witgen

witgen is a library and a CLI that helps you generate wit definitions in a wit file for WebAssembly. Using this lib in addition to wit-bindgen will help you to import/export types and functions from/to wasm module.

Getting started

Goal: Generate a .wit file writing only Rust.

You will need both the library and the CLI.

Preliminaries

• Create a new library project and move to it.

$ cargo new my_wit
$ cd my_wit

• Add witgen as a dependency in your Cargo.toml. Note: must have cargo-edit installed to add dependencies from CLI, e.g. cargo install cargo-edit.

$ cargo add witgen

• Install cargo witgen CLI.

$ cargo install cargo-witgen

Writing code

• Replace the content of your lib.rs by:

use witgen::witgen;

#[witgen]
struct TestStruct {
    inner: String,
}

#[witgen]
enum TestEnum {
    Unit,
    Number(u64),
    StringVariant(String),
}

#[witgen]
fn test(other: Vec<u8>, test_struct: TestStruct, other_enum: TestEnum) -> Result<(String, i64), String> {
    // The following code is not part of the generated `.wit` file.
    // You may add an example implementation or just satisfy the compiler with a `todo!()`.
    Ok((String::from("test"), 0i64))
}

• Then you can launch the CLI (at the root of your package):

$ cargo witgen generate

• It will generate a witgen.wit file at the root of your package:

record test-struct {
    inner: string
}

variant test-enum {
    unit,
    number(u64),
    string-variant(string),
}

test : function(other: list <u8>, test-struct: test-struct, other-enum: test-enum) -> expected<tuple<string, s64>>

• You can find more complete examples here

Limitations

For now using #[witgen] have some limitations:

• You can use the proc macro #[witgen] only on struct, enum, type alias, function
• Generic parameters or lifetime anotations are not supported
• Type &str is not supported (but you can use String)
• Named struct variants in enum are not already supported (examples enum Test { NamedVariant: { inner: String } } but this one is supported enum Test { UnNamedVariant(String, usize) })
• References, Box, Rc, Arc and all types of smart pointers are not supported
• Methods are not supported
• There is no semantic analysis, which means if your function, struct or enum uses a non scalar type, you have to add #[witgen] where this type is declared (it won't fail at the compile time)

Development

It's a very minimal version, it doesn't already support all kinds of types but the main ones are supported. I made it to easily generate .wit files for my need. Feel free to create issues or pull-requests if you need something. I will be happy to help you!