These have been bugging me for a while.

- `literal_text`: `src` is also used and is shorter and better.
- `first_char`: used even when "first" doesn't make sense; `c` is
  shorter and better.
- `curr`: `c` is shorter and better.
- `unescaped_char`: `result` is also used and is shorter and better.
- `second_char`: these have a single use and can be elided.

There is some subtlety here.

It's passed to numerous places where we just need an `is_byte` bool.
Passing the bool avoids the need for some assertions.

Also rename `is_bytes()` as `is_byte()`, to better match `Mode::Byte`,
`Mode::ByteStr`, and `Mode::RawByteStr`.

There are three kinds of "byte" literals: byte literals, byte string
literals, and raw byte string literals. None are allowed to have
non-ASCII chars in them.

Two `EscapeError` variants exist for when that constraint is violated.
- `NonAsciiCharInByte`: used for byte literals and byte string literals.
- `NonAsciiCharInByteString`: used for raw byte string literals.

As a result, the messages for raw byte string literals use different
wording, without good reason. Also, byte string literals are incorrectly
described as "byte constants" in some error messages.

This commit eliminates `NonAsciiCharInByteString` so the three cases are
handled similarly, and described correctly. The `mode` is enough to
distinguish them.

Note: Some existing error messages mention "byte constants" and some
mention "byte literals". I went with the latter here, because it's a
more correct name, as used by the Reference.

Remove a low-value comment, remove a duplicate comment, and correct a
third comment.

It deals with eight cases: ints, floats, and the six quoted types
(char/byte/strings). For ints and floats we have an early return, and
the other six types fall through to the code at the end, which makes the
function hard to read.

This commit rearranges things to avoid the early returns.

It has a single callsite, and is fairly small. The `Float` match arm
already has base-specific checking inline, so this makes things more
consistent.

It's easy to just use `unescape_literal` + `byte_from_char`.

The `usize` isn't needed in the error case.

Stabilize integer logarithms

Stabilizes feature `int_log`.

I've also made the functions const stable, because they don't depend on any unstable const features. `rustc_allow_const_fn_unstable` is just there for `Option::expect`, which could be replaced with a `match` and `panic!`. cc `@rust-lang/wg-const-eval`

closes rust-lang#70887 (tracking issue)

~~blocked on FCP finishing: rust-lang#70887 (comment)
FCP finished: rust-lang#70887 (comment)

…cottmcm

Add documentation examples for `pointer::mask`

The examples are somewhat convoluted, but I don't know how to make this better :(

…=matklad

Unescaping cleanups

Some code improvements, and some error message improvements.

Best reviewed one commit at a time.

r? ```@matklad```

…ce, r=Mark-Simulacrum

Don't intra linkcheck reference

This removes the reference from the intra-doc link checks. This causes problems if any of the reference content needs to change, it causes the linkchecker to break. The reference has its own broken link check (https://github.com/rust-lang/reference/tree/master/style-check) which uses pulldown-cmark on the source to find actual broken links (instead of false-positives like this regex does).

I think the intra-doc link check could potentially be removed completely, since I think rustdoc is now checking for them well enough. However, it may serve as a decent regression check.

…, r=thomcc

Add test for impl of `available_parallelism()`

This checks that `std::thread::available_parallelism()` returns `Ok(NonZeroUsize)`. With this test maintainers of targets(OS) can check if they have implemented this function (or decide to ignore it).

Not sure how to deal with the ignore list right now:

- tier 1's should all implement the feature already ✔️
- tier 2's mostly do as well, but not e.g. `ios`, `wasi` -- should we add them to the ignore list or let the maintainers do that (so that they are informed)?
- tier 3's will certainly sometimes not implement it, my suggestion is to not add them to the ignore list
- I've added only `vxworks`, `redox` and `l4re` to the ignore list because this is documented already in `library/std/src/sys/unix/thread.rs` ([code](https://github.com/rust-lang/rust/blob/9b735a7132acd58b3bd34c084e9ca5b4ca7450a2/library/std/src/sys/unix/thread.rs#L403))

We could also check the value against e.g. `cat /proc/cpuinfo | grep process or | wc`  but I don't know if this is required (how much sense it makes) and if this would play well with e.g. cgroups on Linux.

r? `@thomcc`

…scription-for-as-simd-mut, r=scottmcm

rustdoc: Add mutable to the description

Add mutable the description to differentiate [as_simd](https://github.com/rust-lang/rust/blob/master/library/core/src/slice/mod.rs#L3654) from [as_simd_mut](https://github.com/rust-lang/rust/blob/master/library/core/src/slice/mod.rs#L3654).

…r=fee1-dead

Const Compare for Tuples

Makes the impls for Tuples of ~const `PartialEq` types also `PartialEq`, impls for Tuples of ~const `PartialOrd` types also `PartialOrd`, for Tuples of ~const `Ord` types also `Ord`.

behind the `#![feature(const_cmp)]` gate.

~~Do not merge before rust-lang#104113 is merged because I want to use this feature to clean up the new test that I added there.~~

r? `@fee1-dead`