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//! Declare types for interning various objects.
use once_cell::sync::OnceCell;
use std::hash::Hash;
use std::sync::{Arc, Mutex, MutexGuard, Weak};
use weak_table::WeakHashSet;
/// An InternCache is a lazily-constructed weak set of objects.
///
/// Let's break that down! It's "lazily constructed" because it
/// doesn't actually allocate anything until you use it for the first
/// time. That allows it to have a const [`new`](InternCache::new)
/// method, so you can make these static.
/// It's "weak" because it only holds weak references to its objects;
/// once every strong reference is gone, the object is unallocated.
/// Later, the hash entry is (lazily) removed.
pub(crate) struct InternCache<T: ?Sized> {
/// Underlying hashset for interned objects
cache: OnceCell<Mutex<WeakHashSet<Weak<T>>>>,
}
impl<T: ?Sized> InternCache<T> {
/// Create a new, empty, InternCache.
pub(crate) const fn new() -> Self {
InternCache {
cache: OnceCell::new(),
impl<T: Eq + Hash + ?Sized> InternCache<T> {
/// Helper: initialize the cache if needed, then lock it.
fn cache(&self) -> MutexGuard<'_, WeakHashSet<Weak<T>>> {
let cache = self.cache.get_or_init(|| Mutex::new(WeakHashSet::new()));
cache.lock().expect("Poisoned lock lock for cache")
impl<T: Eq + Hash> InternCache<T> {
/// Intern a given value into this cache.
/// If `value` is already stored in this cache, we return a
/// reference to the stored value. Otherwise, we insert `value`
/// into the cache, and return that.
pub(crate) fn intern(&self, value: T) -> Arc<T> {
let mut cache = self.cache();
if let Some(pp) = cache.get(&value) {
pp
} else {
let arc = Arc::new(value);
cache.insert(Arc::clone(&arc));
arc
impl<T: Hash + Eq + ?Sized> InternCache<T> {
/// Intern an object by reference.
/// Works with unsized types, but requires that the reference implements
/// `Into<Arc<T>>`.
pub(crate) fn intern_ref<'a, V>(&self, value: &'a V) -> Arc<T>
where
V: Hash + Eq + ?Sized,
&'a V: Into<Arc<T>>,
T: std::borrow::Borrow<V>,
{
if let Some(arc) = cache.get(value) {
let arc = value.into();