@PeteVine It depends very much on the workload. Glibc works very well in general, but some cases will be better in jemalloc, and sure, a performance architect may want to control that. I hope people actually measure it though, instead of cargo-culting the idea that one is always better. (No Cargo pun intended.)
AIUI, making your own allocator choice still requires unstable feature flags, right?
Anyway, whatever we do now isn't set in stone. If Rust gets more knobs for controlling defaults, and preferably learns to link jemalloc dynamically, then I won't mind including it.