Generate #[must_use] on functions annotated with __attribute__((warn_unused_result))

[flowbish]

As annotating #[must_use] on functions was stabilized in 1.27, it should be possible to automatically carry the __attribute__((warn_unused_result)) attribute from C/C++ into rust with #[must_use]. This will then require a --rust-target option of 1.27 to allow generation of this code.

Input C/C++ Header

__attribute__((warn_unused_result))
int foo(int);

Actual Results

extern "C" {
    pub fn foo(arg1: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
}

Expected Results

extern "C" {
    #[must_use]
    pub fn foo(arg1: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
}

[emilio]

Yeah, I agree this would be useful. Generally we have a very hard time getting attributes from libclang, but I'd have to check how the libclang AST looks like in that test-case.

[emilio]

Fixed by #1451, thanks a lot @flowbish!