4 ms·

When I read "iterator" I think of an object that points into the vector and can be advanced. For Rust's vector, that is std::slice::Iter (https://doc.rust-lang.

When I read "iterator" I think of an object that points into the vector and can be advanced. For Rust's vector, that is std::slice::Iter (https://doc.rust-lang.org/std/slice/struct.Iter.html https://doc.rust-lang.org/std/slice/struct.Iter.html). When you advance an iterator, you must do a bounds check if the vector is dynamically sized; otherwise, you don't know when to stop. I.e., if I have let mut it = vec.iter(); println!(it.next()); println!(it.next()); println!(it.next()); This needs to do bounds checking on each call to next() to either return Some(a) or None (assuming the length of vec is unknown at compile time). (hhttps://doc.rust-lang.org/beta/src/core/slice/iter/macros.rs.html#133-152 https://doc.rust-lang.org/beta/src/core/slice/iter/macros.rs....) You are right that theoretically a range-based for loop that uses iterators does not need to do bounds checking because a compiler can infer the invariant that the iterator is always valid. In practice I don't know enough about LLVM or rustc to know whether this optimization is actually happening.

dwattttt 4y ago
The Rust docs for the 'for' keyword do say for loops are implemented as sugar for iterator loops. https://doc.rust-lang.org/std/keyword.for.html https://doc.rust-lang.org/std/keyword.for.html Relevant part: > for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common practice within Rust, which is to loop over anything that implements IntoIterator until the iterator returned by .into_iter() returns None (or the loop body uses break). (The other uses of the 'for' keyword it refers to are unrelated to loops)
int_19h 4y ago
That's not an index-out-of-bounds check tho, that's just the regular loop exit condition. In a C-style for(i=0; i<42; ++i) that would be the i<42 part. The check that can (and is) elided by the iterator here is the one that would be in the body of the loop, when you try to use i to index into the array. The nice thing about it is that it doesn't require the compiler to be smart at all.

dwattttt 4y ago

The Rust docs for the 'for' keyword do say for loops are implemented as sugar for iterator loops. https://doc.rust-lang.org/std/keyword.for.html https://doc.rust-lang.org/std/keyword.for.html Relevant part: > for-in-loops, or to be more precise, iterator loops, are a simple syntactic sugar over a common practice within Rust, which is to loop over anything that implements IntoIterator until the iterator returned by .into_iter() returns None (or the loop body uses break). (The other uses of the 'for' keyword it refers to are unrelated to loops)

int_19h 4y ago

That's not an index-out-of-bounds check tho, that's just the regular loop exit condition. In a C-style for(i=0; i<42; ++i) that would be the i<42 part. The check that can (and is) elided by the iterator here is the one that would be in the body of the loop, when you try to use i to index into the array. The nice thing about it is that it doesn't require the compiler to be smart at all.