Providing the way to express wasm(time) types/signatures in Rust code

[yurydelendik]

The problem: the have multiple ways to express signature metadata needed by wasmtime attached to a Rust function. There are several ways to define WASI interface in wasi-common and other project. This patch will try to describe and combine all macros that are needed to connect a native Rust function with its wasmtime cranelift_codegen::ir::Signature. (If wrapper/proxy exists, a native function arguments are expected to be converted automatically to corresponding cranelift_codegen::ir::Type)

TODOs:

• convert WASI signature/syscall as example

[yurydelendik]

Docs:

#[wasmtime_method(module(func1_desc))]
fn func1(a: u8) -> u8 { a + 1 }

will generate/create a function with wasmtime-compatible ABI that looks like, and converts parameters to wasm-friendly types:

extern "C" fn func1(vmctx: *mut VMContext, arg1: u32) -> u32 {
  let c = |a:u8| -> u8 { a + 1 };
  c(arg1 as u8) as u32
}
mod func1_desc { ... }

The func1_desc will have signature() -> ir::Signature and Wrapper class to map function signature to the wasmtime_runtime export.

TODO:

• add macros to hide the Wrapper implementation details

Also,

#[wasmtime_trait(module(trait1_desc))]
trait Trait1 {
  fn func2(a: u8) -> u8;
}

produces:

trait Trait1 { .. }
mod trait1_desc {
  fn get_self(vmctx: *mut VMContext) -> dyn Trait1 { ... }
  extern "C" fn func2(vmctx: *mut VMContext, arg1: u32) -> u32 {
     ...
     get_self(vmctx).func2(arg1 as u8) as u32
  }
}

And similar to function the Wrapper struct will be generated to map InstanceHandle exports to trait.

TODO:

• macro to wrap implementation details on how to wrap InstanceHandle
• more docs and examples (please find "wasmtime-api/examples/hello2.rs" for now and use cargo expand)

[yurydelendik]

hello2.rs and hello3.rs expanded https://gist.github.com/yurydelendik/2c1bd0fa76642712203f9ece3680a104

TODO:

• Why function wrapper has InstanceHandle?
• Instead of dyn Trait try direct implementation call (annotate impl?) -- instead of (https://gist.github.com/yurydelendik/2c1bd0fa76642712203f9ece3680a104#file-hello3-expanded-rs-L133)

[yurydelendik]

* Why function wrapper has InstanceHandle?

The vmctx field is needed when context() is provided #[wasmtime_method(...)]. The context object is connected with instance (e.g. to get global_exports) rather than CL methods vmctx.

[yurydelendik]

added hello4.rs with annotated impl (the gist above has expanded code)

[steveej]

I filed a PR to your PR branch 😄 It adds basic support for a visibility modifier: yurydelendik#1

[yurydelendik]

The zero shims PR addresses the same issue. WasmTy provides flexibility to access callers context. Closing this PR as a duplicate.