Allow rescuing exceptions based on included modules about crystal HOT 1 CLOSED

This seems very reasonable and I wouldn't have been surprised if it already worked this way. It's even a bit unexpected that it doesn't. Esepcially since it's supported in Ruby. So might just be an unintentional omission.

In my mental model, rescue ex : Type is equivalent to an elective type restriction with re-raising on non-match.

rescue ex : Foo
  bar
end

This is more convenient to write than the long form:

rescue ex
  if ex.is_a?(Foo)
    bar
  else
    raise ex
  end
end

The actual type casting seems to work fine with module types. It's just the extra validation that checks for being a descendent of ::Exception which makes it impossible to filter for modules.
So I think the implementation could be pretty trivial:

--- i/src/compiler/crystal/semantic/main_visitor.cr
+++ w/src/compiler/crystal/semantic/main_visitor.cr
@@ -2775,7 +2775,7 @@ module Crystal
         types = node_types.map do |type|
           type.accept self
           instance_type = type.type.instance_type
-          unless instance_type.implements?(@program.exception)
+          unless instance_type.implements?(@program.exception) || instance_type.module?
             type.raise "#{instance_type} is not a subclass of Exception"
           end
           instance_type

from crystal.