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."

"You have 400 megabytes of zeros in <this type>" is probably a pretty easy heuristic to add.