error[E0594]: cannot assign to data in an index of `BTreeMap` --> $DIR/btreemap-index-mut-2.rs:6:9 | LL | map[&0] = 1; | ^^^^^^^^^^^ cannot assign | = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `BTreeMap` help: to modify a `BTreeMap`, use `.get_mut()`, `.insert()` or the entry API | LL | map.insert(&0, 1); | ~~~~~~~~ ~ + LL | map.get_mut(&0).map(|val| { *val = 1; }); | ~~~~~~~~~ ~~~~~~~~~~~~~~~~~~ ++++ LL | let val = map.entry(&0).or_insert(1); | +++++++++ ~~~~~~~ ~~~~~~~~~~~~ + error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0594`.