The Rust Programming Language supports WebAssembly as a compilation target. If you're not familiar with Rust it's recommended to start with its introductory documentation. Compiling to WebAssembly will involve specifying the desired target via the --target flag, and to do this there are a number of "target triples" for WebAssembly compilation in Rust:

  • wasm32-wasi - when using wasmtime this is likely what you'll be using. The WASI target is integrated into the standard library and is intended on producing standalone binaries.
  • wasm32-unknown-unknown - this target, like the WASI one, is focused on producing single *.wasm binaries. The standard library, however, is largely stubbed out since the "unknown" part of the target means libstd can't assume anything. This means that while binaries will likely work in wasmtime, common conveniences like println! or panic! won't work.
  • wasm32-unknown-emscripten - this target is intended to work in a web browser and produces a *.wasm file coupled with a *.js file, and it is not compatible with wasmtime.

For the rest of this documentation we'll assume that you're using the wasm32-wasi target for compiling Rust code and executing inside of wasmtime.

Hello, World!

Cross-compiling to WebAssembly involves a number of knobs that need configuration, but you can often gloss over these internal details by using build tooling intended for the WASI target. For example we can start out writing a WebAssembly binary with cargo wasi.

First up we'll install cargo wasi:

$ cargo install cargo-wasi

Next we'll make a new Cargo project:

$ cargo new hello-world
$ cd hello-world

Inside of src/ you'll see the canonical Rust "Hello, World!" using println!. We'll be executing this for the wasm32-wasi target, so you'll want to make sure you're previously built wasmtime and inserted it into PATH;

$ cargo wasi run
info: downloading component 'rust-std' for 'wasm32-wasi'
info: installing component 'rust-std' for 'wasm32-wasi'
   Compiling hello-world v0.1.0 (/hello-world)
    Finished dev [unoptimized + debuginfo] target(s) in 0.16s
     Running `/.cargo/bin/cargo-wasi target/wasm32-wasi/debug/hello-world.wasm`
     Running `target/wasm32-wasi/debug/hello-world.wasm`
Hello, world!

And we're already running our first WebAssembly code inside of wasmtime!

While it's automatically happening for you as part of cargo wasi, you can also run wasmtime yourself:

$ wasmtime target/wasm32-wasi/debug/hello-world.wasm
Hello, world!

You can check out the introductory documentation of cargo-wasi as well for some more information.

Writing Libraries

Previously for "Hello, World!" we created a binary project which used src/ Not all *.wasm binaries are intended to be executed like commands, though. Some are intended to be loaded into applications and called through various APIs, acting more like libraries. For this use case you'll want to add this to Cargo.toml:

# in Cargo.toml ...

crate-type = ['cdylib']

and afterwards you'll want to write your code in src/ like so:

fn main() {
pub extern "C" fn print_hello() {
    println!("Hello, world!");

When you execute cargo wasi build that'll generate a *.wasm file which has one exported function, print_hello. We can then run it via the CLI like so:

$ cargo wasi build
   Compiling hello-world v0.1.0 (/home/alex/code/hello-world)
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
$ wasmtime --invoke print_hello target/wasm32-wasi/debug/hello_world.wasm
Hello, world!

As a library crate one of your primary consumers may be other languages as well. You'll want to consult the section of this book for using wasmtime from Python and after running through the basics there you can execute our file in Python:

$ cp target/wasm32-wasi/debug/hello_world.wasm .
$ python3
>>> import wasmtime
>>> import hello_world
>>> hello_world.print_hello()
Hello, world!

Note that this form of using #[no_mangle] Rust functions is pretty primitive. You're only able to work with primitive datatypes like integers and floats. While this works for some applications if you need to work with richer types like strings or structs, then you'll want to use the support in wasmtime for interface types.

Exporting Rust functionality

Currently only Rust functions can be exported from a wasm module. Rust functions must be #[no_mangle] to show up in the final binary.

Memory is by default exported from Rust modules under the name memory. This can be tweaked with the -Clink-arg flag to rustc to pass flags to LLD, the WebAssembly code linker.

Tables cannot be imported at this time. When using rustc directly there is no support for anyref and only one function table is supported. When using wasm-bindgen it may inject an anyref table if necessary, but this table is an internal detail and is not exported. The function table can be exported by passing the --export-table argument to LLD (via -C link-arg) or can be imported with the --import-table.

Rust currently does not have support for exporting or importing custom global values.

Importing host functionality

Only functions can be imported in Rust at this time, and they can be imported via raw interfaces like:

fn main() {
struct MyStruct;
#[link(wasm_import_module = "the-wasm-import-module")]
extern "C" {
    // imports the name `foo` from `the-wasm-import-module`
    fn foo();

    // functions can have integer/float arguments/return values
    fn translate(a: i32) -> f32;

    // Note that the ABI of Rust and wasm is somewhat in flux, so while this
    // works, it's recommended to rely on raw integer/float values where
    // possible.
    fn translate_fancy(my_struct: MyStruct) -> u32;

    // you can also explicitly specify the name to import, this imports `bar`
    // instead of `baz` from `the-wasm-import-module`.
    #[link_name = "bar"]
    fn baz();