Would a DelegatedMethod pin be useful?

[grncdr]

Given how frequently method delegation is used in Ruby, it would be nice if Solargraph had a pin type that could express this concept directly. In particular, I want to be able to tell solargraph that "everything about method can be found on another object" so that completion queries can pull docstrings and types etc. from the original method.

To demonstrate why I think this should be part of Solargraph, consider this code from solargraph-rails for handling delegate calls: https://github.com/iftheshoefritz/solargraph-rails/blob/1b322ddc4ca7f71657334402a50d47480e442e5d/lib/solargraph/rails/delegate.rb

This matches a calls like delegate :foo, to: :bar and defines a #foo method on the containing namespace. However, the defined method can't copy the parameters or return type from the original method, because the local convention can't query an ApiMap for pins.

How this could work

If the convention could create a pin like:

# this pin already exists
ns = Solargraph::Pin::Namespace.new(name: "SomeClass", type: :class)

# this the new pin type, this pin states "the foo method is delegated to result of "#bar".
# Note the delegate resolution is late-bound, so there is no dependency on a pin for the #bar method 
Solargraph::Pin::Reference::DelegatedMethod.new(name: "foo", delegate: "SomeClass#bar", scope: :instance, closure: ns)

# as long as this pin exists at query time, we're good.
Solargraph::Pin::Method(name: "bar", scope: :instance, closure: ns, comments: "@return [Bar]")

Then, when asked about "SomeClass#foo", solargraph would compute the delegate (the return type of "SomeClass#bar" -> "Bar") and finally return information about "Bar#foo".

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What do you think? I'd be willing to take a stab at implementing it if you think the design makes sense.

[grncdr]

In case this interests you, take a look at the related PR (#602)