Appendix C — Glossary

• Borrow: a reference (&T or &mut T) that uses a value without owning it.
• Borrow checker: the compiler pass that enforces shared XOR mutable and "no reference outlives its owner", running on MIR.
• Crate: a compilation unit (library or binary). Package: a Cargo.toml with one or more crates.
• Drop: the destructor. It runs when the owner goes out of scope.
• Fat pointer: a two-word pointer: slice (ptr + len) or trait object (ptr + vtable).
• Future: a value that produces a result when polled to completion. Lazy until polled.
• Interior mutability: mutation through &T, made safe by runtime checks or atomics (Cell, RefCell, Mutex, atomics).
• Lifetime: a compile-time name for the region where a borrow must be valid.
• Monomorphization: compiling a generic function once per concrete type.
• Move: transferring ownership. A bitwise copy, after which the source can no longer be used.
• Niche: an invalid bit pattern of a type that an enum uses as its tag, for example None in Option<&T>.
• Orphan rule: you can implement a trait for a type only if your crate defines the trait or the type.
• Pin: a guarantee that a value will not move in memory again; needed for self-referential futures.
• Send / Sync: auto traits for "can be moved to another thread" and "can be shared between threads by reference".
• Typestate: encoding an object's state in its type, so invalid operations do not compile.
• UB (undefined behavior): what happens if unsafe code breaks the rules. The compiler may assume it never happens.
• Zero-cost abstraction: an abstraction that compiles to the same code you would write by hand.