Keep track of original nesting on top level references

[vinistock]

We originally took a shortcut for representing top level constant references. We were simply ignoring (and effectively erasing) its nesting information. This works for almost everything, but it unfortunately falls short for the case of completion.

For example:

module Foo
  CONST = 1
  
  class ::Bar
    # Completion triggered here without a character (CTRL + space)
  end
end

class Bar
  # Completion triggered here without a character (CTRL + space)
end

In this example, both cases of Bar refers to the same class, but the lexical scope in which they are inserted in is different. The first ::Bar has access to Foo::CONST through lexical scope and the second Bar does not.

We can't support this correctly without remembering the original nesting for constant references while at the same time recording that we found a top level reference.

Implementation

I tested two approaches:

1. Using a new enum for ParentScope. This gives us type safety as the compiler forces us to handle each case.
2. I originally thought the new enum would consume more memory, so I also experimented with using a special NameId for a fake name <!top-level!>. To my surprise, based on mem::size_of and our benchmarks on Core, there's no memory difference with the extra enum

Because of this, I went with the new enum, so that we can have extra type safety.

[vinistock]

• Keep track of original nesting on top level references #523 👈 (View in Graphite)
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[Morriar]

Why did this flip?

[vinistock]

I'm honestly not sure, but both Bar and AfterTop have the exact same score, so I think it's okay.