Struct OnceCache 

pub struct OnceCache<K: Eq + Hash + Clone, V: Clone> { /* private fields */ }

Per-key once-init cache. See module docs for the rationale and pattern.

K is the cache key (hashable, cloneable, equatable). V is the cached value type — typically Arc<Something> so the clone on hit is just a refcount bump.

Implementations

impl<K: Eq + Hash + Clone, V: Clone> OnceCache<K, V>

pub fn new() -> Self

Empty cache. Use with LazyLock/OnceLock for a static.

pub fn get_or_init<F>(&self, key: K, init: F) -> Result<V, String>

where F: FnOnce() -> Result<V, String>,

Get the value for key, computing it via init exactly once across all concurrent callers. Subsequent callers for the same key see the cached Result (including errors — see module docs §“Failure semantics”).

init may take seconds (HTTPS download, cert load, file open). The outer Mutex is only held long enough to install the per-key slot; init runs without it, guarded only by the per-key OnceLock.

pub fn len(&self) -> usize

Number of distinct keys currently in the cache. Includes keys whose load is in progress or has failed. Diagnostic only — the cache should not be inspected for routing or behavior decisions.

pub fn is_empty(&self) -> bool

Whether the cache holds no keys.

Trait Implementations

impl<K: Eq + Hash + Clone, V: Clone> Default for OnceCache<K, V>

fn default() -> Self

Returns the “default value” for a type.