Fix missing predicate in case match

[thomasmarshall]

This PR fixes a crash when using Sorbet with Prism on the following code:

case
in x
end

This is invalid because a predicate is necessary for matching on. The original parser desugars this like so:

class <emptyTree><<C <root>>> < (::<todo sym>)
  if ::T.unsafe(::Kernel).raise("Sorbet rewriter pass partially unimplemented")
    begin
      x = ::T.unsafe(::Kernel).raise("Sorbet rewriter pass partially unimplemented")
      ::T.unsafe(::Kernel).raise("Sorbet rewriter pass partially unimplemented")
    end
  else
    <emptyTree>
  end
end

In the Prism translator we can check if the predicate desugars as an empty tree and return the expression early:

class <emptyTree><<C <root>>> < (::<todo sym>)
  if ::T.unsafe(::Kernel).raise("Sorbet rewriter pass partially unimplemented")
    begin
      x = ::T.unsafe(::Kernel).raise("Sorbet rewriter pass partially unimplemented")
      <emptyTree>
    end
  else
    <emptyTree>
  end
end

This is slightly different, we end up with <emptyTree> in place of ::T.unsafe(::Kernel).raise("Sorbet rewriter pass partially unimplemented"), but it seems to me a fine way to handle this. We could try to return a RaiseUnimplemented node here to match the original parser, but that seems wrong in the context—this is invalid code, not an unimplemented feature.

However, the error message for Prism is much better:

test/prism_regression/invalid/case_match_missing_predicate.rb:4: expected a predicate for a case matching statement https://srb.help/2001
     4 |case
        ^^^^
Errors: 1

Compared with this for the original parser:

test/prism_regression/invalid/case_match_missing_predicate.rb:4: unexpected token "case" https://srb.help/2001
     4 |case
        ^^^^

test/prism_regression/invalid/case_match_missing_predicate.rb:6: unexpected token "end" https://srb.help/2001
     6 |end
        ^^^
Errors: 2

[thomasmarshall]

Oops didn't mean to assign you, meant to add you as reviewers.