Don't emit warning when the pattern of one-line pattern matching is just a variable pattern

https://github.com/ruby/dev-meeting-log/blob/master/DevelopersMeeting20201210Japan.md#feature-17371-reintroduce-expr-in-pat-ktsj

parse.y

@@ -502,6 +502,7 @@ static NODE *new_find_pattern(struct parser_params *p, NODE *constant, NODE *fnd
 static NODE *new_find_pattern_tail(struct parser_params *p, ID pre_rest_arg, NODE *args, ID post_rest_arg, const YYLTYPE *loc);
 static NODE *new_hash_pattern(struct parser_params *p, NODE *constant, NODE *hshptn, const YYLTYPE *loc);
 static NODE *new_hash_pattern_tail(struct parser_params *p, NODE *kw_args, ID kw_rest_arg, const YYLTYPE *loc);
+static void warn_one_line_pattern_matching(struct parser_params *p, NODE *node, NODE *pattern);
 
 static NODE *new_kw_arg(struct parser_params *p, NODE *k, const YYLTYPE *loc);
 static NODE *args_with_numbered(struct parser_params*,NODE*,int);
@@ -1661,10 +1662,7 @@ expr		: command_call
 			p->ctxt.in_kwarg = $<ctxt>3.in_kwarg;
 		    /*%%%*/
 			$$ = NEW_CASE3($1, NEW_IN($5, 0, 0, &@5), &@$);
-
+			warn_one_line_pattern_matching(p, $$, $5);
-			if (rb_warning_category_enabled_p(RB_WARN_CATEGORY_EXPERIMENTAL))
-			    rb_warn0L(nd_line($$), "One-line pattern matching is experimental, and the behavior may change in future versions of Ruby!");
-
 		    /*% %*/
 		    /*% ripper: case!($1, in!($5, Qnil, Qnil)) %*/
 		    }
@@ -11690,6 +11688,17 @@ new_hash_pattern_tail(struct parser_params *p, NODE *kw_args, ID kw_rest_arg, co
     return node;
 }
 
+static void
+warn_one_line_pattern_matching(struct parser_params *p, NODE *node, NODE *pattern)
+{
+    enum node_type type;
+    type = nd_type(pattern);
+
+    if (rb_warning_category_enabled_p(RB_WARN_CATEGORY_EXPERIMENTAL) &&
+	!(type == NODE_LASGN || type == NODE_DASGN || type == NODE_DASGN_CURR))
+	rb_warn0L(nd_line(node), "One-line pattern matching is experimental, and the behavior may change in future versions of Ruby!");
+}
+
 static NODE*
 dsym_node(struct parser_params *p, NODE *node, const YYLTYPE *loc)
 {

spec/ruby/command_line/dash_upper_w_spec.rb

@@ -33,11 +33,11 @@ ruby_version_is "2.7" do
   describe "The -W command line option with :no-experimental" do
     before do
       ruby_version_is ""..."3.0" do
-        @src = 'case 0; in a; end'
+        @src = 'case [0, 1]; in [a, b]; end'
       end
 
       ruby_version_is "3.0" do
-        @src = '1 => a'
+        @src = '[0, 1] => [a, b]'
       end
     end
 

spec/ruby/core/warning/element_set_spec.rb

@@ -10,11 +10,11 @@ ruby_version_is '2.7' do
     describe ":experimental" do
       before do
         ruby_version_is ""..."3.0" do
-          @src = 'case 0; in a; end'
+          @src = 'case [0, 1]; in [a, b]; end'
         end
 
         ruby_version_is "3.0" do
-          @src = '1 => a'
+          @src = '[0, 1] => [a, b]'
         end
       end
 

spec/ruby/language/pattern_matching_spec.rb

@@ -43,11 +43,11 @@ ruby_version_is "2.7" do
     describe "warning" do
       before do
         ruby_version_is ""..."3.0" do
-          @src = 'case 0; in a; end'
+          @src = 'case [0, 1]; in [a, b]; end'
         end
 
         ruby_version_is "3.0" do
-          @src = '1 => a'
+          @src = '[0, 1] => [a, b]'
         end
       end