Add `#[repr(transparent)]` to `PathBuf`

[jhpratt]

The fact that PathBuf is internally an OsStr is currently noted as an implementation detail in comments. However, a number of methods clearly indicate (in documentation) that it is stored as such, and one PathBuf::into_os_str() implicitly requires it, as it would be inefficient to do it otherwise.

Would it make sense to add #[repr(transparent)] to PathBuf to externally stabilize this guarantee? It would also allow me (as the maintainer of the standback crate) to use transmute in a guaranteed manner.

[Mark-Simulacrum]

Can you cite which conversion you are trying to provide externally from std which would be enabled by such? I presuming you mean that PathBuf is internally an OsString, not OsStr?

It also looks like both impl From<PathBuf> for OsString and impl From<OsString> for PathBuf have been present since 1.14 and 1.0 respectively, maybe that's enough?

[jhpratt]

Sure. I'm actually backporting the APIs from 1.44 to 1.31 (the MSRV for this crate). Obviously the repr attribute wouldn't apply to older versions, but if any get stabilized later on, it would allow backporting with safety guarantees.

The From impls actually aren't sufficient, as it requires taking ownership when only borrowing is necessary. As such, I'm currently doing unsafe { transmute::<_, &OsString>(self) }.capacity() to backport (and similar for other methods).

Realistically, I don't see why the attribute shouldn't be added, though. I took a quick look through the impls, and it would be nearly impossible to have any other internal representation without removing some of these.

[Mark-Simulacrum]

Ah, I see. Technically you could ptr::read and be quite careful to not drop anything, but I suppose that's not better than transmuting.

I would suggest filing a PR which can then be r?'d for someone on the libs team to approve.

[jhpratt]

Will do. I'll also add it to Path for basically the same reason.

[dtolnay]

Closing per #72841 (review).