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Two holders see one object only when you write `&mut`

Sometimes several parts of a program must change one thing: two checkout lanes selling from the same stock, a connection table, a screen's state. A plain field holds a copy, so each lane would sell the same five units. A field of type &mut Stock holds a reference to one object instead, and &mut Stock { ... } makes that object. Every holder of the reference sees the others' writes, and &mut is not exclusive, so there is no borrow to juggle. A reference keeps its object alive for as long as anyone holds it, so it can be stored and returned.

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struct Stock: units: i32struct Lane: name: string stock: *mut Stock # ← every lane holds the same objectfn sell(lane: Lane, units: i32) -> void $ Console: if lane.stock.units < units: println("${lane.name}: only ${lane.stock.units} left") return lane.stock.units -= units # ← written through the shared reference println("${lane.name}: sold $units")pub fn main() -> void $ Console: stock := &mut Stock { units: 5 } # ← one new object front := Lane { name: "front", stock: stock } back := Lane { name: "back", stock: stock } sell(front, 3) sell(back, 3) println("left: ${stock.units}")# Delete `&mut ` from the `stock :=` line and Run. A plain# value is no reference, so no lane could share it:# type-mismatch: in field: expected *mut Stock, found Stock

Two holders see one object only when you write &mut

Two holders see one object only when you write `&mut`