The Tor Browser / file revision

 # Test printing roots that refer to NULL pointers.

 # Since mozilla.prettyprinters.Pointer declines to create pretty-printers

 # for null pointers, GDB built-in printing code ends up handling them. But

 # as of 2012-11, GDB suppresses printing pointers in replacement values:

 # see: http://sourceware.org/ml/gdb/2012-11/msg00055.html

 #

 # Thus, if the pretty-printer for JS::Rooted simply returns the referent as

 # a replacement value (which seems reasonable enough, if you want the

 # pretty-printer to be completely transparent), and the referent is a null

 # pointer, it prints as nothing at all.

 #

 # This test ensures that the JS::Rooted pretty-printer doesn't make that

 # mistake.

 gdb.execute('set print address on')

 run_fragment('Root.null')

 assert_pretty('null', '0x0')