For Loop: Over a range

fn main() {
    for i in 0..3 {
        print(i)
    }
    for i in 1..=3 {
        print(i)
    }
}

012123

0..3 is 0, 1, 2. 1..=3 is 1, 2, 3. Ranges that leave the end out join without overlap: 0..n then n..m never counts n twice.

One past the end

A list's last index is len() - 1, so a loop to ..=items.len() reads one item too many and panics on its last turn. The compiler warns before it runs:

fn main() {
    const items = ["tea", "milk"]
    for i in 0..=items.len() {
        print(items[i])
    }
}

warning: `i` reaches `items.len()`, one past the last index of `items`, so `items[i]` panics on the last turn: write `..` instead of `..=`, or loop over `items` itself (spec 4.4)teamilkpanic: index 2 is out of range: the list has 2 items

Write 0..items.len(), or better, for item in items, which can't go past the end. A loop that changes the list, or reads it at another index, gets no warning.