2 ms·
Because in the former case, the optimizer has to prove that the length of the array cannot change during the body of the loop, while in the latter case, that's
by pcwalton 9y ago
Because in the former case, the optimizer has to prove that the length of the array cannot change during the body of the loop, while in the latter case, that's guaranteed by the language semantics.
- masklinn 9y agoAlso because the iterator could do something completely different from an iteration by sequential index no?
- mindleyhilner 9y agoDoesn't `let x = xs[i]` immutably borrow from xs? So for the duration of x's lifetime (which is the entire for-body block), xs cannot be changed and therefore its length must remain the same. Though this might be information that rustc knows about but not LLVM.
- pcwalton 9y agoIn the case of "let x = &xs[i]" it would immutably borrow. But we'd need more MIR optimizations to make use of that fact.