Bevy no_std Compatible Library

This example demonstrates how to create a no_std-compatible library crate for use with Bevy. For the sake of demonstration, this library adds a way for a component to be added to an entity after a certain delay has elapsed. Check the Cargo.toml and lib.rs for details around how this is implemented, and how we're able to make a library compatible for all users in the Bevy community.

Testing no_std Compatibility

To check if your library is no_std compatible, it's not enough to just compile with your std feature disabled. The problem is dependencies can still include std even if the top-most crate is declared as #![no_std]. Instead, you need to compile your library without the standard library at all.

The simplest way to compile Rust code while ensuring std isn't linked is to simply use a target without the standard library. Targets with Tier 2 or Tier 3 support often do not have access to std, and therefore can only compile if no_std compatible.

Some recommended targets you can check against are:

• x86_64-unknown-none
  • Representative of desktop architectures.
  • Should be the most similar to typical std targets so it's a good starting point when porting existing libraries.
• wasm32v1-none
  • Newer WebAssembly target with the bare minimum functionality for broad compatibility.
  • Similar to wasm32-unknown-unknown, which is typically used for web builds.
• thumbv6m-none-eabi
  • Representative of embedded platforms.
  • Has only partial support for atomics, making this target a good indicator for atomic incompatibility in your code.

Note that the first time you attempt to compile for a new target, you will need to install the supporting components via rustup:

rustup target add x86_64-unknown-none

Once installed, you can check your library by specifying the appropriate features and target:

cargo check --no-default-features --features libm,critical-section --target x86_64-unknown-none

CI

Checking no_std compatibility can be tedious and easy to forget if you're not actively using it yourself. To avoid accidentally breaking that compatibility, we recommend adding these checks to your CI pipeline. For example, here is a GitHub Action you could use as a starting point:

jobs:
  check-compiles-no-std:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        target:
          - "x86_64-unknown-none"
          - "wasm32v1-none"
          - "thumbv6m-none-eabi"
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
        with:
          targets: ${{ matrix.target }}
      - name: Check Compile
        run: cargo check --no-default-features --features libm,critical-section --target ${{ matrix.target }}