Suggestion: detect calling printf-style C APIs in dodgy ways

Specifically, APIS like:

extern "C" {
    fn foo(some: args, fmt: *const c_char, ...);
}

where they're variadic C functions where the last non-variadic argument is a *const c_char.

If you call it as foo(x, some_cstr.as_ptr()) that's almost always a format string vulnerability. Instead you should use a format arg like b"%s\0".as_ptr().cast() and then pass the string as the first entry in the variadic list.

It's might be enough just to detect the case where there's no variadic args? I don't really know if it's worth trying to do the c-style check that the format args are correct -- these APIs get so little use.

I don't know how doable this is as a lint, but I've seen the mistake a lot.

No files, tests, or existing lint entry points are named. Start by determining how rust-clippy represents variadic extern "C" declarations and calls, then scope detection to a non-variadic *const c_char last parameter and absent variadic arguments. Done means lint behavior and tests covering the proposed call patterns.