Wasm-bindgen: How do you use a Rust struct with a `String` field?

[garrettmaring]

Summary

How do you use a Rust struct with a String field using wasm-bindgen?

The String type seems to be supported for function parameters and return values. https://rustwasm.github.io/docs/wasm-bindgen/reference/types/string.html

Additional Details

#[wasm_bindgen]
pub struct Data {
    id: String,
}

[alexcrichton]

Thanks for the report! For this you'll want to use getters and setters, and that shoul dod the trick!

[garrettmaring]

That worked! Thank you.

[andimarek]

Hi @garrettmaring can you share some details how exactly you solved it with getters and setters? thanks

[garrettmaring]

Sure!

// doesn't work...
#[wasm_bindgen]
struct Data {
    pub id: String,
}

You'll get the error error[E0277]: the trait bound std::string::String: std::marker::Copy is not satisfied.

Since, the String type in Rust isn't implicitly copyable. I had to read up on the difference between Copy and Clone to understand that I couldn't just implement Copy but rather needed to use .clone() to explicitly copy it.

Thankfully, _wasm-bindgen_ gives us a simple way to do it.

#[wasm_bindgen]
struct Data {
    id: String, // ensure that the field is private
}

#[wasm_bindgen]
impl Data {
    #[wasm_bindgen(getter)]
    pub fn id(&self) -> String {
        self.id.clone()
    }

    #[wasm_bindgen(setter)]
    pub fn set_id(&mut self, id: String) {
        self.id = id;
    }
}

There are some interesting things that you can do with getters and setters that are documented here.

[garrettmaring]

@alexcrichton would it be feasible for _wasm-bindgen_ to generate this code if a struct implements Clone?

[alexcrichton]

It's plausible, yeah! It's something though we've avoided doing historically because a Clone implementation can often be accidentally quite expensive, so we tend to prefer to request that users do so manually to ensure they know the cost they're opt-ing into

[alexcrichton]

Now that being said, it'd be a neat feature to do something like #[wasm_bindgen(getter_setter_with_clone)] or something like that so the boilerplate could be drastically reduced