swap_unchecked ?

[leonardo-m]

A slice method similar to slice::get_unchecked_mut that could be added:

#[inline]
pub unsafe fn swap_unchecked(&mut self, a: usize, b: usize) {
    unsafe {
        // Can't take two mutable loans from one vector, so instead
        // just cast them to their raw pointers to do the swap.
        let pa: *mut T = self.get_unchecked_mut(a);
        let pb: *mut T = self.get_unchecked_mut(b);
        ptr::swap(pa, pb);
    }
}

To be added to the std lib only if there's enough usage for this method.

See also:
https://crates.io/crates/unchecked-index

[bluss]

It is relatively easy to implement this outside of libcore (in your own code). The preferred way (current best practice) would be to go through self.as_mut_ptr() and offsetting from there, instead of calling get_unchecked_mut (which uses an exclusive borrow of the slice; no longer exclusive if we keep an active pointer around).

Use ```rust on your code fence to get syntax coloring.

[leonardo-m]

Thank you for the comments bluss, second version then:

#[inline]
pub unsafe fn swap_unchecked(&mut self, i: usize, j: usize) {
    let ptr1 = self.as_mut_ptr().add(i);
    let ptr2 = self.as_mut_ptr().add(j);
    ptr::swap(ptr1, ptr2);
}

[bluss]

I'd propose this for anyone who wants to copy this implementation (this is what was meant with only using one &mut [T]).

// Method on [T]
pub unsafe fn swap_unchecked(&mut self, i: usize, j: usize) {
    debug_assert!(i < self.len());
    debug_assert!(j < self.len());
    let data_ptr = self.as_mut_ptr();
    std::ptr::swap(data_ptr.add(i), data_ptr.add(j));
}

[RalfJung]

@bluss maybe add a debug_assert!(i != j)?

[bluss]

For ptr::swap, equality is ok

[RalfJung]

Ah you are right.

[leonardo-m]

See also:
https://doc.rust-lang.org/nightly/src/core/slice/mod.rs.html#679

[leonardo-m]

Implemented.