wasm-pack/docs/src/tutorial/template-deep-dive/wee_alloc.md

wee_alloc

1. What is wee_alloc?
2. Enabling wee_alloc
3. Rust nightly

What is wee_alloc?

Reducing the size of compiled WebAssembly code is important, since it is often transmitted over the Internet or placed on embedded devices.

wee_alloc is a tiny allocator designed for WebAssembly that has a (pre-compression) code-size footprint of only a single kilobyte.

An analysis suggests that over half of the bare minimum WebAssembly memory footprint is required by Rust's default memory allocator. Yet, WebAssembly code often does not require a sophisticated allocator, since it often just requests a couple of large initial allocations.

wee_alloc trades off size for speed. Although it has a tiny code-size footprint, it is relatively slow if additional allocations are needed.

For even more details, see the wee_alloc repository.

Enabling wee_alloc

In lib.rs, we have the configuration for wee_alloc inside a cfg_if! macro:

cfg_if! {
    if #[cfg(feature = "wee_alloc")] {
        extern crate wee_alloc;
        #[global_allocator]
        static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
    }
}

This code block is intended to initialize wee_alloc as the global memory allocator, but only if the wee_alloc feature is enabled in Cargo.toml.

To do so we need to append "wee_alloc" to the default vector in Cargo.toml. Then, the cfg_if! block is replaced with the contents of the if block, shown above.

[features]
default = ["console_error_panic_hook", "wee_alloc"]

Rust nightly

wee_alloc currently relies on features only available in Rust nightly. As such it requires you to use the nightly toolchain for compilation. If you have Rustup set up, you can install the nightly toolchain as follows:

rustup toolchain add nightly

To use wasm-pack with Rust nightly run:

rustup run nightly wasm-pack build