Grouped and arena allocations work really well in Zig. For awhile, we tried to use this pattern almost everywhere in Bun but it gets really tricky when there’s some GC-managed memory and you want to free things incrementally to reduce RSS. Also, using arenas for arrays that grow wastes memory a lot since it keeps every previous version around (mimalloc arenas are slightly better for this)

Grouped allocations works especially well in parsers & ASTs where the lifetime is very bounded. Since the Rust rewrite, we still use arenas for Bun’s parsers and the bundler but not a ton elsewhere.