The feature in OxCaml that more languages should steal
Jane Street's superset of OCaml lets you flip that around: annotate a function with [@zero_alloc] and the compiler refuses to build if anything in its call tree touches the heap.
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OxCaml, Jane Street’s superset of OCaml, allows you to assert that entire functions do not allocate (on the heap). And if you start allocating within that call tree, the compiler will fail and tell you that you allocated. While you might be able to achieve this with static analysis, there are few mainstream languages that let you do this in the compiler itself. Swift and Clang are the only other exceptions I know of.[^edit]
In most languages (Java, Go, C#, Rust, Zig, OCaml, etc.) the process is reversed: you take a profiler to try and find allocations (usually in loops that happen millions of times). Then you go and eliminate or minimize the allocations. But as soon as you edit a line of code in the hot path, you might forget the context and start allocating again, and you’re back to square one with a profiler.
In Zig (and maybe more recent Rust) you might be able to minimize regression through convention by not passing an allocator to a function. But convention can be ignored and bypassed. Why not let the compiler do the work?
In this article we'll take a look at how OxCaml's [@zero_alloc] assertion works.
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