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A struct is a value, so a function changes yours only if you hand it over with `&mut`

:= binds a name that cannot be reassigned, let declares a variable that can be reassigned as a whole, and let mut also lets its fields change in place. A struct is a value: before := order is a copy, and a parameter order: Order is the function's own copy, so receipt cannot change yours however it is written. A function that should change the caller's order says so in its signature, order: &mut Order, and the caller says so again at the call, &mut order, which needs a let mut variable. To find every place an order can change, search for &mut.

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fn apply_discount(order: &mut Order, percent: i32) -> void: # ← changes the caller's order order.total_cents = order.total_cents * (100 - percent) / 100fn receipt(order: Order) -> string: # ← its own copy: nothing it does reaches the caller "${order.id}: ${order.total_cents} cents"pub fn main() -> void $ Console: let mut order = Order { id: "A-1042", total_cents: 5000 } before := order # ← a copy: later changes do not reach it apply_discount(&mut order, 10) println(receipt(before)) println(receipt(order))# Change `&mut Order` to `&Order` in apply_discount's parameter and Run:# not-writable: this writes through a read-only reference &Order; take the reference with `&mut` to write through it# ── plumbing ──struct Order: id: string total_cents: i32

A struct is a value, so a function changes yours only if you hand it over with &mut

A struct is a value, so a function changes yours only if you hand it over with `&mut`