Cstring from_raw and into_raw safety precisions

[poliorcetics]

Fixes #48525.
Fixes #68456.

This issue had two points:

• The one about from_raw has been addressed (I hope).
• The other one, about into_raw, has only been partially fixed.

About into_raw: the idea was to:

steer users away from using the pattern of CString::{into_raw,from_raw} when interfacing with C APIs that may change the effective length of the string by writing interior NULs or erasing the final NUL

I tried making a Vec<c_char> like suggested but my current solution feels very unsafe and hacky to me (most notably the type cast), I included it here to make it available for discussion:

fn main() {
    use std::os::raw::c_char;

    let v = String::from("abc")
        .bytes()
        // From u8 to i8,
        // I feel like it will be a problem for values of u8 > 255
        .map(|c| c as c_char)
        .collect::<Vec<_>>();

    dbg!(v);
}

[rust-highfive]

r? @dtolnay

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[dtolnay]

@bors r+

[bors]

📌 Commit 87abe17 has been approved by dtolnay

[dtolnay]

@bors rollup

[dtolnay]

I think the canonical way to make a Vec<c_char> in place is:

let (ptr, len, cap) = string.into_bytes().into_raw_parts();
let v = unsafe { Vec::from_raw_parts(ptr as *mut c_char, len, cap) };