Memoize a function with #[drv::memo]. The attribute is liberal about parameter types — owned values, references, struct refs, &str, &[u8], #[derive(drv::Input)] projections — and caches results in a per-memo thread-local slot array keyed by value equality.

#[derive(drv::Input)]
pub struct Config {
    pub threads: u32,
    pub timeout_ms: u64,
}

#[drv::memo(single)]
fn worker_count(c: &Config) -> u32 {
    c.threads * 2
}

let c = Config { threads: 4, timeout_ms: 5000 };
assert_eq!(worker_count(&c), 8);   // computes
assert_eq!(worker_count(&c), 8);   // cache hit, no work

The companion derive #[derive(drv::Input)] is a helper for one specific situation: when you want to cache on a subset of a source struct's fields without cloning the whole struct on every call. See Zero-copy projections.

• Equality-keyed, not hash-keyed. No hashing on the hot path, no HashMap probe. Cache lookup is a linear scan through a fixed-shape slot array with per-field equality.
• Thread-local caches. Every memo owns its own cache — single-writer, lock-free. On Android, drv stores those caches behind one shared thread-local key to avoid the platform's low pthread-key limit. On arm64_32 watchOS, each cache is lazily heap-allocated so large keys, outputs, and LRU capacities do not exceed the 64 KiB TLS offset limit. Other platforms store caches directly in TLS without this indirection.
• Zero allocations on cache hit. A hit is an equality check plus a Clone of the output.
• O(1) cache-hit check for Arc<T> and imbl collections. A pointer-equality fast path skips deep comparison when the field's pointer hasn't changed since the last miss.
• One concept for inputs. #[derive(drv::Input)] is the single opt-in: any struct you want as a memo parameter derives it. Plain structs, borrowed projections, and nested-input bundles all work the same way.

Every memo picks a cache strategy:

• #[drv::memo(single)] — one slot, last-call caching. A hit requires today's inputs to equal the most recent recompute's inputs.
• #[drv::memo(lru = N)] — N slots, least-recently-used eviction. For inputs that cycle between a small number of recurring states.

Every parameter must implement [ToStatic]. That trait is implemented by drv for primitives, String, PathBuf, Arc<T>, std::time::{Instant, Duration, SystemTime}, imbl collections (feature-gated), and — via a reference blanket — &T for any T: ToStatic. Containers (Vec, Option, tuples, arrays, HashMap / HashSet / BTreeMap / BTreeSet) are recursive: they implement ToStatic whenever their elements do, so nested projections like Vec<MyInput<'a>> work without any extra annotation. User types become inputs by adding #[derive(drv::Input)] — supported on structs (named, tuple, unit) and enums.

Bodies see the exact type you declared: strip #[drv::memo] and the function still compiles.

Take the previous Config example. If Config grows a big Vec<Worker> field that worker_count doesn't read, the default &Config form still snapshots the whole struct into the cache slot and every cache-hit check compares the whole thing.

#[derive(drv::Input)] lets you declare a lightweight view that borrows only the fields the memo actually depends on. The derive auto-generates the owned snapshot and the machinery #[drv::memo] uses internally:

#[derive(drv::Input)]
struct TotalInput<'a> {
    pub hits: &'a Vec<u32>,
}

impl<'a> TotalInput<'a> {
    pub fn new(s: &'a Scoreboard) -> Self { Self { hits: &s.hits } }
}

#[drv::memo(single)]
fn total_score<'a>(input: TotalInput<'a>) -> u32 {
    input.hits.iter().sum()
}

let mut game = Scoreboard { hits: vec![100, 250, 50], player_x: 0 };
assert_eq!(total_score(TotalInput::new(&game)), 400);   // computes
game.player_x = 42;                                      // not in TotalInput
assert_eq!(total_score(TotalInput::new(&game)), 400);   // cache hit

Only hits enters the cache key. Changes to player_x don't invalidate; changes to hits do. The projection is whatever code you write — a ::new method, a From<&Source> impl, or an inline struct literal at the call site. drv doesn't prescribe one.

A #[derive(drv::Input)] struct can have another #[derive(drv::Input)] struct as a field — useful for bundling a handful of sub-projections into one memo parameter:

#[derive(drv::Input)]
struct ChildA<'a> { pub a: &'a Vec<u32> }

#[derive(drv::Input)]
struct ChildB<'a> { pub b: &'a Vec<u32> }

#[derive(drv::Input)]
struct Both<'a> {
    pub ca: ChildA<'a>,
    pub cb: ChildB<'a>,
}

#[drv::memo(single)]
fn sum_both<'a>(input: Both<'a>) -> u32 {
    input.ca.a.iter().sum::<u32>() + input.cb.b.iter().sum::<u32>()
}

Vec, Option, tuples, HashMap / BTreeMap values, and arrays are all recursive — a field type like Vec<MyInput<'a>> or HashMap<String, MyInput<'a>> works without any extra annotation. The derive handles generic type parameters, tuple structs, and unit structs as well.

Two sources of per-call work:

1. Per-field equality check on cache-hit lookup.
2. Output Clone on every return.

drv's ToStatic impls for Arc<T> and (under the imbl feature) imbl's persistent collections take a pointer-equality fast path — O(1) when the field hasn't been mutated since the last miss.

Rule of thumb: scalars are free; small Vec / String is fine; for collections with more than a handful of elements, wrap in Arc<T> or reach for imbl.

Enable imbl:

[dependencies]
drv = { version = "0.4", features = ["imbl"] }

Ranked from most to least alike.

• comemo — closest in spirit. Memoises functions with fine-grained dependency tracking via runtime access recording (#[track]). drv's static input struct is cheaper per call but asks you to declare dependencies up front rather than discovering them at runtime.
• salsa — incremental-computation database used by rust-analyzer. Tracks a dependency graph across queries; much more powerful than drv for deeply chained derivations, and much heavier.
• cached / memoize — general-purpose memoisation via Hash of arguments, backed by HashMap under a lock. Work for any hashable input; drv skips hashing entirely and trades generality for hot-path speed and field-level invalidation.
• moka / quick_cache / stretto — concurrent in-memory cache data structures (Caffeine / Ristretto ports). Not memoisation crates — they're backing stores you'd build a cache on top of. drv's thread-local single-writer model is the opposite design choice.

Dual-licensed under MIT or Apache-2.0, at your option.