feat(wit): add map type support

[yordis]

Related Add a 'map<K,V>' type

[alexcrichton]

Thanks for this! Are you looking to get this merged ASAP? Or is this more of a proof-of-concept to support the development of the PR on the spec itself? If you're looking to get this merged sooner-rather-than-later, do you have plans to work on other parts of the stack such as wit-bindgen or Wasmtime?

[yordis]

@alexcrichton no rush at all in my end. I want to learn how to do the whole thing if possible if that is ok with you

[yordis]

Note for self: 0x63 why is that typed in so many places instead of having a enum somewhere?

[alexcrichton]

Ok looked over things, and thanks for this!

In terms of implementing this, adding a new type is a relatively major endeavor that takes awhile as there are a lot of integration points. Given that we'll want to make sure that this work is properly "gated" from external users to avoid, in theory, accidentally hitting unimplemented parts of the type system. The primary way this is done is via WasmFeatures in the wasmparser crate. What I'd recommend is adding a new cm_map feature similar to cm_fixed_size_lists. Effectively you'll want to add a bunch of tests/checks/etc in the same manner as fixed-size-lists, also an unstable feature at this time.

Additionally you'll want to add tests in tests/cli/component-model/*.wast for this new type as well. You should be able to find tests for fixed-size-lists around there and you can structure tests in a similar way.

Other than that though I think this would be reasonable to land. After landing here would come integration work into wasmtime/wit-bindgen/etc.

[alexcrichton]

Looking good, thanks! The remaining things I can think of are:

• This needs a new feature gate in wasmparser's validation to ensure that this isn't accidentally usable-by-default.
• This needs *.wast tests for the component-model side of things to ensure that the type works in the text format, validation, etc. There should also be tests ensuring that if the feature gate is disabled that usage of the type is an error (you can copy fixed-size-list tests for example)
• Tests are currently not passing in CI, so that'll need to be green