Function first_touch 

pub fn first_touch<T>(f: impl FnOnce() -> T) -> T

Run f, which is a key coming into existence for the first time.

allow with a name on it, and the name is the claim. Y7 says no allocation on a command path, and the first SADD to a key that was not there has to make a set somewhere. There is no arrangement of this code that avoids it and no reason to want one: it happens once per key rather than once per command, and a workload that creates a key on every command is one where the allocation is the smallest thing it is paying for.

So the rule this module enforces is the one that is actually true. Nothing on a command path allocates except a key being created, and every place that does is this call, which makes the list of them a grep rather than an argument.

This is the one way to misuse the module. Wrapping steady state work in it to stop an abort would leave the check passing and the rule broken, which is worse than not having the check.