Something I don't understand ( there is so much I don't understand )...

BTreeSet is defined like this:

pub struct BTreeSet<
    T,
    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator + Clone = Global,
> {
    map: BTreeMap<T, SetValZST, A>,
}

https://doc.rust-lang.org/src/alloc/collections/btree/set.rs.html#78-81

SetValZST is defined here:

where it explains:

/// Zero-Sized Type (ZST) for internal `BTreeSet` values.
/// Used instead of `()` to differentiate between:
/// * `BTreeMap<T, ()>` (possible user-defined map)
/// * `BTreeMap<T, SetValZST>` (internal set representation)
#[derive(Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Clone, Default)]
pub(super) struct SetValZST;

But isn't the definition (of map) private? Does it somehow "leak" even though it is private?

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