Week 3 - Ownership, Borrowing, and Region Inference¶

3.1 Conceptual Core¶

Ownership is destructor scheduling: the owner is the entity that will run Drop::drop. There is exactly one.
Borrowing is temporary capability delegation: &T grants read capability, &mut T grants exclusive read+write capability. A capability cannot outlive the resource that backs it (the lifetime constraint).
Lifetimes are not durations. They are region variables that the compiler infers under a system of inequality constraints ('a: 'b means region 'a outlives region 'b). The compiler does not know "how long" anything lives in seconds-only the partial order of regions.

3.2 Mechanical Detail¶

The three borrow-checker rules, stated formally:
At any program point, for any place p: at most one &mut p or any number of &p, never both.
References must be valid for their entire region.
The owner cannot mutate or move the value while a borrow is active (this is what NLL-non-lexical lifetimes-relaxed).
Two-phase borrows (v.push(v.len())): why this compiles even though it looks like aliasing.
Reborrowing: &mut *r produces a fresh &mut with a shorter lifetime. This is the foundation for passing &mut references into functions repeatedly.

3.3 Lab-"Defeat the Borrow Checker, Then Submit"¶

You will be given (as exercise files) ten programs that the borrow checker rejects. For each: 1. Predict which rule is violated before reading the diagnostic. 2. Fix it three different ways (e.g., scope shrinking, split borrow, Cell/RefCell). 3. Pick the idiomatic fix and justify it in a one-line comment-but only if the comment captures non-obvious reasoning. (See feedback rule on comments.)

3.4 Idiomatic & Clippy Drill¶

clippy::needless_lifetimes, clippy::redundant_clone, clippy::ptr_arg. The first two are about elision; the third is about API ergonomics.

3.5 Production Hardening Slice¶

Run cargo clippy --workspace --all-targets -- -D warnings in CI from week 1 forward. This is non-negotiable.