Index Struct.new

[vinistock]

Another step for #392

This PR starts handling Struct.new, creating all definitions related to it. There are a few things regarding behaviour that's interesting:

• If the first argument is a capitalized string, then Ruby creates an alias for the constant under the Struct namespace

Foo = Struct.new("Bar")
Foo             # => Foo
Struct::Bar # => Foo

• All other arguments become both an attribute accessor and an instance variable
• An initialize method is always created (even if there are no fields)
• The keyword_init parameter changes the signature of initialize to use keyword optional arguments instead of positional optional arguments
• All structs automatically inherit from Struct

Note: there might be overlap with the handling for Data.define, but I didn't want to make this PR too large. I'll refactor if there's an opportunity for it

[vinistock]

• Index Struct.new #485 👈 (View in Graphite)
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[Morriar]

Can't we leave this in handle_class_definition since it's the same in both cases?

[vinistock]

It's not always the same. In visit_class_node, this pushes a Nesting::LexicalScope.

[Morriar]

It looks like we're again trying to do some desugaring/rewriting by introducing a synthetic constant alias. Shouldn't we create a Struct definition instead and create the proper declaration in the resolver?

I think it's another symptom of #471 (review) we're trying to work around.

[vinistock]

I put a topic for us to discuss in our next meeting.