Implement Mutation- and BorrowOfLayoutConstrainedField in thir-unsafeck

Since nobody has so far claimed Mutation- and BorrowOfLayoutConstrainedField in rust-lang/project-thir-unsafeck#7, I have taken the liberty of implementing them in thir-unsafeck.

r? `@LeSeulArtichaut`

Rollup of 11 pull requests

Successful merges:

 - #86344 (Split MaybeUninit::write into new feature gate and stabilize it)
 - #86811 (Remove unstable `io::Cursor::remaining`)
 - #86846 (stdio_locked: add tracking issue)
 - #86887 (rustdoc: remove dead code in `clean`)
 - #87007 (Fix rust-analyzer install when not available.)
 - #87035 (Fix implementors display)
 - #87065 (Fix ICE with unsized type in const pattern)
 - #87070 (Simplify future incompatible reporting.)
 - #87077 (⬆ rust-analyzer)
 - #87078 (Rustdoc: suggest removing disambiguator if linking to field)
 - #87089 (CTFE engine: small cleanups)

Failed merges:

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

CTFE engine: small cleanups

I noticed these while preparing a large PR, and figured I'd better send them ahead to not muddy the diff unnecessarily.

- remove remaining use of Pointer in Allocation API (I missed those in https://github.com/rust-lang/rust/pull/85472)
- remove unnecessary deallocate_local hack (this logic does not seem necessary any more)

r? `@oli-obk`

Rustdoc: suggest removing disambiguator if linking to field

This fixes #85615.

`@rustbot` label T-rustdoc

⬆ rust-analyzer

`@bors` r+

Simplify future incompatible reporting.

This simplifies the implementation of the future incompatible reporting system. Instead of having a separate field in the future_incompatible definition, this reuses the `FutureIncompatibilityReason` enum. It also drops the "date" field. Cargo does not use the date field, and there isn't much of a need for this to be structured, and I am skeptical that the date can be predicted reliably. The date or release version can be listed in the lint text if desired.

Fix ICE with unsized type in const pattern

Fixes #87046. The `deref_const()` query currently contains the following check:
https://github.com/rust-lang/rust/blob/e9a387d6cf5961a7f2dcb671da3147bd413355c4/compiler/rustc_mir/src/const_eval/mod.rs#L191-L204

i.e. this will cause an ICE for every unsized type except slices. An error is reported with my changes if such a type is used as a const pattern (this should not be a breaking change, since so far, this has caused an ICE).

Fix implementors display

Part of https://github.com/rust-lang/rust/issues/86632.

This PR does a few things:
 * It fixes of the JS rendered implementors.
 * It generates anchors for JS rendered implementors to make it coherent with the others.
 * It adds a test to ensure that we won't have the same issue again.
 * It changes the way we render the rustdoc-gui crates to simplify it a bit and also to allow to have dependencies without going through compiletest.

Before:

![Screenshot from 2021-07-10 13-30-13](https://user-images.githubusercontent.com/3050060/125174172-b4048700-e1c3-11eb-8f0e-c46081371d4f.png)

After:

![Screenshot from 2021-07-10 21-11-15](https://user-images.githubusercontent.com/3050060/125174173-b49d1d80-e1c3-11eb-8740-1dbbff70c2eb.png)

I plan to add the `[src]` links in another PR because this one is already big enough.

cc `@Mark-Simulacrum` (for the bootstrap changes)

r? `@Nemo157`

Fix rust-analyzer install when not available.

This changes it so that `x.py install` won't fail if rust-analyzer isn't available. This was changed in #86568 to handle the case where installing on stable/beta, and `extended=true`, to skip rust-analyzer. But I neglected to update the install part to also ignore it.

Fixes #86999

rustdoc: remove dead code in `clean`

Builds on https://github.com/rust-lang/rust/pull/86886 and should not be merged before.

stdio_locked: add tracking issue

Add the tracking issue number #86845 to the stability attributes for the implementation in #86799.

r? `@joshtriplett`
`@rustbot` label +A-io +C-cleanup +T-libs-api

Remove unstable `io::Cursor::remaining`

Adding `io::Cursor::remaining` in #86037 caused a conflict with the implementation of `bytes::Buf` for `io::Cursor`, leading to an error in nightly, see https://github.com/rust-lang/rust/issues/86369#issuecomment-867723485.

This fixes the error by temporarily removing the `remaining` function.

r? `@yaahc`

Split MaybeUninit::write into new feature gate and stabilize it

This splits off the `MaybeUninit::write` function from the `maybe_uninit_extra` feature gate into a new `maybe_uninit_write` feature gate and stabilizes it.

Earlier work to improve the documentation of the write function: #86220

Tracking issue: #63567

Update to LLVM 12.0.1

LLVM 12.0.1 has been released, rebase our fork on top of it.

shrinking the deprecated span

ref: https://github.com/rust-lang/rust/pull/85617#issuecomment-854947988

part of #85403

r? `@estebank`

The reason is that if we use method_span directly, it will cause the in_derive_expansion judgment to fail.

Remove `multiple-definitions` test

r? `@joshtriplett`

Temporary solution to #87084.

Fix rendering of reexported macros 2.0 and fix visibility of reexported items

So, this PR grew a bit out of focus, it does the following things:

* Fixes #86276.
* Fixes visibility display for reexported items: it now takes the visibility of the "use" statement rather than the visibility of the reexported item itself).
* Fixes the display of reexported items if "--document-private-items" option is used. Before, they were simply skipped.
* Fixes inconsistency on typedef items: they didn't display their visibility contrary to other items.

I added tests to check everything listed above.

cc `@camelid` `@ollie27` (in case one of you want to review?)

r? `@jyn514`

Improve error reporting for modifications behind `&` references

I had a look at #84210 and noticed that #85823 has effectively already fixed #84210.

However, the string matching in #85823 is _very_ crude and already breaks down when a variable name starts with `mut`. I have made this a bit more robust; further improvements could definitely be made but are complicated by the lack of information provided by an earlier pass:
https://github.com/rust-lang/rust/blob/ce331ee6ee010438d1a58c7da8ced4f26d69a20e/compiler/rustc_mir_build/src/build/matches/mod.rs#L2103-L2107

I have also fixed a missing comma in the error message.

Report an error if resolution of closure call functions failed

This pull request fixes #86238. The current implementation seems to assume that resolution of closure call functions (I'm not sure what the proper term is; I mean `call` of `Fn` etc.) can never fail:
https://github.com/rust-lang/rust/blob/60f1a2fc4b535ead9c85ce085fdce49b1b097531/compiler/rustc_typeck/src/check/callee.rs#L590-L595

But actually, it can, if the `fn`/`fn_mut`/`fn_once` lang items are not defined, or don't have an associated `call`/`call_mut`/`call_once` function, leading to the ICE described in #86238. I have therefore turned the `span_bug!()` into an error message, which prevents the ICE.

Rollup of 8 pull requests

Successful merges:

 - #73936 (Rustdoc: Change all 'optflag' arguments to 'optflagmulti')
 - #86926 (Update regex crates)
 - #86951 ([docs] Clarify behaviour of f64 and f32::sqrt when argument is negative zero)
 - #87031 (Update reference.md)
 - #87037 (cleanup(rustdoc): remove unused function getObjectNameById)
 - #87045 (Fix tracking issue for `bool_to_option`)
 - #87049 (Account for `submodules = false` in config.toml when updating LLVM submodule)
 - #87061 (Do not suggest adding a semicolon after `?`)

Failed merges:

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

Do not suggest adding a semicolon after `?`

Fixes #87051. I have only modified `report_return_mismatched_types()`, i.e. my changes only affect suggestions to add `;` for return type mismatches, but this never makes sense after `?`, because the function cannot return `()` if `?` is used (it has to return a `Result` or an `Option`), and a semicolon won't help if the expected and actual `Err` types differ, even if the expected one is `()`.

Account for `submodules = false` in config.toml when updating LLVM submodule

Fixes https://github.com/rust-lang/rust/issues/86954.

r? ``@Mark-Simulacrum`` cc ``@durin42``

Fix tracking issue for `bool_to_option`

The previous tracking issue was closed in favor of the current.

cleanup(rustdoc): remove unused function getObjectNameById

This function was used in an earlier version, when idx's were used to serialize function inputs and outputs. That's not done any more, so removed the JS-side support for it.

Update reference.md

I ran into a link to the outdated src/doc/reference.md here: https://users.rust-lang.org/t/conditional-compilation-for-debug-release/1098/6
Apparently the Rust reference has moved again, so the link gave a 404 error. This should fix it.

[docs] Clarify behaviour of f64 and f32::sqrt when argument is negative zero

From IEEE 754 section 6.3:
> Except that squareRoot(−0) shall be −0, every numeric squareRoot result shall have a positive sign.