Developing the split-str 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 split.wit file we created earlier. Copy it into this directory if it is not already there.

Implementing and Compiling

Much of this will be similar to the techniques we used in the power-of example.

Edit the Cargo.toml file so it looks like this:

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

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

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

And, for the implementation, edit the src/lib.rs file and replace its contents with this:

wit_bindgen_rust::export!("split.wit");
struct Split;
use crate::split::Subphrase;

impl split::Split for Split {

    fn split_str(phrase: String, delim: String) -> Vec<Subphrase> {
        phrase
            .split(&delim)
            .scan(0, |idx, s| {
                let current = Subphrase {
                    str: s.to_string(),
                    idx: *idx as i32
                };
                *idx += (s.0 + delim.0) as i32;
                Some(current)
            })
            .collect()
    }
}

Fortunately, with Rust, we have some nice language features that help make our code concise. Unlike in C/C++, we don’t need to explicitly free the parameters. Due to Rust’s move semantics, they will be freed implicitly when the function ends. Allocation for the output vector is also managed “under the hood” by Rust’s robust data structures and runtime. Finally, the declaration of the Subphrase struct happens via the wit_bindgen_rust::export macro, so we don’t need to do it.

Let’s compile the Wasm module now:

cargo wasi build --lib

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

Next, we'll do some testing.