[]Keyword for

The for keyword.

The for keyword is used in many syntactic locations:

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 an iterator until that iterator returns None (or break is called).

for i in 0..5 {
    println!("{}", i * 2);
}

for i in std::iter::repeat(5) {
    println!("turns out {} never stops being 5", i);
    break; // would loop forever otherwise
}

'outer: for x in 5..50 {
    for y in 0..10 {
        if x == y {
            break 'outer;
        }
    }
}Run

As shown in the example above, for loops (along with all other loops) can be tagged, using similar syntax to lifetimes (only visually similar, entirely distinct in practice). Giving the same tag to break breaks the tagged loop, which is useful for inner loops. It is definitely not a goto.

A for loop expands as shown:

for loop_variable in iterator {
    code()
}Run
{
    let mut _iter = std::iter::IntoIterator::into_iter(iterator);
    loop {
        match _iter.next() {
            Some(loop_variable) => {
                code()
            },
            None => break,
        }
    }
}Run

More details on the functionality shown can be seen at the IntoIterator docs.

For more information on for-loops, see the Rust book or the Reference.