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:
foris used in for-in-loops (see below).foris used when implementing traits as inimpl Trait for Type(seeimplfor more info on that).foris also used for higher-ranked trait bounds as infor<'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 {i} never stops being 5");
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:
{
let result = match IntoIterator::into_iter(iterator) {
mut iter => loop {
match iter.next() {
None => break,
Some(loop_variable) => { code(); },
};
},
};
result
}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.