inKeyword

Explanation

in binds the loop variable to the iterator source in a for loop, as in for x in 0..10 { ... }.

Outside of for ... in ..., in appears in only one other place: the restricted-visibility syntax pub(in crate::some::path). In both spots it is part of a fixed grammar (for PATTERN in EXPR { BLOCK }, pub(in PATH)), never a general membership-test operator the way in works in Python. Testing membership in a Rust collection is a method call instead (collection.contains(&value)).

Usage examples

Binding a loop variable to a range

for x in 0..10 { // <- `in` binds `x` to each value produced by `0..10`
    println!("{x}");
}

Restriction: in only exists as part of fixed grammar — the for PATTERN in EXPR { ... } loop and pub(in path) restricted visibility — it is not a standalone membership-test operator the way in works in Python.

Working with collections

in binds whatever pattern precedes it to each item an iterator chain produces — including a destructured tuple, when the source yields pairs.

let inventory = [("widget", 12), ("gadget", 0), ("gizmo", 5)];

for (name, count) in inventory.into_iter().filter(|&(_, c)| c > 0) {
    // <- `in` binds `(name, count)` to each item the adaptor chain yields
    println!("{name}: {count} in stock");
}

Destructuring directly in the pattern before in avoids a separate destructuring let inside the loop body — the Iterator::filter adaptor narrows what's seen before in ever binds it. in itself has no meaning outside the for grammar and pub(in path) visibility — it isn't a general membership operator — so there's nothing further to say about it in isolation; see for for the loop it belongs to.

Explanation

Embedded support: Full

in is pure grammar bound to the for loop form (and pub(in path) visibility) — it carries no behavior of its own, so there's nothing that changes under #![no_std] regardless of what kind of iterator source sits on its right: a range over register offsets, a fixed-size array, or a heapless collection all bind through in the same way.

Usage examples

Binding a loop variable while scanning a channel range

for offset in 0..NUM_CHANNELS {
    // <- `in` binds `offset` to each value the range produces
    read_channel(offset);
}

Destructuring a pin/level pair on each iteration

let pins = [(0u8, true), (1u8, false), (2u8, true)];

for (pin, level) in pins {
    // <- `in` binds the destructured tuple to each pair the array yields
    set_output(pin, level);
}