Refactor significantly:

- return "warn" sub to its previous state
- put .WARN logic into the control exception handler (this was the
  place I was looking for earlier, it could use a different name)
- the .WARN methods are now also expected to receive a Backtrace
  object as the second positional argument
- this restores the capability of having a CONTROL block catching
  warnings if $*WARNINGS is something else than CX::Warn

src/core.c/Exception.pm6

@@ -452,33 +452,29 @@ my class CX::Warn does X::Control {
         nqp::p6box_n(nqp::div_In($nu,$de))
     }
 
-    method WARN(str $message --> Nil) is hidden-from-backtrace {
-        my $ex := nqp::newexception();
-        nqp::setmessage($ex,$message);
-        nqp::setextype($ex,nqp::const::CONTROL_WARN);
-        nqp::throw($ex);
+    method WARN(str $message, $bt --> Nil) is hidden-from-backtrace {
+        note "$message\n$bt.first-none-setting-line()";
     }
 }
 my class CX::Warn::Quietly {
-    method WARN($ --> Nil) is hidden-from-backtrace { }
+    method WARN($, $ --> Nil) is hidden-from-backtrace { }
 }
 my class CX::Warn::Fatal {
-    method WARN(str $message --> Nil) is hidden-from-backtrace {
+    method WARN(str $message, $bt --> Nil) is hidden-from-backtrace {
         X::AdHoc.new(payload => $message).throw;
     }
 }
 my class CX::Warn::Verbose {
-    method WARN(str $message --> Nil) is hidden-from-backtrace {
-        my $bt := Backtrace.new;
-        note $message ~ "\n" ~ Backtrace.new.Str;
+    method WARN( str $message, $bt --> Nil) is hidden-from-backtrace {
+        note "$message\n$bt";
     }
 }
 my class CX::Warn::Collect {
     my $lock     := Lock.new;
     my $messages := nqp::hash;
 
-    method WARN(str $message --> Nil) is hidden-from-backtrace {
-        my str $key = $message ~ "\n" ~ Backtrace.new.Str;
+    method WARN(str $message, $bt --> Nil) is hidden-from-backtrace {
+        my str $key = "$message\n$bt";
         $lock.protect: {
             nqp::bindkey($messages,$key,
               nqp::add_i(nqp::ifnull(nqp::atkey($messages,$key),0),1)
@@ -641,15 +637,15 @@ do {
         nqp::if(
           nqp::iseq_i($type,nqp::const::CONTROL_WARN),
           nqp::stmts(
-            (my Mu $err := $*ERR),
-            (my str $msg = nqp::getmessage($ex)),
-            $err.say(nqp::if(nqp::chars($msg),$msg,"Warning")),
-            $err.print($backtrace.first-none-setting-line),
+            (nqp::istype((my $class := $*WARNINGS),Failure)
+              ?? CX::Warn
+              !! $class
+            ).WARN(nqp::getmessage($ex) || "Warning", $backtrace),
             nqp::resume($ex)
           )
         );
 
-        my $label = $type +& nqp::const::CONTROL_LABELED ?? "labeled " !! "";
+        my $label := $type +& nqp::const::CONTROL_LABELED ?? "labeled " !! "";
         if $type +& nqp::const::CONTROL_LAST {
             X::ControlFlow.new(illegal => "{$label}last", enclosing => 'loop construct', :$backtrace).throw;
         }

src/core.c/control.pm6

@@ -263,10 +263,11 @@ multi sub die(|cap ( *@msg ) --> Nil) {
 
 proto sub warn(|) {*}
 multi sub warn(*@msg --> 0) {
-    (nqp::istype((my $class := $*WARNINGS),Failure)
-      ?? CX::Warn
-      !! $class
-    ).WARN(@msg.join || "Warning: something's wrong")
+    my str $message = @msg.join || "Warning: something's wrong";
+    my $ex := nqp::newexception(); 
+    nqp::setmessage($ex,$message);
+    nqp::setextype($ex,nqp::const::CONTROL_WARN);
+    nqp::throw($ex);
 }
 multi sub warn(Junction:D $j) { $j.THREAD: &warn }