Co-authored-by: Rémy Rakic <remy.rakic+github@gmail.com>

Manually tested via


```
// spawn a new cgroup scope for the current user
$ sudo systemd-run -p CPUQuota="300%" --uid=$(id -u) -tdS


// quota.rs
#![feature(available_parallelism)]
fn main() {
    println!("{:?}", std::thread::available_parallelism()); // prints Ok(3)
}
```


Caveats

* cgroup v1 is ignored
* funky mountpoints (containing spaces, newlines or control chars) for cgroupfs will not be handled correctly since that would require unescaping /proc/self/mountinfo
  The escaping behavior of procfs seems to be undocumented. systemd and docker default to `/sys/fs/cgroup` so it should be fine for most systems.
* quota will be ignored when `sched_getaffinity` doesn't work
* assumes procfs is mounted under `/proc` and cgroupfs mounted and readable somewhere in the directory tree

this avoids parsing mountinfo which can be huge on some systems and
something might be emulating cgroup fs for sandboxing reasons which means
it wouldn't show up as mountpoint

additionally the new implementation operates on a single pathbuffer, reducing allocations

… are now taken into account

To avoid the strange style where comments force `else` onto its own
line.

The commit also removes several else-after-return constructs, which can
be hard to read.

There are three `Option` fields in `MatcherPos` that are only used in
tandem. This commit combines them, making the code slightly easier to
read. (It also makes clear that the `sep` field arguably should have
been `Option<Option<Token>>`!)

Because `inner_parse_loop` has only one way to not succeed, not three.

Signed-off-by: cuishuang <imcusg@gmail.com>

Use cgroup quotas for calculating `available_parallelism`

Automated tests for this are possible but would require a bunch of assumptions. It requires root + a recent kernel, systemd and maybe docker. And even then it would need a helper binary since the test has to run in a separate process.

Limitations

* only supports cgroup v2 and assumes it's mounted under `/sys/fs/cgroup`
* procfs must be available
* the quota gets mixed into `sched_getaffinity`, so if the latter doesn't work then quota information gets ignored too

Manually tested via

```
// spawn a new cgroup scope for the current user
$ sudo systemd-run -p CPUQuota="300%" --uid=$(id -u) -tdS

// quota.rs
#![feature(available_parallelism)]
fn main() {
    println!("{:?}", std::thread::available_parallelism()); // prints Ok(3)
}
```

strace:

```
sched_getaffinity(3041643, 32, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47]) = 32
openat(AT_FDCWD, "/proc/self/cgroup", O_RDONLY|O_CLOEXEC) = 3
statx(0, NULL, AT_STATX_SYNC_AS_STAT, STATX_ALL, NULL) = -1 EFAULT (Bad address)
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0444, stx_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "0::/system.slice/run-u31477.serv"..., 128) = 36
read(3, "", 92)                         = 0
close(3)                                = 0
statx(AT_FDCWD, "/sys/fs/cgroup/system.slice/run-u31477.service/cgroup.controllers", AT_STATX_SYNC_AS_STAT, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0444, stx_size=0, ...}) = 0
openat(AT_FDCWD, "/sys/fs/cgroup/system.slice/run-u31477.service/cpu.max", O_RDONLY|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "300000 100000\n", 20)          = 14
read(3, "", 6)                          = 0
close(3)                                = 0
openat(AT_FDCWD, "/sys/fs/cgroup/system.slice/cpu.max", O_RDONLY|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "max 100000\n", 20)             = 11
read(3, "", 9)                          = 0
close(3)                                = 0
openat(AT_FDCWD, "/sys/fs/cgroup/cpu.max", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
sched_getaffinity(0, 128, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47]) = 40
```

r? ``````@joshtriplett``````
cc ``````@yoshuawuyts``````

Tracking issue and previous discussion: rust-lang#74479

improve comments for `simplify_type`

Should now correctly describe what's going on. Experimented with checking the invariant for projections
but that ended up requiring fairly involved changes. I assume that it is not possible to get unsoundness here,
at least for now and I can pretty much guarantee that it's impossible to trigger it by accident.

r? ````@nikomatsakis```` cc rust-lang#92721

…trochenkov

`parse_tt` cleanups

I've been looking closely at this code, and saw some opportunities to improve its readability.

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

rustdoc: Add test for higher kinded functions generated by macros

Fixes rust-lang#75564.

The problem has been solved apparently so adding a test to prevent a regression.

r? ``@notriddle``

Doc: Fix use of quote instead of backstick in Adapter::map.

A little commit to fix documentation rendering and semantics in https://doc.rust-lang.org/std/iter/struct.Map.html#notes-about-side-effects `"` where used around an expression instead \`.

Screenshot on doc.rust-lang.org:
![2022-03-03 11-21-43_backstick](https://user-images.githubusercontent.com/2827553/156546536-569b7692-7ac4-4388-8e93-c1628ddc6a0f.png)

Looking forward: Maybe reworking the doc to use assert_eq like the upper paragraph:
```
let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x| x + 1).rev().collect();

assert_eq!(v, [4, 3, 2]);
```

add tests for rust-lang#94502

cc rust-lang#94552

Fix invalid lint_node_id being put on a removed stmt

This pull-request remove a invalid `assign_id!` being put on an stmt node.

The problem is that this node is being removed away by a cfg making it unreachable when triggering a buffered lint.
The comment in the other match arm already tell to not assign a id because it could have a `#[cfg()]` so this is just respecting the comment.

Fixes rust-lang#94523
r? ``````@petrochenkov``````

all: fix some typos

Signed-off-by: cuishuang <imcusg@gmail.com>

…or-ignoring-remaining-parts, r=Dylan-DPC

Remove a unnecessary `..` pattern