Define c_char using cfg_if rather than repeating 40-line cfg

[dtolnay]

Libstd has a 40-line cfg that defines the targets on which c_char is unsigned, and then repeats the same cfg with not(…) for the targets on which c_char is signed.

This PR replaces it with a cfg_if! in which an else takes care of the signed case.

I confirmed that x.py doc library/std inlines the type alias because c_char_definition is not a publicly accessible path:

[rust-highfive]

r? @kennytm

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[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit db5a2ae has been approved by Mark-Simulacrum

[fee1-dead]

Looks like this caused a clippy test to fail in a rollup: #91650 (comment). The stderr now tells users to remove cast to c_char which would actually break compilation for some platforms.

[dtolnay]

I filed rust-lang/rust-clippy#8093 about the false positive and added a workaround in this PR. Tested by:

//#[cfg(all())]  // unnecessary_cast goes away if uncommented
type T = u8;

fn main() {
    let _ = 0 as T;
}

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 4e8b91a has been approved by Mark-Simulacrum