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    // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! 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) }
        }
    }

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