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Keyword for

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 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;
        }
    }
}

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

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.

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/keyword.for.html