This adds the typeid and `vcall_visibility` metadata to vtables when the
-Cvirtual-function-elimination flag is set.

The typeid is generated in the same way as for the
`llvm.type.checked.load` intrinsic from the trait_ref.

The offset that is added to the typeid is always 0. This is because LLVM
assumes that vtables are constructed according to the definition in the
Itanium ABI. This includes an "address point" of the vtable. In C++ this
is the offset in the vtable where information for RTTI is placed. Since
there is no RTTI information in Rust's vtables, this "address point" is
always 0. This "address point" in combination with the offset passed to
the `llvm.type.checked.load` intrinsic determines the final function
that should be loaded from the vtable in the
`WholeProgramDevirtualization` pass in LLVM. That's why the
`llvm.type.checked.load` intrinsics are generated with the typeid of the
trait, rather than with that of the function that is called. This
matches what `clang` does for C++.

The vcall_visibility metadata depends on three factors:

1. LTO level: Currently this is always fat LTO, because LLVM only
   supports this optimization with fat LTO.
2. Visibility of the trait: If the trait is publicly visible, VFE
   can only act on its vtables after linking.
3. Number of CGUs: if there is more than one CGU, also vtables with
   restricted visibility could be seen outside of the CGU, so VFE can
   only act on them after linking.

To reflect this, there are three visibility levels: Public, LinkageUnit,
and TranslationUnit.

Add the intrinsic

declare {i8*, i1} @llvm.type.checked.load(i8* %ptr, i32 %offset, metadata %type)

This is used in the VFE optimization when lowering loading functions
from vtables to LLVM IR. The `metadata` is used to map the function to
all vtables this function could belong to. This ensures that functions
from vtables that might be used somewhere won't get removed.

This function computes a Itanium-like typeid for a trait_ref. This is
required for the VFE optimization in LLVM. It is used to map
`llvm.type.checked.load` invocations, that is loading the function from
a vtable, to the vtables this function could be from.

It is important to note that `typeid`s are not unique. So multiple
vtables of the same trait can share `typeid`s.

To apply the optimization the `Virtual Function Elim` module flag has to
be set. To apply this optimization post-link the `LTOPostLink` module
flag has to be set.

Adds the virtual-function-elimination unstable compiler flag and a check
that this flag is only used in combination with -Clto. LLVM can only
apply this optimization with fat LTO.

⬆ rust-analyzer

r? `@ghost`

Remove RegionckMode in favor of calling new skip_region_resolution

Simple cleanup. We can skip a bunch of stuff for places where NLL does the region checking, so skip earlier.

r? rust-lang/types

Sync rustfmt subtree

Rollup of 4 pull requests

Successful merges:

 - #95211 (Improve parser diagnostics)
 - #95243 (Add Apple WatchOS compile targets)
 - #97385 (Add WIP stable MIR crate)
 - #97508 (Harden bad placeholder checks on statics/consts)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Harden bad placeholder checks on statics/consts

Resubmission of #89161
Fixes https://github.com/rust-lang/rust/issues/88643

In #83739, I added a check for trait objects on statics/consts but it wasn't robust. `is_suggestable_infer_ty` fn does a more strict check and finds more bad placeholders. See https://github.com/rust-lang/rust/pull/89161#issuecomment-934690300 for the more detailed explanation.

r? `@pnkfelix` as you're the reviewer of the previous PR

Add WIP stable MIR crate

r? ``@pnkfelix``

Discussion about this happend in the SMIR meeting yesterday. Some info can be found at https://rust-lang.zulipchat.com/#narrow/stream/320896-project-stable-mir/topic/dev.20plan.20mtg/near/283774691

Add Apple WatchOS compile targets

Hello,

I would like to add the following target triples for Apple WatchOS as Tier 3 platforms:

armv7k-apple-watchos
arm64_32-apple-watchos
x86_64-apple-watchos-sim
There are some pre-requisites Pull Requests:
https://github.com/rust-lang/compiler-builtins/pull/456 (merged)
https://github.com/alexcrichton/cc-rs/pull/662 (pending)
https://github.com/rust-lang/libc/pull/2717 (merged)

There will be a subsequent PR with standard library changes for WatchOS.  Previous compiler and library changes were in a single PR (https://github.com/rust-lang/rust/pull/94736) which is now closed in favour of separate PRs.

Many thanks!
Vlad.

### Tier 3 Target Requirements

Adds support for Apple WatchOS compile targets.

Below are details on how this target meets the requirements for tier 3:

>   tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

`@deg4uss3r` has volunteered to be the target maintainer. I am also happy to help if a second maintainer is required.

> Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.

Uses the same naming as the LLVM target, and the same convention as other Apple targets.

> Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.

I don't believe there is any ambiguity here.

> Tier 3 targets may have unusual requirements to build or use, but must not create legal issues or impose onerous legal terms for the Rust project or for Rust developers or users.

I don't see any legal issues here.

> The target must not introduce license incompatibilities.
> Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
> The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.
> If the target supports building host tools (such as rustc or cargo), those host tools must not depend on proprietary (non-FOSS) libraries, other than ordinary runtime libraries supplied by the platform and commonly used by other binaries built for the target. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.
> Targets should not require proprietary (non-FOSS) components to link a functional binary or library.
> "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

I see no issues with any of the above.

> Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.
> This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.

Only relevant to those making approval decisions.

> Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.

core and alloc can be used. std support will be added in a subsequent PR.

> The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running tests (even if they do not pass), the documentation must explain how to run tests for the target, using emulation if possible or dedicated hardware if necessary.

Use --target=<target> option to cross compile, just like any target. Tests can be run using the WatchOS simulator (see https://developer.apple.com/documentation/xcode/running-your-app-in-the-simulator-or-on-a-device).

> Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via `@)` to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.
> Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.

I don't foresee this being a problem.

> Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target.
> In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

No other targets should be affected by the pull request.

Improve parser diagnostics

This pr fixes https://github.com/rust-lang/rust/issues/93867 and contains a couple of diagnostics related changes to the parser.
Here is a short list with some of the changes:
- don't suggest the same thing that is the current token
- suggest removing the current token if the following token is one of the suggestions (maybe incorrect)
- tell the user to put a type or lifetime after where if there is none (as a warning)
- reduce the amount of tokens suggested (via the new eat_noexpect and check_noexpect methods)

If any of these changes are undesirable, i can remove them, thanks!

Rollup of 10 pull requests

Successful merges:

 - #97709 (Normalize consts' tys when relating with `adt_const_params`)
 - #97875 (Remove the `infer_static_outlives_requirements` feature)
 - #97960 (interpret: unify offset_from check with offset check)
 - #97999 (Make `type_changing_struct_update` no longer an incomplete feature)
 - #98043 (Remove unnecessary `to_string` and `String::new`)
 - #98044 ([issues:97981] del unrelated comment)
 - #98049 (Document an edge case of `str::split_once`)
 - #98050 (Add some more regression tests for #67945)
 - #98054 (Fix error message for `download-ci-llvm`)
 - #98057 (Update miri)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Update miri

r? ``@ghost``

fixes #98036

Fix error message for `download-ci-llvm`

The error message when `download-ci-llvm` fails includes too many newlines right now:

```
curl: (22) The requested URL returned error: 404

error: failed to download llvm from ci

help: old builds get deleted after a certain time

help: if trying to compile an old commit of rustc, disable `download-ci-llvm` in config.toml:

[llvm]

download-ci-llvm = false

Build completed unsuccessfully in 0:01:15
```

This PR fixes the message to include the right amount of newlines:

```
curl: (22) The requested URL returned error: 404

error: failed to download llvm from ci

help: old builds get deleted after a certain time
help: if trying to compile an old commit of rustc, disable `download-ci-llvm` in config.toml:

[llvm]
download-ci-llvm = false

Build completed unsuccessfully in 0:00:07
```

r? `@jyn514`

Add some more regression tests for #67945

Closes #67945, added two tests from https://github.com/rust-lang/rust/issues/67945#issuecomment-572617285, other snippets were already added in #71952 and #77439
r? `@compiler-errors`

Document an edge case of `str::split_once`

[issues:97981] del unrelated comment

fixes https://github.com/rust-lang/rust/issues/97981
summary: del unrelated comment

Remove unnecessary `to_string` and `String::new`

https://github.com/rust-lang/rust/pull/96468/commits/73fa217bc11fbac76f730223f6766c8e03513b5e changed the type of the `suggestion` argument to `impl ToString`. This patch removes unnecessary `to_string` and `String::new`.

cc: `````@davidtwco`````

Make `type_changing_struct_update` no longer an incomplete feature

After #97705, I don't see what would make it incomplete anymore. `check_expr_struct_fields` seems to now implement the RFC to the letter.

r? ``````@nikomatsakis``````
cc ``````@rust-lang/types``````

interpret: unify offset_from check with offset check

`offset` does the check with a single `check_ptr_access` call while `offset_from` used two calls. Make them both just one one call.

I originally intended to actually factor this into a common function, but I am no longer sure if that makes a lot of sense... the two functions start with pretty different precondition (e.g. `offset` *knows* that the 2nd pointer has the same provenance).

I also reworded the UB messages a little. Saying it "cannot" do something is not how we usually phrase UB (as far as I know). Instead it's not *allowed* to do that.

r? ``````@oli-obk``````

Remove the `infer_static_outlives_requirements` feature

Closes #54185
r? ``@pnkfelix``

Normalize consts' tys when relating with `adt_const_params`

Fixes #97007

issues97981

Remove an unnecessary space in doc