And in fact, I've written that exact thing in a C# network stack, about 7 years ago. No bug, as I did properly check the size of the data, but totally possible to have messed up. It's not even hard code to write, just a simple object pool to return byte arrays. Which is natural, as in .NET, high performance often ends up as an exercise in removing every heap allocation possible.

I would argue that the whole Rust ecosystem pretty strongly discourages you from writing broken low-level infrastructure code. The idea behind unsafe code blocks, the generics system, and Cargo is to encourage you to use off-the-shelf tools instead of rolling your own, often broken, solutions. The C ecosystem, by contrast, tends to encourage rolling your own solutions because managing dependencies in a cross-platform manner is such a pain (and the language doesn't help due to the lack of generics and so forth). I suspect that any free-list library in Cargo that didn't zero out buffers would be fixed pretty quickly (and, as eddyb rightly points out downthread, it would be pretty hard to write a free list system that works with multiple types that doesn't require initialization before use--Rust in general abhors uninitialized memory).

There are parallels between the philosophy of crypto libraries like NaCl and Keyczar and the Rust philosophy. Don't write your own low-level infrastructure code. Use somebody else's.