`HashSet::insert` does not replace the value with equal value.

Fixes rust-lang#107581.

…=Amanieu

Implement `TryFrom<&OsStr>` for `&str`

Recently when trying to work with `&OsStr` I was surprised to find this `impl` missing.

Since the `to_str` method already existed the actual implementation is fairly non-controversial, except for maybe the choice of the error type. I chose an opaque error here instead of something like `std::str::Utf8Error`, since that would already make a number of assumption about the underlying implementation of `OsStr`.

As this is a trait implementation, it is insta-stable, if I'm not mistaken?
Either way this will need an FCP.
I chose "1.64.0" as the version, since this is unlikely to land before the beta cut-off.

`@rustbot` modify labels: +T-libs-api

API Change Proposal: rust-lang#99031 (accepted)

Specify behavior of HashSet::insert

`HashSet::insert` does not replace the value with equal value.

Fixes rust-lang#107581.

…nieu

Stabilize String::leak

Stabilizes the following API:

```Rust
impl String {
    pub fn leak(self) -> &'static mut str;
}
```

closes rust-lang#102929

blocked by having an FCP for stabilization.

Update runtime guarantee for `select_nth_unstable`

rust-lang#106933 changed the runtime guarantee for `select_nth_unstable` from O(n) to O(n log n), since the old guarantee wasn't actually met by the implementation at the time. Now with rust-lang#107522, `select_nth_unstable` should be truly linear in runtime, so we can revert its runtime guarantee to O(n). Since rust-lang#106933 was considered a bug fix, this will probably need an FCP because it counts as a new API guarantee.

r? `@Amanieu`

…nfo, r=b-naber

Don't print unsupported split-debuginfo modes with `-Zunstable-options`

Currently unsupported `split-debuginfo` options are enabled by `-Zunstable-options`, for projects that have `-Zunstable-options` for other reasons this can be [an unexpected interaction](rust-lang/rust-clippy#10516 (comment))

This PR makes it so that `--print split-debuginfo -Zunstable-options` doesn't print unsupported modes, so that a cargo config of e.g.

```toml
[profile.dev]
split-debuginfo = "unpacked"
```

Would not cause an unsupported mode to be enabled on `x86_64-pc-windows-msvc`

…=b-naber

Properly check associated consts for infer placeholders

We only reported an error if it was in a "suggestable" position (according to `is_suggestable_infer_ty`) -- this isn't correct for infer tys that can show up in other places in the constant's type, like behind a dyn trait.

fixes rust-lang#112491