Panic on instantiation with resource with implicit use dependency

[rylev]

This is a bit complicated of a repro, but here's a failing test to follow along with: [repro].

Take the following package that we want to instantiate:

package foo:baz;

world my-world {
    use foo:dependency/types.{x};
    import my-func: func() -> x;
    import foo:dependency/types;
}

And here's the wit for the foo:dependency package:

package foo:dependency;

interface types {
    resource x;
}

world dependency-world {
    export types;
}

If when instantiation my-world, the foo:dependency/types and my-func imports are satisfied directly, a panic happens here:

wac/crates/wac-graph/src/encoding.rs

Line 711 in ac04147

instance: state.current.instances[iid],

Presumably, this happens because the my-func import still has a dependency on foo:dependency/types (because of the use of resource x) but this dependency gets lost along the way.

This is no longer error when we no longer explicitly provide the foo:dependency/types import when instantiating my-world. Then the instantiation some how knows that it depends on foo:dependency/types and it needs to be imported.

I imagine what's happening here is that the explicit import is assumed to satisfy my-func's dependency on foo:dependency/types but this assumption (which seems to be written as a comment here) is violated.

[rylev]

Doing some further investigation, a fix for this particular example becomes apparent, but I'm not 100% sure if the fix is correct in general.

The issue is that when the aggregator remaps resources, it does not keep track of whether the resource's owning interface needs to be implicitly imported or not. The tracking for implicitly imported types is kept in the names collection. After remapping a type, it is put into the names collection, and those imports are then encoded in wac_graph as imports.

However, resources can have a dependency on an interface which needs to also be implicitly imported if it's not otherwise imported. One possible solution then would be to make sure that if there is an owning interface, that that interface is also in the names collection as well.

The naive way to do this is by changing the mapping of owner interfaces seen here:

wac/crates/wac-types/src/aggregator.rs

Lines 552 to 558 in ac04147

	Ok(ResourceAlias {
	owner: a
	.owner
	.map(|id| self.remap_interface(types, id, checker))
	.transpose()?,
	source: self.remap_resource(types, a.source, checker)?,
	})

To something like this:

let owner = a
  .owner
  .map(|id| self.remap_interface(types, id, checker))
  .transpose()?;
if let Some(owner) = owner {
  let name = self.types()[owner].id.clone().expect("interface has no id");
  self.names.insert(name, ItemKind::Instance(owner));
}
Ok(ResourceAlias {
  owner,
  source: self.remap_resource(types, a.source, checker)?,
})