[tylerhou]

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). 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....)

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.

[int_19h]

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]

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

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)