Keyword for

Expand description

Iteration with in, trait implementation with impl, or higher-ranked trait bounds (for<'a>).

The for keyword is used in many syntactic locations:

for is used in for-in-loops (see below).
for is used when implementing traits as in impl Trait for Type (see impl for more info on that).
for is also used for higher-ranked trait bounds as in for<'a> &'a T: PartialEq<i32>.

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).

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 result = match IntoIterator::into_iter(iterator) {
        mut iter => loop {
            match iter.next() {
                None => break,
                Some(loop_variable) => { code(); },
            };
        },
    };
    result
}

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.