[][src]Attribute Macro topo::nested


Gives a function a unique [CallId] in its caller's topology by applying #[track_caller] to the function and wrapping its body in [call] or [call_in_slot] if the slot parameter is given.

fn widget() -> topo::CallId {

// each call to the nested function gets a unique CallId in its parent's scope
assert_ne!(widget(), widget());

// nesting can be overridden by giving the function its own root
assert_eq!(topo::root(widget), topo::root(widget));


By default, #[nested] functions use for their slot the number of times the current source location has been called during the span of the current CallId. It is the behavior offered by the [call] shorthand.

To override the slot of a nested function, use the slot parameter, which is then passed directly as the first argument to [call_in_slot]:

#[topo::nested(slot = "name")]
fn get_name_id(name: &str, _value: &str) -> topo::CallId {

// reusing the same slot will get the same CallId
let bob = get_name_id("bob", "hello");
let bob_again = get_name_id("bob", "hello");
assert_eq!(bob, bob_again);

// the same name in a nested call returns a *new* CallId
let bob_nested = topo::call(|| get_name_id("bob", "hello"));
assert_ne!(bob, bob_nested);

// different names produce different slots, even when other args are the same
let alice_hello = get_name_id("alice", "hello");
assert_ne!(bob, alice_hello);

// changing non-slot arguments doesn't affect the CallId produced
let alice_goodbye = get_name_id("alice", "goodbye");
assert_eq!(alice_hello, alice_goodbye);

See [call_in_slot] and [CallId]'s documentation for more information on how slots are used.