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//! Declare types for interning various objects.
use std::hash::Hash;
use std::sync::{Arc, Mutex, MutexGuard, OnceLock, 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 struct InternCache<T: ?Sized> {
/// Underlying hashset for interned objects
//
// TODO: If WeakHashSet::new is someday const, we can do away with OnceLock here.
cache: OnceLock<Mutex<WeakHashSet<Weak<T>>>>,
}
impl<T: ?Sized> InternCache<T> {
/// Create a new, empty, InternCache.
pub const fn new() -> Self {
InternCache {
cache: OnceLock::new(),
impl<T: ?Sized> Default for InternCache<T> {
fn default() -> Self {
Self::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 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 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();
#[cfg(test)]
mod test {
// @@ begin test lint list maintained by maint/add_warning @@
#![allow(clippy::bool_assert_comparison)]
#![allow(clippy::clone_on_copy)]
#![allow(clippy::dbg_macro)]
#![allow(clippy::mixed_attributes_style)]
#![allow(clippy::print_stderr)]
#![allow(clippy::print_stdout)]
#![allow(clippy::single_char_pattern)]
#![allow(clippy::unwrap_used)]
#![allow(clippy::unchecked_time_subtraction)]
#![allow(clippy::useless_vec)]
#![allow(clippy::needless_pass_by_value)]
//! <!-- @@ end test lint list maintained by maint/add_warning @@ -->
use super::*;
#[test]
fn interning_by_value() {
// "intern" case.
let c: InternCache<String> = InternCache::new();
let s1 = c.intern("abc".to_string());
let s2 = c.intern("def".to_string());
let s3 = c.intern("abc".to_string());
assert!(Arc::ptr_eq(&s1, &s3));
assert!(!Arc::ptr_eq(&s1, &s2));
assert_eq!(s2.as_ref(), "def");
assert_eq!(s3.as_ref(), "abc");
fn interning_by_ref() {
let c: InternCache<str> = InternCache::new();
let s1 = c.intern_ref("abc");
let s2 = c.intern_ref("def");
let s3 = c.intern_ref("abc");
assert_eq!(&*s2, "def");
assert_eq!(&*s3, "abc");