debug(std_scope_buffer) then.
I'd say from my experience (and I have ton of such crap in std.regex), however that I'm against such code in any shape and form.
Reasons:
* Gets out of sync real damn quick.
* Makes no sense as debugging specific problem typically requires new hooks anyway.
* printf debugging illustrated? what example do we set out to establish?

If we aim at _tracing_ here then kinds of events to collect should be identified and messages well formed. It would however be a completely new initiative to add tracing to Phobos.