#core #extensions #corelib

no-std r-ex

Zero-bloat Rust core library extensions

2 stable releases

Uses new Rust 2021

1.0.1 Nov 7, 2022
1.0.0 Nov 6, 2022

#368 in Rust patterns

Download history 10/week @ 2022-10-31 65/week @ 2022-11-07 22/week @ 2022-11-14 11/week @ 2022-11-21

108 downloads per month
Used in 2 crates

MIT license

294 lines


Github Crates.io Codacy grade Crates.io

Zero-bloat Rust core library extensions that improve the experience of using the Rust's type system in everyday tasks.


  • Zero bloat. Each extension sits behind its own feature flag, and no features are enabled by default.

  • Zero risk. Any feature that may panic or uses unsafe code has -unsafe added to its feature name.

  • Zero feature interdependencies. Each extension is standalone, and compatible with all the others, so that they can be freely mixed and matched.

  • Zero external dependencies.

  • No-std. This project focuses exclusively on the core library. Would you like to see a similar project for the standard library? Let me know.

  • Stable Rust only.

  • 100% code coverage.


Copy between arrays

With r-ex (code never panics):

let mut buf = [0u8; 4];

    .carved_mut()?          // automatically select range
    .copy_from(&[1, 2]);    // type-safe & size-safe copy never panics
                            // buf = [1, 2, 0, 0]

Without r-ex (can panic):

let mut buf = [0u8; 4];

    .get_mut(0..2)?             // explicitly specify range
    .copy_from_slice(&[1, 2]);  // copy_from_slice panics if sizes mismatch
                                // buf = [1, 2, 0, 0]

Write a big-endian value to a buffer at offset

With r-ex:

fn write_big_endian(dest: &mut [u8], offset: usize, val: u32) {
    dest.carve_mut(offset)? // automatically select range
        .set_be(val);       // supports big-endian and little-endian values; never panics

Without r-ex:

fn write_big_endian(dest: &mut [u8], offset: usize, val: u32) {
    dest.get_mut(offset..offset + core::mem::size_of_val(&val))?    // explicitly specify range
        .copy_from_slice(&val.to_be_bytes())                        // copy_from_slice can panic


The Rust core library is awesome. It's clear, concise, non-bloated. This is what we like it for, and we'd all like to keep it this way. And yet, we often find ourselves writing utils or misc modules with a handful of small, general-purpose extensions to the core library. Sometime these few dozen lines make for the most genius, powerful, or useful code in the whole project. To think that these moments of brilliance and ingenuity sit buried down in some internal module, unexposed to the public, is saddening.

This is where the r-ex project comes in. It is home to these little but precious core library extensions that would otherwise remain unnoticed.

Contributor guidelines

Pull requests are welcome. Please make sure your contribution adheres to the Principles section above.

No runtime deps


  • carve
  • copy-from
  • endian
  • from-arr
  • from-ref
  • splice