We can generate non-whitelisted structs using opaque items

[emilio]

In #452 I'm removing an assertion that was hitting in stylo, concretely with the BaseErrorResult typedef.

This codegen was being called from CompInfo::codegen as expected (it's an inner typedef). The above item was clearly whitelisted (it'd have hit before otherwise).

Upon reflection, ErrorResult is marked as opaque, so we immediately stop tracing it (never whitelist the inner item).

When we arrive to generate code for ErrorResult, we generate opaque structs, but we still generate the inner types for it (which is desirable IMO).

What should we do here @fitzgen? Tracing across opaque stuff doesn't seem desirable, but neither does not generating sub-items for opaque items. Presumably we could change the assertion to something like "is whitelisted or any parent is opaque", or something like that.

[fitzgen]

If we are going to generate inner types regardless if the parent is opaque or not, then the parent's TypeCollector impl should trace inner types (but not everything else, such as fields and methods), regardless whether it is opaque or not.

[amanjeev]

Triage comment: @emilio @fitzgen is this issue still relevant? I cannot find TypeCollector in the codebase anymore.