How to debug GC leak

[Andrew Benton via Digitalmars-d-learn]

I'm looking for a memory leak in my use of vibe.d JSON objects.  
The gc profiler built into the runtime only shows allocation 
count and allocation bytes, so I can make guesses around what 
might have leaked, but I can't be sure where or when, or exactly 
what leaked.

I'm sure someone else has run into this problem before.  How did 
you solve it?  I had hoped to also get a view into what was freed 
and when.