1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133

//! bip (`Box` in place) provides a fully generic in-place `map` for
//! the `Box` type, taking care to be panic-safe and not leak memory.
//!
//! [Available on
//! crates.io](http://crates.io/crates/bip). [Source](https://github.com/huonw/bip).

#![feature(alloc)]

use std::rt::heap;
use std::{mem,ptr};

// avoid memory leaks by freeing the memory, without
// running the destructor of the contents
struct Dropper<T> {
    ptr: *mut T
}

impl<T> Drop for Dropper<T> {
    fn drop(&mut self) {
        unsafe {
            heap::deallocate(self.ptr as *mut u8, mem::size_of::<T>(), mem::align_of::<T>());
        }
    }
}

/// Execute `f` on the data in `x`, replacing the output into the same
/// allocation.
///
/// This is semantically equivalent to `Box::new(f(*x))`, but avoids
/// the allocation by reusing the memory of `x` directly. `map` will
/// not cause unsafety or leak memory if `f` panics.
///
/// `T` and `U` must have the same size, and the alignment (measured
/// by `std::mem::min_align_of`) of `T` must be at least as large as
/// that of `U`. A violation of either of these requirements will
/// result in a runtime panic.
///
/// # Example
///
/// ```rust
/// let x = Box::new(1_i32);
/// let address = &*x as *const _ as usize;
///
/// let new_x = bip::map_in_place(x, |a| a as f32 + 1.0);
/// assert_eq!(*new_x, 2.0);
///
/// assert_eq!(address, &*new_x as *const _ as usize);
/// ```
pub fn map_in_place<T, U, F>(x: Box<T>, f: F) -> Box<U> where F: FnOnce(T) -> U {
    assert!(mem::size_of::<T>() == mem::size_of::<U>(),
            "map_in_place: `T` and `U` are of different sizes");
    assert!(mem::align_of::<T>() >= mem::align_of::<U>(),
            "map_in_place: alignment of `U` is too large");

    unsafe {
        let dropper = Dropper {
            ptr: mem::transmute(x)
        };
        let old = ptr::read(dropper.ptr);
        let new = f(old);
        ptr::write(dropper.ptr as *mut U, new);

        let ret: Box<U> = mem::transmute(dropper.ptr);

        mem::forget(dropper);
        ret
    }
}

#[cfg(test)]
mod tests {
    use super::map_in_place;
    use std::thread;
    use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};

    #[derive(PartialEq, Debug)]
    struct NotCopy(i32);
    #[derive(PartialEq, Debug)]
    struct NotCopy2(f32);

    #[test]
    fn smoke() {
        assert_eq!(map_in_place(Box::new(NotCopy(1)), |x| NotCopy2(x.0 as f32 + 1.0)),
                   Box::new(NotCopy2(2.0)));
    }

    #[test]
    fn in_place() {
        let x = Box::new(NotCopy(1));
        let address_x = &*x as *const NotCopy;
        let y = map_in_place(x, |x| NotCopy(x.0 + 1));
        let address_y = &*y as *const _;

        assert_eq!(address_x, address_y);
    }

    #[test]
    fn destructor_count() {
        static COUNT: AtomicUsize = ATOMIC_USIZE_INIT;

        struct Foo { _x: u8 }
        impl Drop for Foo {
            fn drop(&mut self) {
                COUNT.fetch_add(1, Ordering::SeqCst);
            }
        }
        struct Bar { _x: u8 }
        impl Drop for Bar {
            fn drop(&mut self) {
                COUNT.fetch_add(10_000, Ordering::SeqCst);
            }
        }

        COUNT.store(0, Ordering::SeqCst);
        let value = Box::new(Foo { _x: 1 });

        let _ = thread::spawn(|| {
            map_in_place(value, |_| -> Bar { panic!() });
        }).join();
        assert_eq!(COUNT.load(Ordering::SeqCst), 1);
    }

    #[test]
    #[should_panic]
    fn mismatching_sizes() {
        map_in_place(Box::new(1i32), |_| 0i16);
    }
    #[test]
    #[should_panic]
    fn insufficient_alignment() {
        map_in_place(Box::new([0u8; 8]), |_| 0u64);
    }
}