Because the compiler doesn’t know what read/write are doing to the buffer. And since it’s declared as [0; 4096], the compiler wouldn’t be able to do anything ot

Because the compiler doesn’t know what read/write are doing to the buffer. And since it’s declared as [0; 4096], the compiler wouldn’t be able to do anything other than 0’ing the entire 4kib region on every read instead of what’s dirtied if it attempted to automatically hoist. BorrowedBuf is an attempt to let you declare [MaybeUninit::uninit(); 4096] which the compiler could hoist although there it doesn’t matter either since the allocation of the uninit array is just an adjustment of the stack pointer.