getrandom

getrandom is a popular crate that allows retrieving random data from system resources. It is used by bevy and many other crates. It provides multiple backends which retrieve the random data from different sources.

This works great, except when targeting the web. Unlike most other platforms, there is no compilation target available that guarantees web APIs to exist. Just because you're building for wasm32-unknown-unknown doesn't necessarily mean you are making a web app -- you could also be building a standalone Wasm application.

So the target cannot be used to automatically activate the web backend. Features are also inadequate: They are additive, so if any dependency pulled in the web feature, the backend would be used everywhere or the build would be broken. Considering the security-sensitive nature of random data, this was deemed unacceptable.

So in addition to a feature to make the backend available, getrandom requires you to pass a RUSTFLAG to the compiler to activate the backend. This guarantees that the backend can only be configured once (by "outer" package).

Configuring the web backend

Since you are the application developer, you know that you are building for the web and not just any Wasm target. This allows you to set up the getrandom backend properly, for example like this:

[target.'cfg(all(target_family = "wasm", any(target_os = "unknown", target_os = "none")))'.dependencies]
getrandom = { version = "0.3", features = ["wasm_js"] }
getrandom_02 = { version = "0.2", features = ["js"], package = "getrandom" }

This activates the necessary feature flags for getrandom, accounting for both v0.2 and v0.3 (as they can both be in the dependency tree, depending on the Bevy version used).

Additionally, you need to add --cfg getrandom_backend="wasm_js" to your RUSTFLAGS. This can be done in several places:

• Setting rustflags in .cargo/config.toml
• Setting build.rustflags in Cargo.toml
• Setting the RUSTFLAGS env variable when running cargo

However, note that the rustflags are not merged, but overwritten. So if you have e.g. rustflags defined in your ~/.cargo/config.toml to optimize your compile times, but then have again rustflags defined in your workspace to set the getrandom backend, only the workspace rustflags will be used.

Automated by the CLI

The Bevy CLI automatically sets the RUSTFLAGS env variable to configure the web getrandom backend when bevy build web or bevy run web is used and you haven't configured the backend yourself. This allows more projects to work out-of-the-box for the web and simplifies the configuration.

You can still explicitly define the backend if you wish (or need to).

The feature configuration is not done automatically, but the CLI will give you a snippet to set it up when needed.