Rust Type conversions

Previously we've been seeing mostly abridged versions of type conversions when values enter Rust. Here we'll go into some more depth about how this is implemented. There are two categories of traits for converting values, traits for converting values from Rust to JS and traits for the other way around.

From Rust to JS

First up let's take a look at going from Rust to JS:


# #![allow(unused_variables)]
#fn main() {
pub trait IntoWasmAbi: WasmDescribe {
    type Abi: WasmAbi;
    fn into_abi(self, extra: &mut Stack) -> Self::Abi;
}
#}

And that's it! This is actually the only trait needed currently for translating a Rust value to a JS one. There's a few points here:

  • We'll get to WasmDescribe later in this section
  • The associated type Abi is what will actually be generated as an argument to the wasm export. The bound WasmAbi is only implemented for types like u32 and f64, those which can be placed on the boundary and transmitted losslessly.
  • And finally we have the into_abi function, returning the Abi associated type which will be actually passed to JS. There's also this Stack parameter, however. Not all Rust values can be communicated in 32 bits to the Stack parameter allows transmitting more data, explained in a moment.

This trait is implemented for all types that can be converted to JS and is unconditionally used during codegen. For example you'll often see IntoWasmAbi for Foo but also IntoWasmAbi for &'a Foo.

The IntoWasmAbi trait is used in two locations. First it's used to convert return values of Rust exported functions to JS. Second it's used to convert the Rust arguments of JS functions imported to Rust.

From JS to Rust

Unfortunately the opposite direction from above, going from JS to Rust, is a bit more complicated. Here we've got three traits:


# #![allow(unused_variables)]
#fn main() {
pub trait FromWasmAbi: WasmDescribe {
    type Abi: WasmAbi;
    unsafe fn from_abi(js: Self::Abi, extra: &mut Stack) -> Self;
}

pub trait RefFromWasmAbi: WasmDescribe {
    type Abi: WasmAbi;
    type Anchor: Deref<Target=Self>;
    unsafe fn ref_from_abi(js: Self::Abi, extra: &mut Stack) -> Self::Anchor;
}

pub trait RefMutFromWasmAbi: WasmDescribe {
    type Abi: WasmAbi;
    type Anchor: DerefMut<Target=Self>;
    unsafe fn ref_mut_from_abi(js: Self::Abi, extra: &mut Stack) -> Self::Anchor;
}
#}

The FromWasmAbi is relatively straightforward, basically the opposite of IntoWasmAbi. It takes the ABI argument (typically the same as IntoWasmAbi::Abi) and then the auxiliary stack to produce an instance of Self. This trait is implemented primarily for types that don't have internal lifetimes or are references.

The latter two traits here are mostly the same, and are intended for generating references (both shared and mutable references). They look almost the same as FromWasmAbi except that they return an Anchor type which implements a Deref trait rather than Self.

The Ref* traits allow having arguments in functions that are references rather than bare types, for example &str, &JsValue, or &[u8]. The Anchor here is required to ensure that the lifetimes don't persist beyond one function call and remain anonymous.

The From* family of traits are used for converting the Rust arguments in Rust exported functions to JS. They are also used for the return value in JS functions imported into Rust.

Global stack

Mentioned above not all Rust types will fit within 32 bits. While we can communicate an f64 we don't necessarily have the ability to use all the bits. Types like &str need to communicate two items, a pointer and a length (64 bits). Other types like &Closure<Fn()> have even more information to transmit.

As a result we need a method of communicating more data through the signatures of functions. While we could add more arguments this is somewhat difficult to do in the world of closures where code generation isn't quite as dynamic as a procedural macro. Consequently a "global stack" is used to transmit extra data for a function call.

The global stack is a fixed-sized static allocation in the wasm module. This stack is temporary scratch space for any one function call from either JS to Rust or Rust ot JS. Both Rust and the JS shim generated have pointers to this global stack and will read/write information from it.

Using this scheme whenever we want to pass &str from JS to Rust we can pass the pointer as the actual ABI argument and the length is then placed in the next spot on the global stack.

The Stack argument to the conversion traits above looks like:


# #![allow(unused_variables)]
#fn main() {
pub trait Stack {
    fn push(&mut self, bits: u32);
    fn pop(&mut self) -> u32;
}
#}

A trait is used here to facilitate testing but typically the calls don't end up being virtually dispatched at runtime.