Add location slice to deconstruct_keys

It is nice to be able to slice out a location in pattern matching
without having to explicitly get the location object and call
slice manually.

Also, construct the response for deconstruct_keys lazily in order
to avoid reifying location objects if we don't have to.

Author: kddnewton

templates/lib/prism/node.rb.erb

@@ -551,7 +551,16 @@ module Prism
 
     #: (Array[Symbol]? keys) -> Hash[Symbol, untyped]
     def deconstruct_keys(keys) # :nodoc:
-      { <%= (["node_id: node_id", "location: location"] + node.fields.map { |field| "#{field.name}: #{field.name}" }).join(", ") %> }
+      <%- deconstruct = [:node_id, :location, *node.fields.map { |field| field.name.to_sym }, *node.fields.select { |field| field.is_a?(Prism::Template::LocationField) || field.is_a?(Prism::Template::OptionalLocationField) }.map { |field| field.name.delete_suffix("_loc").to_sym }].uniq -%>
+      (keys || %i[<%= deconstruct.join(" ") %>]).each_with_object(
+        {} #: Hash[Symbol, untyped]
+      ) do |key, deconstructed|
+        case key
+        <%- deconstruct.each do |key| -%>
+        when <%= "%-24s then" % [key.inspect] %> deconstructed[<%= key.inspect %>] = self.<%= key %>
+        <%- end -%>
+        end
+      end
     end
 
     # See `Node#type`.

test/prism/ruby/deconstruct_keys_test.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+require_relative "../test_helper"
+
+module Prism
+  class DeconstructKeysTest < TestCase
+    def test_deconstruct_keys
+      node = Prism.parse_statement("1.to_s")
+
+      deconstruct_all = node.deconstruct_keys(nil)
+      assert_equal deconstruct_all[:node_id], node.node_id
+      assert_equal deconstruct_all[:message], "to_s"
+
+      deconstruct_receiver = node.deconstruct_keys([:receiver])
+      assert_equal 1, deconstruct_receiver[:receiver].value
+      refute_includes deconstruct_receiver.keys, :message
+
+      deconstruct_invalid = node.deconstruct_keys([:invalid])
+      refute_includes deconstruct_invalid.keys, :invalid
+    end
+  end
+end