Function generic_static_promise_method 

pub(crate) unsafe extern "C" fn generic_static_promise_method(
    cx: *mut JSContext,
    argc: c_uint,
    vp: *mut JSVal,
) -> bool

https://searchfox.org/mozilla-central/rev/7279a1df13a819be254fd4649e07c4ff93e4bd45/dom/bindings/BindingUtils.cpp#3300

Safety

cx must point to a valid, non-null JSContext. vp must point to a VALID, non-null JSVal.