E0428: Mutate borrowed value

Explanation

Code attempts to mutate storage held under a shared loan, such as a borrow parameter or a collection borrowed by for. The restriction covers the borrowed binding itself and every field or element projected from it, including passing any such place inout.

Likely cause

Code assigns through a shared-borrowed parameter or mutates a collection while iterating it, possibly by forwarding borrowed storage to a mutating operation. Use inout when caller-visible mutation is intended, perform the mutation outside the iteration, or limit the operation to reads.

Examples

Mutate a field through a shared borrow

struct Item { value: i32 }
fn change(borrow item: Item) { item.value = 42; }
fn main() -> i32 {
    let item = Item { value: 0 };
    change(borrow item);
    item.value
}

Read through a shared borrow

struct Item { value: i32 }
fn read(borrow item: Item) -> i32 { item.value }
fn main() -> i32 {
    let item = Item { value: 42 };
    read(borrow item)
}

References

• Borrowed storage is read-only

• For iteration holds a shared loan