Writes warning message msg to $stderr, followed by a newline if
the message does not end in a newline. This method is called by Ruby for
all emitted warnings.
static VALUE
rb_warning_s_warn(VALUE mod, VALUE str)
{
Check_Type(str, T_STRING);
rb_must_asciicompat(str);
rb_write_error_str(str);
return Qnil;
}