Fix exact import in custom descriptors

[yurydelendik]

Allows exact function imports such as:

 (import "" "" (func (exact (type $f))))
 (import "" "" (func (exact (type $f) (param) (result))))

The patch adds FuncExact to external kind and uses heap type reference (instead of index) in the entity function references field. I tried to match changes to the overview explanations in the Exact Function Imports sections.

I need help with:

• component changes: is it make sense to have the similar change there too?
• wasm-smith or wasm-mutate changes

[alexcrichton]

I'm a bit wary to match the spec exactly here in AST-representation where I would predict that idiomatically in Rust it'd be nicer to have Thing::{Func,FuncExact}(u32) as opposed to Thing::Func(ThingType { id: u32, exact: bool }). Did you try that already though and find that it was unworkable? If not, mind trying that out?

As for components/wasm-smith/wasm-encoder, where possible it's nice to keep things working but if major surgery or refactorings are required feel free to return an error and/or panic in those situations. I can take a look at any particular location in review and see if anything can't be semi-easily-handled or if it needs to do something else.

[yurydelendik]

in Rust it'd be nicer to have Thing::{Func,FuncExact}(u32) as opposed to Thing::Func(ThingType { id: u32, exact: bool }). Did you try that already though and find that it was unworkable? If not, mind trying that out?

Yep, I can try that.

[alexcrichton]

Nice that looks like it worked out well to me at least, with green CI should be good to go 👍

[alexcrichton]

Judging by WebAssembly/testsuite#146 it looks like some instruction names may be getting an update too? (fine to land after this of course)

[yurydelendik]

Judging by WebAssembly/testsuite#146 it looks like some instruction names may be getting an update too? (fine to land after this of course)

Yes, this is my next item. The newest update to the proposal will rollback some code around struct.new validation and adds struct.new_desc