Comment by majormajor

It would be super useful since I think this is pretty likely to be surprising to many users. But the profiler would need to be a particularly-specific refinement of even that: you need to make it obvious that it's not "95% of your Option<Thing>s are None, and your Option<Things> are using X bytes", but that "95% of the bytes used for your Option<Thing>s are used for None versions." Otherwise you could just assume that your non-None ones are just that chunky, or you have that many of them... I haven't seen a profiler with that level of insight, unfortunately.

Perhaps because this feels like a fairly rust-specific gotcha. Especially if you're coming from languages where there's often not much syntactical distinction made between "this is a pointer because I don't want to be copying it" and "this is a pointer because it's optional."

For instance, it's not until now that I actually understood what the sibling comment about the Enum type size discrepancy lint meant: "This lint obviously cannot take the distribution of variants in your running program into account. It is possible that the smaller variants make up less than 1% of all instances, in which case the overhead is negligible and the boxing is counter-productive. Always measure the change this lint suggests." I had always accidentally read this backwards, thinking it meant something more to the effect of "if most of the instances are actually small, then it's not a problem here, but be aware that some of them are much larger so some of your calls to things with this could end up passing much larger types."