3 files changed, 70 insertions, 1 deletions
diff --git a/lib/prism/translation/parser/builder.rb b/lib/prism/translation/parser/builder.rb
index ee3da1ee67..d3b51f4275 100644
--- a/lib/prism/translation/parser/builder.rb
+++ b/lib/prism/translation/parser/builder.rb
@@ -6,7 +6,55 @@ module Prism
       # A builder that knows how to convert more modern Ruby syntax
       # into whitequark/parser gem's syntax tree.
       class Builder < ::Parser::Builders::Default
+        # It represents the `it` block argument, which is not yet implemented in the Parser gem.
+        def itarg
+          n(:itarg, [:it], nil)
+        end
 
+        # The following three lines have been added to support the `it` block parameter syntax in the source code below.
+        #
+        #   if args.type == :itarg
+        #     block_type = :itblock
+        #     args = :it
+        #
+        # https://github.com/whitequark/parser/blob/v3.3.7.1/lib/parser/builders/default.rb#L1122-L1155
+        def block(method_call, begin_t, args, body, end_t)
+          _receiver, _selector, *call_args = *method_call
+
+          if method_call.type == :yield
+            diagnostic :error, :block_given_to_yield, nil, method_call.loc.keyword, [loc(begin_t)]
+          end
+
+          last_arg = call_args.last
+          if last_arg && (last_arg.type == :block_pass || last_arg.type == :forwarded_args)
+            diagnostic :error, :block_and_blockarg, nil, last_arg.loc.expression, [loc(begin_t)]
+          end
+
+          if args.type == :itarg
+            block_type = :itblock
+            args = :it
+          elsif args.type == :numargs
+            block_type = :numblock
+            args = args.children[0]
+          else
+            block_type = :block
+          end
+
+          if [:send, :csend, :index, :super, :zsuper, :lambda].include?(method_call.type)
+            n(block_type, [ method_call, args, body ],
+              block_map(method_call.loc.expression, begin_t, end_t))
+          else
+            # Code like "return foo 1 do end" is reduced in a weird sequence.
+            # Here, method_call is actually (return).
+            actual_send, = *method_call
+            block =
+              n(block_type, [ actual_send, args, body ],
+                block_map(actual_send.loc.expression, begin_t, end_t))
+
+            n(method_call.type, [ block ],
+              method_call.loc.with_expression(join_exprs(method_call, block)))
+          end
+        end
       end
     end
   end
diff --git a/lib/prism/translation/parser/compiler.rb b/lib/prism/translation/parser/compiler.rb
index c145cfa52c..8453c9383a 100644
--- a/lib/prism/translation/parser/compiler.rb
+++ b/lib/prism/translation/parser/compiler.rb
@@ -1138,7 +1138,7 @@ module Prism
         # -> { it }
         # ^^^^^^^^^
         def visit_it_parameters_node(node)
-          builder.args(nil, [], nil, false)
+          builder.itarg
         end
 
         # foo(bar: baz)
diff --git a/test/prism/ruby/parser_test.rb b/test/prism/ruby/parser_test.rb
index b32b52f3d8..274730d641 100644
--- a/test/prism/ruby/parser_test.rb
+++ b/test/prism/ruby/parser_test.rb
@@ -6,6 +6,7 @@ begin
   verbose, $VERBOSE = $VERBOSE, nil
   require "parser/ruby33"
   require "prism/translation/parser33"
+  require "prism/translation/parser34"
 rescue LoadError
   # In CRuby's CI, we're not going to test against the parser gem because we
   # don't want to have to install it. So in this case we'll just skip this test.
@@ -148,6 +149,22 @@ module Prism
       end
     end
 
+    def test_it_block_parameter_syntax
+      it_fixture_path = Pathname(__dir__).join("../../../test/prism/fixtures/it.txt")
+
+      buffer = Parser::Source::Buffer.new(it_fixture_path)
+      buffer.source = it_fixture_path.read
+      actual_ast = Prism::Translation::Parser34.new.tokenize(buffer)[0]
+
+      it_block_parameter_sexp = parse_sexp {
+        s(:itblock,
+          s(:send, nil, :x), :it,
+          s(:lvar, :it))
+      }
+
+      assert_equal(it_block_parameter_sexp, actual_ast.to_sexp)
+    end
+
     private
 
     def assert_equal_parses(fixture, compare_asts: true, compare_tokens: true, compare_comments: true)
@@ -246,5 +263,9 @@ module Prism
         "actual: #{actual_comments.inspect}"
       }
     end
+
+    def parse_sexp(&block)
+      Class.new { extend AST::Sexp }.instance_eval(&block).to_sexp
+    end
   end
 end