I wish I could tell the compiler to compile a function only if it can do it without using memory allocations. Often I would rather see a compilation error than the GC making performance of a function unpredictable.
Of course borrow checker would be helpful, but I guess that's too hard to implement, that's why something like this would be a good balance.
I also think Julia would benefit a lot from a list of "we want these but haven't had time for them". People like contributing things they know are desired, and with Julia it's hard to know whether people want or will accept something until you make the PR.