Hello, World!

Let's see an example of how to run the WASI version of "Hello, World!". This'll end up looking very familiar to the Rust version of "Hello, World!" as well. First up let's create a new project with Cargo:

$ cargo new wasi-hello-world
     Created binary (application) `wasi-hello-world` package
$ cd wasi-hello-world

This creates a wasi-hello-world folder which has a default Cargo.toml and src/main.rs. The main.rs is the entry point for our program and currently contains println!("Hello, world!");. Everything should be set up for us to execute (no code needed!) so let's run the code inside of the wasi-hello-world directory:

$ cargo wasi run
error: failed to find `wasmtime` in $PATH, you'll want to install `wasmtime` before running this command

Oh dear, that failed very quickly! For this command though we need to have a way to actually execute the WebAssembly binary that Rust will produce. The cargo wasi subcommand by default supports wasmtime, and the error message should have instructions of how to install wasmtime. You can also view installation instructions on the wasmtime website.

Once we've got wasmtime installed, make sure it's working via:

$ wasmtime --version

Note that you may have to open a new shell for this to ensure PATH changes take effect.

Ok, now that we've got a runtime installed, let's retry executing our binary:

$ cargo wasi run
info: downloading component 'rust-std' for 'wasm32-wasi'
info: installing component 'rust-std' for 'wasm32-wasi'
   Compiling wasi-hello-world v0.1.0 (/code/wasi-hello-world)
    Finished dev [unoptimized + debuginfo] target(s) in 0.15s
     Running `/.cargo/bin/cargo-wasi target/wasm32-wasi/debug/wasi-hello-world.wasm`
     Running `target/wasm32-wasi/debug/wasi-hello-world.wasm`
Hello, world!

Success! The command first used rustup to install the Rust wasm32-wasi target automatically, and then we executed cargo to build the WebAssembly binary. Finally wasmtime was used and we can see that Hello, world! was printed by our program.

After this we're off to the races in developing our crate. Be sure to check out the rest of this book for more information about what you can do with cargo wasi. Additionally if this is your first time using Cargo, be sure to check out Cargo's introductory documentation as well