Add `cstr!` macro for creating `&'static CStr`s.

[reitermarkus]

Proposal

Problem statement

Now that core::ffi::CStr is stabilized, there should be an ergonomic way to create &CStr constants.

Motivation, use-cases

On stable there is currently no good way to create a &'static CStr, the simplest solution needs unsafe, i.e.

const MY_C_STRING: &CStr = unsafe { CStr::from_bytes_with_nul_unchecked(b"Hello, world!\0") };

On nightly, it is possible since rust-lang/rust#100291 without unsafe, but these are even more verbose or don't provide meaningful error messages:

#![feature(const_cstr_methods)]

const MY_C_STRING: &CStr = if let Ok(s) = CStr::from_bytes_with_nul(b"Hello, world!\0") {
    s
} else {
    panic!("contains interior nul bytes")
};

#![feature(const_cstr_methods)]
#![feature(const_option)]

const MY_C_STRING: &CStr = CStr::from_bytes_with_nul(b"Hello, world!\0").ok().unwrap();

Solution sketches

With a cstr!, the example above becomes both easier to write and easier to read, i.e.

const MY_C_STRING: &CStr = cstr!("Hello, world!");

Links and related work

• The cstr_core crate contains such a macro: https://docs.rs/cstr_core/0.2.6/cstr_core/macro.cstr.html
• The cstr crate provides such a macro: https://docs.rs/cstr/0.2.10/cstr/index.html
• Pull request adding this as core::ffi::cstr is here: Add cstr! macro. rust#101607

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[thomcc]

I'm broadly in favor of this -- it's worth considering what syntax we'd want for this under rust-lang/rfcs#3267 (although, I guess I'm unsure the future that RFC has).

[m-ou-se]

I've nominated rust-lang/rust#101607 for lang team discussion, to discuss the c"abc" alternative.

[m-ou-se]

I've opened rust-lang/rfcs#3348 as an alternative.

[SUPERCILEX]

I actually just ran into this! I'll echo Thom about broadly wanting this and say that I'd prefer Mara's solution over a macro, but would settle for either.

[m-ou-se]

Closing this, because rust-lang/rfcs#3348 has been accepted.