String interning for attribute names and identifiers.

Converts heap-allocated String comparisons into cheap u32 comparisons. Every unique string is stored exactly once; lookups and comparisons use the [Symbol] handle (a u32 index).

Performance Impact

Attrset key operations (GetAttr, HasAttr, MakeAttrs, UpdateAttrs) go from O(n) string comparison to O(1) integer comparison. This is the single highest-ROI optimization for Nix evaluation because nixpkgs is dominated by attrset operations.

Storage

Strings are stored as Rc<str>. The forward map's key and the reverse strings vector share the same allocation — no double-allocation on intern, and resolve_rc returns a cheap Rc::clone instead of a full String::from copy. Hashing uses FxHashMap (rustc-hash) — ~2x faster than SipHash for the short strings typical of Nix keys and identifiers.

Thread-Local Helpers

For convenience in single-threaded evaluation, this crate provides [intern()] and [resolve()] free functions that operate on a thread-local [Interner] instance. [prewarm()] pre-interns a curated set of hot nixpkgs symbols so common names get low indices and the hashmap's initial resize cost is paid upfront.