reinterpret

Low level utility functions to reinterpret arrays of data in Rust

Overview

This crate provides convenient low-level utility functions for reinterpreting data. This includes Vecs and slices. These functions are intrinsically unsafe but are very useful in performance critical code since they avoid additional copies.

The goal of this crate is to provide some memory safety along the boundaries of Vecs and slices to reduce the boilerplate for reinterpreting data.

These functions check that the source and target arrays have the same size, however they don't check for safety of converting the contained types.

It is possible to write safe wrappers for converting collections of concrete types using these functions, however this is out of the scope of this crate.

Examples

use reinterpret::*;
let points: Vec<[f64;2]> = vec![
    [0.1, 1.0],
    [1.2, 1.4],
    [0.5, 3.2],
];
let coordinates: Vec<f64> = vec![0.1, 1.0, 1.2, 1.4, 0.5, 3.2];

let point_coordinates: &[f64] = unsafe { reinterpret_slice(&points) };
assert_eq!(*point_coordinates, *coordinates.as_slice()); // Same data.
assert_eq!(point_coordinates, coordinates.as_slice()); // Same location in memory.

let coordinate_points: &[[f64;2]] = unsafe { reinterpret_slice(&coordinates) };
assert_eq!(*coordinate_points, *points.as_slice()); // Same data.
assert_eq!(coordinate_points, points.as_slice()); // Same location in memory.

Panics

Panics can occur in surprising circumstances when converting Vecs. The rule of thumb is to ensure that the element size of the target Vec is smaller than or equal to the source element size.

To illustrate this consider converting between a Vec of u8 and a Vec of u16. The following code will not cause panics for any size of the source Vec.

use reinterpret::*;
let vec_u16 = vec![1u16,2,3,4,5];
let vec_u8: Vec<u8> = unsafe { reinterpret_vec(vec_u16) };

However converting to a Vec<[u8; 3]> can cause a panic if the size of the source capacity cannot be divided into the number of bytes of a [u8; 3]. For example the following code will panic because the capacity of vec_u8 after adding an additional 4 bytes becomes 8, which is not divisible by 3 and so the resulting Vec cannot be converted to a Vec<[u8; 3]>.

vec_u8.extend(vec![1,2,3,4]);
let vec_u16_round_trip: Vec<[u8; 3]> = unsafe { reinterpret_vec(vec_u8) };

There is no way of ensuring that this type of conversion will succeed, however we can try to improve the situation by calling shrink_to_fit on the source Vec and hope that this will produce a capacity equal to the length of the Vec as follows:

vec_u8.extend(vec![1,2,3,4]);
vec_u8.shrink_to_fit();
let vec_u16_round_trip: Vec<[u8; 3]> = unsafe { reinterpret_vec(vec_u8) };

This approach relies on the specifics of Vec implementations and is heavily discouraged in library code.

Undefined Behavior

There are ways to misuse these functions without causing panics that may produce undefined behavior. For instance:

use reinterpret::*;
let a = 1;
let b = 2;
let v = vec![&a, &b];
let mut m: Vec<&mut usize> = unsafe { reinterpret_vec(v) };
*m[0] = 100; // Mutating an immutable variable a!

assert_eq!(a, 100);

It is the users' responsibility to avoid these types of scenarios.

License

This repository is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT License (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.