Function is_type 

pub unsafe fn is_type(
    value: *const c_void,
    source: *const c_void,
    target: *const c_void,
) -> bool

Is value of type target?

x is T, without producing the result. The source is left untouched, so this is safe to ask of a value someone else owns.

Safety

value must address an initialised value of source’s type, and both metadata pointers must be valid.