Rust

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/main.rs 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/main.rs. 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 ...

[lib]
crate-type = ['cdylib']

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


#![allow(unused)]
fn main() {
#[no_mangle]
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.

WebAssembly Interface Types

Note: support for interface types has temporarily removed from Wasmtime. This documentation is somewhat up to date but will no longer work with recent versions of Wasmtime. For more information see https://github.com/bytecodealliance/wasmtime/issues/677

Working with WebAssembly modules at the bare-bones level means that you're only dealing with integers and floats. Many APIs, however, want to work with things like byte arrays, strings, structures, etc. To facilitate these interactions the WebAssembly Interface Types Proposal comes into play. The wasmtime runtime has support for interface types, and the Rust toolchain has library support in a crate called wasm-bindgen.

Note: WebAssembly Interface Types is still a WebAssembly proposal and is under active development. The toolchain may not match the exact specification, and during development you'll generally need to make sure tool versions are all kept up to date to ensure everything aligns right. This'll all smooth over as the proposal stabilizes!

To get started with WebAssembly interface types let's write a library module which will generate a greeting for us. The module itself won't do any printing, we'll simply be working with some strings.

To get starts let's add this to our Cargo.toml:

[lib]
crate-type = ['cdylib']

[dependencies]
wasm-bindgen = "0.2.54"

Using this crate, we can then update our src/lib.rs with the following:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn greet(name: &str) -> String {
    format!("Hello, {}!", name)
}

Then we can build this with:

$ cargo wasi build --release
    Updating crates.io index
...
    Finished dev [unoptimized + debuginfo] target(s) in 9.57s
 Downloading precompiled wasm-bindgen v0.2.54

and we have our new wasm binary!

Note: for now when using wasm-bindgen you must use --release mode to build wasi binaries with interface types.

We can then test out support for this with the CLI:

$ wasmtime --invoke greet ./target/wasm32-wasi/release/hello_world.wasm "Wasmtime CLI"
warning: using `--invoke` with a function that takes arguments is experimental and may break in the future
warning: using `--invoke` with a function that returns values is experimental and may break in the future
Hello, Wasmtime CLI!

Here we can see some experimental warnings, but we got our error message printed out! The first CLI parameter, "Wasmtime CLI", was passed as the first argument of the greet function. The resulting string was then printed out to the console.

Like before, we can also execute this with Python:

$ cp target/wasm32-wasi/release/hello_world.wasm .
$ python3
>>> import wasmtime
>>> import hello_world
>>> hello_world.greet('python interpreter')
'Hello, python interpreter!'
>>>

Note that wasm-bindgen was originally developed for JS and usage in a browser, but a subset of its implementation (such as arguments which are strings) are supported for WebAssembly interface types. You can also check out the reference documentation for wasm-bindgen for more information about how it works. Note that the wasm-bindgen support for wasm interface type is still in its nascent phase and is likely to be greatly improved in the future.

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, but if you're using #[wasm_bindgen] that will happen automatically for you.

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:


#![allow(unused)]
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();
}
}

When you're using wasm-bindgen you would instead use:

use wasm_bindgen::prelude::*;

#[wasm_bindgen(module = "the-wasm-import-module")]
extern "C" {
    fn foo();
    fn baz();
    // ...
}

Note that unless you're using interface types you likely don't need wasm-bindgen.