for - Rust
Description: API documentation for the Rust `for` keyword in crate `std`.
Keywords: rust, rustlang, rust-lang, for

# [−]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;
            }
        }
    }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.