Skip compiling constants with alias from another gem in namespace

[egiurleo]

Motivation

When compiling a gem, it is possible for the gem to define an alias that includes an alias from another gem in its namespace.

For example, let's say we have gems foo and bar:

# foo.rb
module Foo; end
F = Foo

# bar.rb
module Foo
  module Bar; end
end
F::B = Foo::Bar

Then the RBIs might look something like this:

# foo.rbi
module Foo; end
F = Foo

# bar.rbi
module Foo::Bar; end
F::B = Foo::Bar
Foo::B = Foo::Bar

There are two issues with this RBI:

1. There will be a Sorbet error because the alias F is defined in the RBI for both gems (once in the line F = Foo and another time in the line F::B = Foo::Bar)
2. The alias for Foo::Bar is defined twice in bar.rbi

This commit will cause the RBI for gem bar to skip over the line F::B = Foo::Bar, which will prevent the alias F from being defined twice in RBI.

More Context

I found this issue when trying to generate RBI for the eventmachine and em-synchrony gems. eventmachine defines an alias EM = EventMachine, and em-synchrony defines another alias EM::S = EventMachine::Synchrony. The resulting RBI files cause a Sorbet error because EM is defined twice.

Implementation

I implemented this change by adding an additional check to the skip_alias? method that determines whether the namespace of a constant contains an alias and skips compiling that constant if so.

Tests

I added one test that fails without this change and passes with it. All other tests continue to pass. I have tested this in Shopify core and verified no new Sorbet errors.

[Morriar]

Should we extract this as a private constant_name_differs_from_actual_namespace method to facilitate reading? Maybe just adding a comment in the block would help.

[egiurleo]

What do you think of the comment I added?

[Morriar]

Perfect 👌