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`for p in &mut players` changes the elements in place, and the list holds still while you do

A plain for p in players binds each element as a read-only copy, so a loop that writes p.score would change nothing, and the compiler rejects it. To change the elements, loop over &mut players: each step binds a writable reference to the element's slot. While such a reference lives, the list may not grow, shrink, or move its elements, because the reference would point at moved memory. A push in the loop's body panics with collection-pinned, and replacing or sorting elements is still fine. &mut players[0] passed to a call holds the list still for that call only, and no name may keep it: write players[0].score += 1 instead. bonus takes &mut List[Player] so that it changes the caller's list, not a copy.

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fn bonus(players: &mut List[Player], points: i32) -> void: for p in &mut players: # ← a writable reference per element; the list is pinned p.score += pointsfn leader(players: List[Player]) -> Player: let mut best = players[0] for p in players: # ← a snapshot of the list: read-only copies if p.score > best.score: best = p bestpub fn main() -> void $ Console: let mut players = [Player { name: "ada", score: 10 }, Player { name: "grace", score: 12 }] bonus(&mut players, 5) players[0].score += 1 # ← the element is a place best := leader(players) println("${best.name}: ${best.score}") println(players[0].score)# Change the loop in `bonus` to `for p in players:` and Run:# not-writable: `p` is not `let mut`, so it may only be reassigned as a whole; bind it with `let mut` to change a field# ── plumbing ──struct Player: name: string score: i32

for p in &mut players changes the elements in place, and the list holds still while you do

`for p in &mut players` changes the elements in place, and the list holds still while you do