6.10 Summary

This chapter introduced Rust’s core memory management philosophy, centered around ownership, borrowing, and lifetimes:

  • Ownership: Every value has one owner; when the owner goes out of scope, the value is dropped. Ownership transfers via move semantics for types managing resources (like heap data), both in assignments and function calls/returns.
  • Copy vs. Clone: Simple value types use cheap copy semantics (Copy trait), leaving the original variable valid. Types managing resources require explicit, potentially expensive cloning (Clone trait) for deep copies.
  • Borrowing: References (&T, &mut T) allow temporary access to data without taking ownership. Governed by strict compile-time rules (one mutable OR multiple immutable) that prevent data races and other aliasing bugs, even in single-threaded code. Method calls often use automatic referencing/dereferencing for convenience.
  • Lifetimes: Ensure references never outlive the data they point to, preventing dangling references. Often inferred (elision), but sometimes require explicit annotation ('a) to clarify relationships for the compiler.
  • Slices (&str, &[T]): Non-owning references (borrows) to contiguous sequences of data (like parts of Strings or arrays), enabling flexible function APIs.
  • Smart Pointers (Box, Rc, Arc, RefCell): Provide patterns like heap allocation, shared ownership (single/multi-threaded), and interior mutability, abstracting over raw pointers while maintaining specific safety guarantees. Used for specific scenarios beyond standard stack/collection usage.
  • Unsafe Rust: Allows bypassing some safety checks within designated blocks for low-level control and FFI, requiring manual programmer verification of safety.
  • C Interoperability: Rust provides a robust FFI for calling C code and being called by C.

Mastering ownership, borrowing, and lifetimes is fundamental to writing effective, safe, and performant Rust code. It allows Rust to offer memory safety comparable to garbage-collected languages without the runtime overhead, making it highly suitable for the systems programming tasks familiar to C programmers.