`str::as_str` but for `CStr`, `OsStr`, `Path`, and `[T]`

[GrigorenkoPV]

Proposal

Add CStr::as_c_str, OsStr::as_os_str, Path::as_path, [T]::as_slice, and [T]::as_mut_slice, analogous to str::as_str (rust-lang/rust#130366) to aid dereferencing Box<CStr/OsStr/...> without CString, OsString, ... being the only ones with the convenient .as_thing() method.

Background

str::as_str was proposed by t-libs-api in rust-lang/rust#129550 (comment) as a response to a PR adding Box<str>::as_str.

The idea of analogous additions for other "slicy" types was brought up in rust-lang/rust#130366 (comment)

Solution sketch

rust-lang/rust#145933

Questions

• Should this reuse the same feature gate as str::as_str?
• Should the [T]:: methods be moved to a separate feature gate?
• Bikeshed the feature gate names.
• Technically slices already have an as_slice method, but with no mut version and it comes from an unstable trait with a closed tracking issue and docs saying that hopefully this trait will be removed in the future: https://doc.rust-lang.org/stable/std/primitive.slice.html#impl-SlicePattern-for-%5BT%5D