1. What is wee_alloc?
2. Enabling wee_alloc

WebAssembly code is frequently transmitted over the wire to users, so compiled code size is often important to ensure an application loads quickly and is responsive.

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. It has a tiny code-size footprint, but it is not competitive in terms of performance with the default global allocator, for example.

For even more details, see the wee_alloc repository, or general documentation about shrinking code size of WebAssembly binaries.

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

# #![allow(unused_variables)]
#fn main() {
cfg_if! {
    if #[cfg(feature = "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 at compile time. The feature can be enabled by passing extra options while building:

$ wasm-pack build --features wee_alloc

or alternatively you could turn it on by default in Cargo.toml:

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