Manually tested by printing to stderr in both branches (sp in os
stack, or not), and triggering a GC in a filterSource function,
e.g.:
let
generateTree = n: if n == 0 then "ha" else { left = generateTree (n - 1); right = generateTree (n - 1); };
in
builtins.deepSeq (generateTree 18) ...
Note that the darwin still uses the strategy of disabling GC, despite
having an implementation that compiles. The proper solution will be
enabled and tested later.
