Ensure that `write` and `write_all` can be inlined and that their
commonly executed fast paths can be as short as possible.

`write_vectored` would likely benefit from the same optimization, but I
omitted it because its implementation is more complex, and I don't have
a benchmark on hand to guide its optimization.

We use a Vec as our internal, constant-sized buffer, but the overhead of
using methods like `extend_from_slice` can be enormous, likely because
they don't get inlined, because `Vec` has to repeat bounds checks that
we've already done, and because it makes considerations for things like
reallocating, even though they should never happen.

Optimize for the common case where the input write size is less than the
buffer size. This slightly increases the cost for pathological write
patterns that commonly fill the buffer exactly, but if a client is doing
that frequently, they're already paying the cost of frequent flushing,
etc., so the cost is of this optimization to them is relatively small.

I came up with this idea ages ago, but rustdoc used to ICE on it. Now it
doesn't.

This extracts a new `parse_cfg` function that's used between both.

- Treat `#[doc(cfg(x), cfg(y))]` the same as `#[doc(cfg(x)]
  #[doc(cfg(y))]`. Previously it would be completely ignored.
- Treat `#[doc(inline, cfg(x))]` the same as `#[doc(inline)]
  #[doc(cfg(x))]`. Previously, the cfg would be ignored.
- Pass the cfg predicate through to rustc_expand to be validated

Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>

…ully

Fix rust-lang#84772.

This should fix linking of other C code (and soon Rust-generated code)
on aarch64 musl.

And also add backticks around `MaybeUninit`.

That PR caused multiple test failures when Rust's channel is changed
from nightly to anything else. The commit will have to be landed again
after the test suite is fixed.

using allow_internal_unstable (as recommended)

Fixes: rust-lang#84836

```shell
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc     src/test/run-make-fulldeps/coverage/no_cov_crate.rs
error[E0554]: `#![feature]` may not be used on the dev release channel
 --> src/test/run-make-fulldeps/coverage/no_cov_crate.rs:2:1
  |
2 | #![feature(no_coverage)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
```

I just need this until rustbuild supports -Cpanic=abort std directly.

… r=m-ou-se

Optimize BufWriter

…, r=m-ou-se

Stablize {HashMap,BTreeMap}::into_{keys,values}

I would propose to stabilize `{HashMap,BTreeMap}::into_{keys,values}`( aka. `map_into_keys_values`).

Closes rust-lang#75294.

Unify rustc and rustdoc parsing of `cfg()`

This extracts a new `parse_cfg` function that's used between both.

- Treat `#[doc(cfg(x), cfg(y))]` the same as `#[doc(cfg(x)]
  #[doc(cfg(y))]`. Previously it would be completely ignored.
- Treat `#[doc(inline, cfg(x))]` the same as `#[doc(inline)]
  #[doc(cfg(x))]`. Previously, the cfg would be ignored.
- Pass the cfg predicate through to rustc_expand to be validated

Technically this is a breaking change, but doc_cfg is still nightly so I don't think it matters.

Fixes rust-lang#84437.

r? ``````@petrochenkov``````

Cleanup of `wasm`

Some more cleanup of `sys`, this time `wasm`

- Reuse `unsupported::args` (functionally equivalent implementation, just an empty iterator).
- Split out `atomics` implementation of `wasm::thread`, the non-`atomics` implementation is reused from `unsupported`.
- Move all of the `atomics` code to a separate directory `wasm/atomics`.

`@rustbot` label: +T-libs-impl
r? `@m-ou-se`

Simplify chdir implementation and minimize unsafe block

…=Mark-Simulacrum

Add `needs-unwind` and beginning of support for testing `panic=abort` std to compiletest

For the Fuchsia platform we build libstd with `panic=abort` and would like a way to run tests with that enabled. This adds low-level support for this directly to compiletest.

In the future I'd like to add high-level support in rustbuild, e.g. having target-specific flags that allow configuring a panic strategy. (Side note: It would be nice if we could also build multiple configurations for the same target, but I'm getting ahead of myself.)

This plus rust-lang#84500 have everything that's needed to get ui tests passing on fuchsia targets.

Part of rust-lang#84766. Note that this change only includes the header on tests which need an unwinder to _build_, not those which need it to _run_.

r? `@Mark-Simulacrum`

Allow using `core::` in intra-doc links within core itself

I came up with this idea ages ago, but rustdoc used to ICE on it. Now it doesn't.

Helps with rust-lang#73445. Doesn't fix it completely since `extern crate self as std;` in std still gives strange errors.

…ns, r=Amanieu

Update compiler-builtins to 0.1.42 to get fix for outlined atomics

This should fix linking of other C code (and soon Rust-generated code)
on aarch64 musl.

…=jonas-schievink

⬆️ rust-analyzer

…, r=nagisa

Disallows `#![feature(no_coverage)]` on stable and beta (using standard crate-level gating)

Fixes: rust-lang#84836

Removes the function-level feature gating solution originally implemented, and solves the same problem using `allow_internal_unstable`, so normal crate-level feature gating mechanism can still be used (which disallows the feature on stable and beta).

I tested this, building the compiler with and without `CFG_DISABLE_UNSTABLE_FEATURES=1`

With unstable features disabled, I get the expected result as shown here:

```shell
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc     src/test/run-make-fulldeps/coverage/no_cov_crate.rs
error[E0554]: `#![feature]` may not be used on the dev release channel
 --> src/test/run-make-fulldeps/coverage/no_cov_crate.rs:2:1
  |
2 | #![feature(no_coverage)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
```

r? `@Mark-Simulacrum`
cc: `@tmandry` `@wesleywiser`

Handle incorrect placement of parentheses in trait bounds more gracefully

Fix rust-lang#84772.

CC ```@jonhoo```

CTFE engine: rename copy → copy_intrinsic, move to intrinsics.rs

The `copy` name is confusing for this function because we also have `copy_op` which is pretty different. I hope `copy_intrinsic` is clearer. Also `step.rs` should really just contain the main loop and opcode dispatch, so move this helper function to a more appropriate place.

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

…chenkov

Only compute Obligation `cache_key` once  in `register_obligation_at`

…etrochenkov

E0583: Include secondary path in error message

Fixes rust-lang#84819.

Fix typo in `MaybeUninit::array_assume_init` safety comment

And also add backticks around `MaybeUninit`.

…rk-Simulacrum

Revert PR 83866

rust-lang#83866 caused multiple test failures when Rust's channel is changed from nightly to anything else. The PR will have to be landed again after the test suite is fixed.

The two kinds of test failures were:

* Rustdoc tests failed because the links in the generated HTML didn't point at nightly anymore, see rust-lang#84909 (comment)
* Rustdoc UI tests failed because the links included in error messages didn't point at nightly anymore, see rust-lang#84909 (comment)

r? `@Mark-Simulacrum`
cc `@jyn514`