[ruby/prism] Make `it = it` assign `nil` to match parse.y behavior [Bug #21139]

Currently Prism returns `42` for code like this:
```ruby
42.tap { it = it; p it } # => 42
```

But parse.y returns `nil`:
```ruby
42.tap { it = it; p it } # => nil
```

In parse.y, it on the right-hand side is parsed as a local variable.
In Prism, it was parsed as the implicit block parameter it, which caused this inconsistent behavior.

This change makes the right-hand side it to be parsed as a local variable, aligning with parse.y's behavior.

Bug ticket: https://bugs.ruby-lang.org/issues/21139

https://github.com/ruby/prism/commit/cf3bbf9d2c

3 files changed, 28 insertions, 1 deletions

diff --git a/prism/prism.c b/prism/prism.c
index 4d2c372d5d..85098c52d8 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -16459,7 +16459,14 @@ parse_variable(pm_parser_t *parser) {
             pm_node_list_append(&current_scope->implicit_parameters, node);
 
             return node;
-        } else if ((parser->version >= PM_OPTIONS_VERSION_CRUBY_3_4) && pm_token_is_it(parser->previous.start, parser->previous.end)) {
+        } else if ((parser->version != PM_OPTIONS_VERSION_CRUBY_3_3) && pm_token_is_it(parser->previous.start, parser->previous.end)) {
+            if (match1(parser, PM_TOKEN_EQUAL)) {
+                pm_constant_id_t name_id = pm_parser_local_add_location(parser, parser->previous.start, parser->previous.end, 0);
+                pm_node_t *node = (pm_node_t *) pm_local_variable_read_node_create_constant_id(parser, &parser->previous, name_id, 0, false);
+
+                return node;
+            }
+
             pm_node_t *node = (pm_node_t *) pm_it_local_variable_read_node_create(parser, &parser->previous);
             pm_node_list_append(&current_scope->implicit_parameters, node);
 
diff --git a/test/prism/fixtures/it_assignment.txt b/test/prism/fixtures/it_assignment.txt
new file mode 100644
index 0000000000..523b0ffe1e
--- /dev/null
+++ b/test/prism/fixtures/it_assignment.txt
@@ -0,0 +1 @@
+42.tap { it = it; p it }
diff --git a/test/prism/ruby/parser_test.rb b/test/prism/ruby/parser_test.rb
index 156e8f9e9f..7bf2e59a86 100644
--- a/test/prism/ruby/parser_test.rb
+++ b/test/prism/ruby/parser_test.rb
@@ -183,6 +183,25 @@ module Prism
       assert_equal(it_block_parameter_sexp, actual_ast.to_sexp)
     end
 
+    def test_it_assignment_syntax
+      it_assignment_fixture_path = Pathname(__dir__).join('../../../test/prism/fixtures/it_assignment.txt')
+
+      buffer = Parser::Source::Buffer.new(it_assignment_fixture_path)
+      buffer.source = it_assignment_fixture_path.read
+      actual_ast = Prism::Translation::Parser34.new.tokenize(buffer)[0]
+
+      it_assignment_sexp = parse_sexp {
+        s(:block,
+           s(:send, s(:int, 42), :tap),
+           s(:args),
+           s(:begin,
+              s(:lvasgn, :it, s(:lvar, :it)),
+              s(:send, nil, :p, s(:lvar, :it))))
+      }
+
+      assert_equal(it_assignment_sexp, actual_ast.to_sexp)
+    end
+
     private
 
     def assert_equal_parses(fixture, compare_asts: true, compare_tokens: true, compare_comments: true)