Add #[must_use] to from_value conversions

[jkugelman]

I added two methods to the list myself. Clippy did not flag them because they take mut args, but neither modifies their argument.

core::str           const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str;
std::ffi::CString   unsafe fn from_raw(ptr: *mut c_char) -> CString;

I put a custom note on from_raw:

#[must_use = "call `drop(from_raw(ptr))` if you intend to drop the `CString`"]
pub unsafe fn from_raw(ptr: *mut c_char) -> CString {

Parent issue: #89692

r? @joshtriplett

[joshtriplett]

If we find any significant number of crates that are using one of these methods to convert to a type with Drop as a means of freeing it, we should consider reverting the addition of must_use to that method to avoid excessive disruption. Such a code pattern would be unusual but legitimate. We're allowed to add warnings to existing code, but we typically take into account how many warnings we're adding to the ecosystem.

[joshtriplett]

@bors r+

[bors]

📌 Commit cf2bcd1 has been approved by joshtriplett

[BurntSushi]

Is there any particular reason why this was added to CString::from_raw but not, say, Box::from_raw?

[jkugelman]

@BurntSushi No, I don't believe so. I don't remember excluding it. I'm guessing I simply didn't catch Box::from_raw.