Warn it (#9152)

https://bugs.ruby-lang.org/issues/18980

NEWS.md

@@ -257,6 +257,10 @@ changelog for details of the default gems or bundled gems.
   removed. Environment variables `RUBY_GC_HEAP_%d_INIT_SLOTS` should be
   used instead.  [[Feature #19785]]
 
+* `it` calls without arguments in a block with no ordinary parameters are
+  deprecated. `it` will be a reference to the first block parameter in Ruby 3.4.
+  [[Feature #18980]]
+
 ## Stdlib compatibility issues
 
 * `racc` is promoted to bundled gems.
@@ -374,6 +378,7 @@ changelog for details of the default gems or bundled gems.
 [Feature #18515]: https://bugs.ruby-lang.org/issues/18515
 [Feature #18551]: https://bugs.ruby-lang.org/issues/18551
 [Feature #18885]: https://bugs.ruby-lang.org/issues/18885
+[Feature #18980]: https://bugs.ruby-lang.org/issues/18980
 [Feature #18949]: https://bugs.ruby-lang.org/issues/18949
 [Bug #19012]:     https://bugs.ruby-lang.org/issues/19012
 [Bug #19150]:     https://bugs.ruby-lang.org/issues/19150

parse.y

@@ -12785,6 +12785,11 @@ gettable(struct parser_params *p, ID id, const YYLTYPE *loc)
         }
 # endif
         /* method call without arguments */
+        if (dyna_in_block(p) && id == rb_intern("it")
+            && !(DVARS_TERMINAL_P(p->lvtbl->args) || DVARS_TERMINAL_P(p->lvtbl->args->prev))
+            && p->max_numparam != ORDINAL_PARAM) {
+            rb_warn0("`it` calls without arguments will refer to the first block param in Ruby 3.4; use it() or self.it");
+        }
         return NEW_VCALL(id, loc);
       case ID_GLOBAL:
         return NEW_GVAR(id, loc);

test/ruby/test_syntax.rb

@@ -1719,6 +1719,17 @@ def test_numbered_parameter
     assert_valid_syntax("p { [_1 **2] }")
   end
 
+  def test_it
+    assert_no_warning(/`it`/) {eval('if false; it; end')}
+    assert_no_warning(/`it`/) {eval('def foo; it; end')}
+    assert_warn(/`it`/)       {eval('0.times { it }')}
+    assert_no_warning(/`it`/) {eval('0.times { || it }')}
+    assert_no_warning(/`it`/) {eval('0.times { |_n| it }')}
+    assert_warn(/`it`/)       {eval('0.times { it; it = 1; it }')}
+    assert_no_warning(/`it`/) {eval('0.times { it = 1; it }')}
+    assert_no_warning(/`it`/) {eval('it = 1; 0.times { it }')}
+  end
+
   def test_value_expr_in_condition
     mesg = /void value expression/
     assert_syntax_error("tap {a = (true ? next : break)}", mesg)