I'm not sure if I'm the target audience for this (low-latency trading), but here's my thought - code which would allocate in a fast path is a strict no-go for me, and this runs fairly close to that in a few regards:

> It seems easy to accidentally make allocating code allocate by changing some variable names (b = fnc(a) - oops, allocation)

> I would be extremely wary of trusting a compiler to realize what sort of allocations are optimizable and not - I already have problems with inlining decisions, poor code layout, etc in fairly optimized c++/rust code compiled by gcc+llvm.

Replace allocation with any other expensive work that duplicating a datastructure would result in, and you have the same story. I suspect many of the people that would be evaluating this compared to C/C++/Rust have similar performance concerns.