Developing the power-of example in Rust

Initialize Your Source Tree

To setup our project, we'll need to do a few things:

1. Ensure that you are in a new directory. It can be called anything you want.

2. In the work directory, run the following to set up a skeltal Rust source tree:

   cargo init --vcs none --lib
   

3. Now, create a special configuration file to tell utilities like the rust-analyzer that the default build target of this project is Wasm. Run this command:

   mkdir .cargo && echo -e "[build]\ntarget = \"wasm32-wasi\"\n" > .cargo/config.toml   
   

Copy the WIT File

We’ll need the power.wit file we created earlier. Copy it into this directory if it is not already there.

Implementing and Compiling

Now, edit the file called Cargo.toml so that it looks like the following:

[package]
name = "power"
version = "0.1.0"
edition = "2018"

[dependencies]
wit-bindgen-rust = { git = "https://github.com/bytecodealliance/wit-bindgen.git", rev = "60e3c5b41e616fee239304d92128e117dd9be0a7" }

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

Now we’re almost ready to roll. Edit the file src/lib.rs and replace its content with this:

wit_bindgen_rust::export!("power.wit");
struct Power;

impl power::Power for Power {
    fn power_of(base: i32, exp: i32) -> i32 {
        let mut res = 1;
        for _i in 0..exp {
            res *= base;
        }
        res
    }
}

The syntax at the top of the code is boiler-plate. The export macro generates code that declares a trait named after our WIT file (and some other things). So, our main job is here is to implement this trait. If you are curious what the macro actually generates, you can run cargo expand and it will show you the fully expanded source code.

The WIT IDL is heavily inspired by the Rust language syntax, so it was pretty easy to derive the Rust function signature we needed from the IDL:

• Replaced hyphens with underscores

• Changed s32 types to i32

Now we can compile the program into a wasm module using this command:

cargo wasi build --lib

The new Wasm file should be written to target/wasm32-wasi/debug/power.wasm.

Next, we'll do some testing.