Allow allow enums like `Result<T, E>` to be used across FFI if the
T/E can be niche optimized and the non-niche-optimized type is FFI safe.

Fields are disallowed so checking the top attribute is enough.

Tests both `T` and `E` for niche variant optimization lints

Couldn't find documentation supporting that single-variant
`#[repr(Rust)]` enums with RHS assigned work as expected with this
change.

```rust
enum Variants {
  A = 17,
} // Would this be zero sized optimized guaranteed?
```

…sion-of-nodejs, r=RalfJung

bump actions/cache to v4

fixes rust-lang#3518.

josh rustc-pull: check that no new root commits get created

A second root was a bad sign in Miri (judging from the description in rust-lang/miri#2583) and seems to be a [bad sign in RA](rust-lang/rust-analyzer#17025 (comment)). So let's add this to the sanity checks.

…numbers of elements

Handle post-merge comments of AVX2 PR

file descriptors: make write take &mut self

Use the interpreted program's TZ variable in localtime_r

This requires a bit of wiring and a new dependency, but the tests should correctly pass now regardless of what the host's time zone is.

Fixes rust-lang/miri#3522

Don’t print `Preparing a sysroot` when `-q`/`--quiet` is passed

Resolves rust-lang#3530.

This also fixes a typo in `cargo miri --help` that I found while trying to run the `--print-sysroot` example.

…initialized'

Tree Borrows: first apply transition, then check protector with new 'initialized'