Fix rb_warn and add location and "warning: " prefix

lib/cext/ABI_check.txt

@@ -1 +1 @@
-2
+3

lib/truffle/truffle/cext.rb

@@ -2319,6 +2319,12 @@ def rb_eval_cmd_kw(cmd, args, kw_splat)
     end
   end
 
+  def rb_tr_warn(message)
+    location = caller_locations(1, 1)[0]
+    message_with_prefix = location.label + ': warning: ' + message
+    Warning.warn(message_with_prefix)
+  end
+
   def rb_warning_category_enabled_p(category)
     Warning[category]
   end

spec/ruby/optional/capi/kernel_spec.rb

@@ -156,26 +156,16 @@ class CApiKernelSpecs::Exc < StandardError
   end
 
   describe "rb_warn" do
-    before :each do
-      @stderr, $stderr = $stderr, IOStub.new
-      @verbose = $VERBOSE
-    end
-
-    after :each do
-      $stderr = @stderr
-      $VERBOSE = @verbose
-    end
-
     it "prints a message to $stderr if $VERBOSE evaluates to true" do
-      $VERBOSE = true
-      @s.rb_warn("This is a warning")
-      $stderr.should =~ /This is a warning/
+      -> {
+        @s.rb_warn("This is a warning")
+      }.should complain(/warning: This is a warning/, verbose: true)
     end
 
     it "prints a message to $stderr if $VERBOSE evaluates to false" do
-      $VERBOSE = false
-      @s.rb_warn("This is a warning")
-      $stderr.should =~ /This is a warning/
+      -> {
+        @s.rb_warn("This is a warning")
+      }.should complain(/warning: This is a warning/, verbose: false)
     end
   end
 

src/main/c/cext/truffleruby.c

@@ -35,7 +35,7 @@ int rb_tr_obj_equal(VALUE first, VALUE second) {
 }
 
 void rb_tr_warn_va_list(const char *fmt, va_list args) {
-  RUBY_INVOKE(rb_mKernel, "warn", rb_vsprintf(fmt, args));
+  RUBY_CEXT_INVOKE("rb_tr_warn", rb_vsprintf(fmt, args));
 }
 
 VALUE rb_tr_zlib_crc_table(void) {