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    //! Closure type (equivalent to `&mut dyn FnMut(A) -> R`) that's `repr(C)`.

    #[repr(C)]
    pub struct Closure<'a, A, R> {
        call: unsafe extern "C" fn(&mut Env, A) -> R,
        env: &'a mut Env,
    }

    extern "C" {
        type Env;
    }

    impl<'a, A, R> !Sync for Closure<'a, A, R> {}
    impl<'a, A, R> !Send for Closure<'a, A, R> {}

    impl<'a, A, R, F: FnMut(A) -> R> From<&'a mut F> for Closure<'a, A, R> {
        fn from(f: &'a mut F) -> Self {
            unsafe extern "C" fn call<A, R, F: FnMut(A) -> R>(env: &mut Env, arg: A) -> R {
                (*(env as *mut _ as *mut F))(arg)
            }
            Closure { call: call::<A, R, F>, env: unsafe { &mut *(f as *mut _ as *mut Env) } }
        }
    }

    impl<'a, A, R> Closure<'a, A, R> {
        pub fn call(&mut self, arg: A) -> R {
            unsafe { (self.call)(self.env, arg) }
        }
    }