alloc/collections/btree/
set_val.rs

1/// Zero-Sized Type (ZST) for internal `BTreeSet` values.
2/// Used instead of `()` to differentiate between:
3/// * `BTreeMap<T, ()>` (possible user-defined map)
4/// * `BTreeMap<T, SetValZST>` (internal set representation)
5#[derive(Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Clone, Default)]
6pub(super) struct SetValZST;
7
8/// A trait to differentiate between `BTreeMap` and `BTreeSet` values.
9/// Returns `true` only for type `SetValZST`, `false` for all other types (blanket implementation).
10/// `TypeId` requires a `'static` lifetime, use of this trait avoids that restriction.
11///
12/// [`TypeId`]: std::any::TypeId
13pub(super) trait IsSetVal {
14    fn is_set_val() -> bool;
15}
16
17// Blanket implementation
18impl<V> IsSetVal for V {
19    default fn is_set_val() -> bool {
20        false
21    }
22}
23
24// Specialization
25impl IsSetVal for SetValZST {
26    fn is_set_val() -> bool {
27        true
28    }
29}