Explain Rust code

What this Rust sample shows

traits

Available names one method, and impl Available for Stock gives Stock that method. first_available takes a slice of any type T that implements the trait; here that is Stock, and an item with a quantity of 0 doesn't count as available.

lifetimes

'a ties the result to the slice: the &'a T that comes back points at the item inside items itself, not at a copy. Rust would infer the same lifetimes here: a version without 'a compiles too, because only one reference goes in.

borrowing

first_available takes &[T], a shared borrow, and reserve takes &mut Stock, a mutable one. While the item first_available(&items) returned is still in use, reserve(&mut items[0]) doesn't compile: rustc stops with error E0502, cannot borrow items as mutable because it is also borrowed as immutable.

Good to know

• With an empty item first, then box (quantity 2) and bag (quantity 1), first_available skips the empty one and returns box.
• reserve returns true twice for box, then false, and the quantity stays at 0. The next first_available call returns bag.
• find gives None for an empty slice or when nothing is available, so the caller has to handle the Option before it can read sku.

Watch out: The if stock.quantity == 0 check matters because quantity is a u32, which can't go below 0. Without it, stock.quantity -= 1 on 0 panics with attempt to subtract with overflow in a debug build, and in a build with -O, where overflow checks are off by default, it quietly wraps to 4294967295.