… to use the feature

…rom tests

Rebase commit

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

to avoid panic for invalid Unicode scalar values

Improve the documentation of drain members

hopefully fixes rust-lang#92765

Stabilize `#[cfg(panic = "...")]`

[Stabilization PR](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr) for rust-lang#77443

…imulacrum

rustdoc-json: buffer output

It turns out we were doing syscalls for each part of the json syntax

Before:
```
...
[pid 1801267] write(5, "\"", 1)         = 1
[pid 1801267] write(5, ",", 1)          = 1
[pid 1801267] write(5, "\"", 1)         = 1
...
```

After:

```
[pid 1974821] write(5, "{\"root\":\"0:0\",\"crate_version\":nu"..., 1575) = 1575
```

In one benchmark (one struct, almost all time in `std`), this gives ~2x perf

r? `@CraftSpider`

`@rustbot` modify labels: +A-rustdoc-json +T-rustdoc -A-testsuite

Add debug assertions to validate NUL terminator in c strings

The `unchecked` variants from the stdlib usually perform the check anyway if debug assertions are on (for example, `unwrap_unchecked`). This PR does the same thing for `CStr` and `CString`, validating the correctness for the NUL byte in debug mode.

pre rust-lang#89862 cleanup

changes used in rust-lang#89862 which can be landed without the rest of this PR being finished.

r? `@estebank`

Use a `Field` in `ConstraintCategory::ClosureUpvar`

As part of rust-lang#90317, we do not want `HirId` to implement `Ord`, `PartialOrd`. This line of code has made that difficult

https://github.com/rust-lang/rust/blob/1b27144afc77031ba9c05d86c06c64485589775a/compiler/rustc_borrowck/src/region_infer/mod.rs#L2184

since it sorts a [`ConstraintCategory::ClosureUpvar(HirId)`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.ConstraintCategory.html#variant.ClosureUpvar).

This PR makes that variant take a [`Field`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Field.html) instead.

r? `@nikomatsakis`

…=davidtwco

Fix ScalarInt to char conversion

to avoid panic for invalid Unicode scalar values