merge revision(s) 131c31a9209c61f84d318aa18b61f468f48b8219: [Backport #19081]

	[Bug #19081] Show the caller location in warning for Ractor

	The internal location in ractor.rb is not usefull at all.
	```
	$ ruby -e 'Ractor.new {}'
	<internal:ractor>:267: warning: Ractor is experimental, ...
	```
	---
	 bootstraptest/test_ractor.rb | 6 ++++++
	 ractor.c                     | 6 ------
	 ractor.rb                    | 4 ++++
	 3 files changed, 10 insertions(+), 6 deletions(-)

1 files changed, 4 insertions, 0 deletions

diff --git a/ractor.rb b/ractor.rb
index ef36b2937f..bc3d33cae2 100644
--- a/ractor.rb
+++ b/ractor.rb
@@ -262,6 +262,10 @@ class Ractor
   def self.new(*args, name: nil, &block)
     b = block # TODO: builtin bug
     raise ArgumentError, "must be called with a block" unless block
+    if __builtin_cexpr!("RBOOL(ruby_single_main_ractor)")
+      warn("Ractor is experimental, and the behavior may change in future versions of Ruby! " \
+           "Also there are many implementation issues.", uplevel: 0, category: :experimental)
+    end
     loc = caller_locations(1, 1).first
     loc = "#{loc.path}:#{loc.lineno}"
     __builtin_ractor_create(loc, name, args, b)