Awesome to see the contributed back their custom allocator that lets you allocate from a fixed block: https://github.com/dropbox/rust-alloc-no-stdlib Solid stuff from the looks of it.
Fancy frame allocators would actually use a fairly standard malloc-style interface, but transparently place allocations of different sizes in their own regions. The allocator would internally use the slow system malloc to create and expand the fixed blocks it uses for its own allocations. During development, the frame allocator would be used to profile and optimize the actual object allocation sizes being made. E.g. sometimes it might make sense to pad some objects to consolidate the number of allocation-size regions. As the project progresses, the initial block allocations for each allocation size would be adjusted so that the system malloc was called very infrequently, usually only at application startup.