#swap #references

no-std omniswap

swap values between possibly-overlapping references

1 unstable release

Uses new Rust 2021

0.1.0 Aug 20, 2022

#696 in Rust patterns

MIT/Apache

14KB
163 lines

omniswap: a crate to swap values between possibly-overlapping references

Motivating Example

You cannot simply use std::mem::swap to replace values within an array:

let mut a = [1, 2, 3];
// You cannot prove their disjointness!
std::mem::swap(&mut a[0], &mut a[2]);

You get the following message:

error[E0499]: cannot borrow `a[_]` as mutable more than once at a time
 --> src/main.rs:4:31
  |
4 |     std::mem::swap(&mut a[0], &mut a[2]);
  |     -------------- ---------  ^^^^^^^^^ second mutable borrow occurs here
  |     |              |
  |     |              first mutable borrow occurs here
  |     first borrow later used by call
  |
  = help: consider using `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices

You can use the dedicated <[T]>::swap instead:

let mut a = [1, 2, 3];
a.swap(0, 2);

But how about two-dimensional arrays?

let mut a = [[1, 2], [3, 4]];
// You cannot prove their disjointness!
std::mem::swap(&mut a[0][0], &mut a[1][1]);

This is not as simple as the first one.

Solution

This crate solves the problem by providing a generic framework for sentinel-based swapping.

The idea is simple: it leaves a dummy value behind to safely move values around:

let mut a = [[1, 2], [3, 4]];
let tmp = std::mem::replace(&mut a[0][0], 0);
let tmp = std::mem::replace(&mut a[1][1], tmp);
a[0][0] = tmp;
# assert_eq!(a, [[4, 2], [3, 1]]);

However, in Rust, the best sentinel value differs between types.

The macro swap! automatically chooses the best sentinel and provides the same interface as std::mem::swap:

let mut a = [[1, 2], [3, 4]];
omniswap::swap!(&mut a[0][0], &mut a[1][1]);
# assert_eq!(a, [[4, 2], [3, 1]]);

Usage

Simply use swap! where you want to use std::mem::swap:

let mut x = 42;
let mut y = 84;
omniswap::swap!(&mut x, &mut y);

See swap! for detailed usages.

Other APIs

The crate provides the following variants:

  • rotate! -- swaps more than two values at once

The crate also exposes take! and Replace. These are primitives used in swap! and rotate!.

No runtime deps