Fixes rust-lang#35729

According to recommendations in
rust-lang#35729 (comment)

…sistent docs order

See rust-lang#47115 (comment)

The old text was: "The precise definition is: a type T is Sync if &T is Send."

Since we've also got
```
impl<'a, T> Send for &'a T 
where
    T: Sync + ?Sized,
```
I purpose we can change the `if` to `if and only if` to make it more precise.

Fixes rust-lang#47275. These two macros are similar, but different, and could do with documentation links to each other.

It now details why using compile_error!() is different from just not having the final macro_rules!() branch.

This commit adds a new section to the Documentation Test docs, which briefly mentions indented code blocks, and links to the CommonMark specification for both.

I’m not sure about saying "fenced code blocks the more popular choice in the Rust community” because it seems like I’m speaking for everyone, but I can’t think of a better way to phrase it!

If a struct pattern has a field error return an error.

…are now exactly the same as that produced by AST borrowck. Bravo!

It's never used in a meaningful way.

We already got the open file descriptor at this point.
Don't make the kernel resolve the path again.

This reverts commit 068fc44.

Because they are small and hot.

std::fs::DirEntry.metadata(): use fstatat instead of lstat when possible

When reading a directory with `read_dir`, querying metadata for a resulting `DirEntry` is done by building the whole path and then `lstat`ing it, which requires the kernel to resolve the whole path. Instead, one
can use the file descriptor to the enumerated directory and use `fstatat`. This make the resolving step
unnecessary.
This PR implements using `fstatat` on linux, android and emscripten.

## Compatibility across targets
`fstatat` is POSIX.
* Linux >= 2.6.19 according to https://linux.die.net/man/2/fstatat
* android according to https://android.googlesource.com/platform/bionic/+/master/libc/libc.map.txt#392
* emscripten according to https://github.com/kripken/emscripten/blob/7f89560101843198787530731f40a65288f6f15f/system/include/libc/sys/stat.h#L76

The man page says "A similar system call exists on Solaris." but I haven't found it.

## Compatibility with old platforms
This was introduced with glibc 2.4 according to the man page. The only information I could find about the minimal version of glibc rust must support is this discussion https://internals.rust-lang.org/t/bumping-glibc-requirements-for-the-rust-toolchain/5111/10
The conclusion, if I understand correctly, is that currently rust supports glibc >= 2.3.4 but the "real" requirement is Centos 5 with glibc 2.5. This PR would make the minimal version 2.4, so this should be fine.

## Benefit
I did the following silly benchmark:
```rust
use std::io;
use std::fs;
use std::os::linux::fs::MetadataExt;
use std::time::Instant;

fn main() -> Result<(), io::Error> {
    let mut n = 0;
    let mut size = 0;
    let start = Instant::now();
    for entry in fs::read_dir("/nix/store/.links")? {
        let entry = entry?;
        let stat = entry.metadata()?;
        size += stat.st_size();
        n+=1;
    }
    println!("{} files, size {}, time {:?}", n, size, Instant::now().duration_since(start));
    Ok(())
}
```
On warm cache, with current rust nightly:
```
1014099 files, size 76895290022, time Duration { secs: 2, nanos: 65832118 }
```
(between 2.1 and 2.9 seconds usually)
With this PR:
```
1014099 files, size 76895290022, time Duration { secs: 1, nanos: 581662953 }
```
(1.5 to 1.6 seconds usually).

approximately 40% faster :)

On cold cache there is not much to gain because path lookup (which we spare) would have been a cache hit:
Before
```
1014099 files, size 76895290022, time Duration { secs: 391, nanos: 739874992 }
```
After
```
1014099 files, size 76895290022, time Duration { secs: 388, nanos: 431567396 }
```
## Testing
The tests were run on linux `x86_64`
```
python x.py test src/tools/tidy
./x.py test src/libstd
```
and the above benchmark.
I did not test any other target.

Update build instructions

It get stuck at the cloning step.
`./x.py build `
Updating only changed submodules
Updating submodule src/llvm
Submodule 'src/llvm' (https://github.com/rust-lang/llvm.git) registered for path 'src/llvm'
Cloning into '/home/username/rust/src/llvm'...

…uillaumeGomez

Add doc link from discriminant struct to function.

None

typeck: Do not pass the field check on field error

If a struct pattern has a field error return an error.

Fixes: rust-lang#51102

Stabilize SliceIndex trait.

CC rust-lang#35729

According to recommendations in
rust-lang#35729 (comment)

…dtolnay

Move slice::exact_chunks directly above exact_chunks_mut for more con…

…sistent docs order

See rust-lang#47115 (comment)

… r=GuillaumeGomez

Link panic and compile_error docs

This adds documentation links between `panic!()` and `compile_error!()` as per rust-lang#47275, which points out that they’re similar. It also adds a sentence to the `compile_error()` docs I thought could be added.

Mention spec and indented blocks in doctest docs

Fixes rust-lang#49717.

This commit adds a new section to the Documentation Test docs, which briefly mentions indented code blocks, and links to the CommonMark specification for both.

I’m not sure about saying "fenced code blocks the more popular choice in the Rust community” because it seems like I’m speaking for everyone, but I can’t think of a better way to phrase it!

…derr-files, r=oli-obk

Remove two redundant .nll.stderr files

It turns out that the diagnostics generated from NLL for these cases are now exactly the same as that produced by AST borrowck, and thus we can just fallback on those `.stderr` files that already exist for AST-borrowck.

Bravo!

(it is a good idea to remove these files, because it slightly reduces the amount of time humans will spend reviewing the .nll.stderr fileset...)

((it *might* be worthwhile trying to change the `compiletest` code to even issue a warning when two such files have equivalent contents... but I am not going so far as to try to implement that right now...))

…lwoerister

Two minor `obligation_forest` tweaks.

Pretty minimal improvements, but improvements nonetheless.

fs: copy: Use File::set_permissions instead of fs::set_permissions

We already got the open file descriptor at this point.
Don't make the kernel resolve the path again.

The old text was: "The precise definition is: a type `T` is `Sync` if `&T` is Send."

Since we've also got
```
impl<'a, T> Send for &'a T
where
    T: Sync + ?Sized,
```
I purpose we can change the `if` to `if and only if` to make it more precise.