-cf2c46e82222ee4b1037a68fff8a1af3c4f1de7a6b3fd798aacbf3c0475abe3d)
- [x] **New note regarding `assert!`**: [Modified but not "wrong". Maybe can be a good thing?](https://github.com/rust-lang/rust/pull/79328/files#diff-9e0d7c89ed0224e2b62060c957177c27db43c30dfe3c2974cb6b5091cda9cfb5)
- [x] **Inverted report location**: [Modified but not "wrong". Locations were inverted.](https://github.com/rust-lang/rust/pull/79328/files#diff-f637ce7c1f68d523a165aa9651765df05e36c4d7d279194b1a6b28b48a323691)
- [x] **`src/test/ui/point-to-type-err-cause-on-impl-trait-return.rs` has weird errors**: [Not sure why this is happening.](https://github.com/rust-lang/rust/pull/79328/files#diff-c823c09660f5b112f95e97e8ff71f1797b6c7f37dbb3d16f8e98bbaea8072e95)
- [x] **Missing diagnostic**: [???](https://github.com/rust-lang/rust/pull/79328/files#diff-6b8ab09360d725ba4513933827f9796b42ff9522b0690f80b76de067143af2fc)

Encode optimized MIR of generators when emitting metadata

Fix #49660 - Adds checks to ensure existence of arithmetic trait implementations

The first 2 commits fix an issue with the existing `wrapping.rs` tests. It wasn't referred to from the module, so the file was being ignored. This is fixed in https://github.com/rust-lang/rust/pull/78259/commits/872dc60ed203d16d43140c1d1623474cf8a9aedc This surfaced a bug in its macro which is fixed in https://github.com/rust-lang/rust/pull/78259/commits/8ddad18283e304753e09ef651209b4a6b54148b0

Lastly, commit https://github.com/rust-lang/rust/pull/78259/commits/64d695b753bfe09797b5f95c7b9b72948da01710 is the actual tests for fixing #49660

The following checks are done:

* `Add`, `Sub`, `Mul`, `Div`, `Rem`
  * `T op T`, `T op &T`, `&T op T` and `&T op &T`
  * for all integer and floating point types
* `AddAssign`, `SubAssign`, `MulAssign`, `DivAssign`, `RemAssign`
  * `&mut T op T` and `&mut T op &T`
  * for all integer and floating point types
* `Neg`
  * `op T` and `op &T`
  * for all signed integer and floating point types
* `Not`
  * `op T` and `op &T`
  * for `bool`
* `BitAnd`, `BitOr`, `BitXor`
  * `T op T`, `T op &T`, `&T op T` and `&T op &T`
  * for all integer types and bool
* `BitAndAssign`, `BitOrAssign`, `BitXorAssign`
  * `&mut T op T` and `&mut T op &T`
  * for all integer types and bool
* `Shl`, `Shr`
  * `L op R`, `L op &R`, `&L op R` and `&L op &R`
  * for all pairs of integer types
* `ShlAssign`, `ShrAssign`
  * `&mut L op R`, `&mut L op &R`
  * for all pairs of integer types

NOTE: I'd like some feedback on improving the macros. I'm not familiar with the idioms and patterns there and composing them has been a challenge for me.

[EDIT]: updated links to commits after rebase.

These tests invoke the various op traits using all accepted types they
are implemented for as well as for references to those types.

This fixes #49660 and ensures the following implementations exist:

* `Add`, `Sub`, `Mul`, `Div`, `Rem`
  * `T op T`, `T op &T`, `&T op T` and `&T op &T`
  * for all integer and floating point types
* `AddAssign`, `SubAssign`, `MulAssign`, `DivAssign`, `RemAssign`
  * `&mut T op T` and `&mut T op &T`
  * for all integer and floating point types
* `Neg`
  * `op T` and `op &T`
  * for all signed integer and floating point types
* `Not`
  * `op T` and `op &T`
  * for `bool`
* `BitAnd`, `BitOr`, `BitXor`
  * `T op T`, `T op &T`, `&T op T` and `&T op &T`
  * for all integer types and bool
* `BitAndAssign`, `BitOrAssign`, `BitXorAssign`
  * `&mut T op T` and `&mut T op &T`
  * for all integer types and bool
* `Shl`, `Shr`
  * `L op R`, `L op &R`, `&L op R` and `&L op &R`
  * for all pairs of integer types
* `ShlAssign`, `ShrAssign`
  * `&mut L op R`, `&mut L op &R`
  * for all pairs of integer types

AFAIK it isn't currently possible to do this. It is also more in line with other tests in the surrounding modules.

Box Item::Attributes

This reduces the size of Item from 128 to 40 bytes. I think this is as small as it needs to get 🎉

Builds on https://github.com/rust-lang/rust/pull/80339 and should not be merged before.

r? `@GuillaumeGomez`

Add as_ref and as_mut method for std::ops::range::Bound, patterned off
of the methods of the same name on Option.

Properly handle `SyntaxContext` of dummy spans in incr comp

Fixes #80336

Due to macro expansion, we may end up with spans with an invalid
location and non-root `SyntaxContext`. This commits preserves the
`SyntaxContext` of such spans in the incremental cache, and ensures
that we always hash the `SyntaxContext` when computing the `Fingerprint`
of a `Span`

Previously, we would discard the `SyntaxContext` during serialization to
the incremental cache, causing the span's `Fingerprint` to change across
compilation sessions.

Rework diagnostics for wrong number of generic args (fixes #66228 and #71924)

This PR reworks the `wrong number of {} arguments` message, so that it provides more details and contextual hints.

Fixes #80336

Due to macro expansion, we may end up with spans with an invalid
location and non-root `SyntaxContext`. This commits preserves the
`SyntaxContext` of such spans in the incremental cache, and ensures
that we always hash the `SyntaxContext` when computing the `Fingerprint`
of a `Span`

Previously, we would discard the `SyntaxContext` during serialization to
the incremental cache, causing the span's `Fingerprint` to change across
compilation sessions.

Consistently avoid constructing optimized MIR when not doing codegen

The optimized MIR for closures is being encoded unconditionally, while
being unnecessary for cargo check. This turns out to be especially
costly with MIR inlining enabled, since it triggers computation of
optimized MIR for all callees that are being examined for inlining
purposes https://github.com/rust-lang/rust/pull/77307#issuecomment-751915450.

Skip encoding of optimized MIR for closures, enum constructors, struct
constructors, and trait fns when not doing codegen, like it is already
done for other items since 49433.

Add allow-by-default lint on implicit ABI in extern function pointers and items

This adds a new lint, missing_abi, which lints on omitted ABIs on extern blocks, function declarations, and function pointers.

It is currently not emitting the best possible diagnostics -- we need to track the span of "extern" at least or do some heuristic searching based on the available spans -- but seems good enough for an initial pass than can be expanded in future PRs.

This is a pretty large PR, but mostly due to updating a large number of tests to include ABIs; I can split that into a separate PR if it would be helpful, but test updates are already in dedicated commits.

This is not particularly pretty but the current situation is a mess
and I don't think I'm making it significantly worse.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

As discussed in #79982.

I think the "new interfaces", ie the new trait and impl, must be
insta-stable.  This seems OK because we are, in fact, adding a new
restriction to the stable API.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Co-authored-by: NJoshua Nelson <joshua@yottadb.com>

Co-authored-by: NJoshua Nelson <joshua@yottadb.com>

Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Necessary to handle WIFSTOPPED.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

This is essential for proper reporting of child process status on Unix.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

We need to be clear that this never returns WSTOPSIG.  That is, if
WIFSTOPPED, the return value is None.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

A unix wait status can contain, at least, exit statuses, termination
signals, and stop signals.

WTERMSIG is only valid if WIFSIGNALED.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html

It will not be easy to experience this bug with `Command`, because
that doesn't pass WUNTRACED.  But you could make an ExitStatus
containing, say, a WIFSTOPPED, from a call to one of the libc wait
functions.

(In the WIFSTOPPED case, there is WSTOPSIG.  But a stop signal is
encoded differently to a termination signal, so WTERMSIG and WSTOPSIG
are by no means the same.)
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Try to avoid locals when cloning into Box/Rc/Arc

For generic `T: Clone`, we can allocate an uninitialized box beforehand,
which gives the optimizer a chance to create the clone directly in the
heap. For `T: Copy`, we can go further and do a simple memory copy,
regardless of optimization level.

The same applies to `Rc`/`Arc::make_mut` when they must clone the data.

Stabilize split_inclusive

### Contents of this MR

This stabilises:

 * `slice::split_inclusive`
 * `slice::split_inclusive_mut`
 * `str::split_inclusive`

Closes #72360.

### A possible concern

The proliferation of `split_*` methods is not particularly pretty.  The existence of `split_inclusive` seems to invite the addition of `rsplit_inclusive`, `splitn_inclusive`, etc.  We could instead have a more general API, along these kinds of lines maybe:
```
   pub fn split_generic('a,P,H>(&'a self, pat: P, how: H) -> ...
       where P: Pattern
       where H: SplitHow;

   pub fn split_generic_mut('a,P,H>(&'a mut self, pat: P, how: H) -> ...
       where P: Pattern
       where H: SplitHow;

   trait SplitHow {
       fn reverse(&self) -> bool;
       fn inclusive -> bool;
       fn limit(&self) -> Option<usize>;
   }

   pub struct SplitFwd;
   ...
   pub struct SplitRevInclN(pub usize);
```
But maybe that is worse.

### Let us defer that? ###

This seems like a can of worms.  I think we can defer opening it now; if and when we have something more general, these two methods can become convenience aliases.  But I thought I would mention it so the lang API team can consider it and have an opinion.