Bindgen should issue an error, if forward declared types are used in function signatures.

E.g. in the bindgen test func_return_must_use forward declared types are used in the function signature.

Bindgen should refuse to generate a function signature where forward declared types are used directly and not via a pointer.

struct MustUseStruct;

struct MustUseStruct return_struct();

#[repr(C)]
#[derive(Debug, Copy, Clone)]
#[must_use]
pub struct MustUseStruct {
    _unused: [u8; 0],
}

unsafe extern "C" {
    #[must_use]
    pub fn return_struct() -> MustUseStruct;
}

Start with the bindgen test func_return_must_use and reproduce the case using a forward-declared type in a function signature. Trace the signature-generation path to determine where this case is handled. Done means bindgen reports an error for direct use of a forward-declared type while still permitting pointer use.