error: usage of `contains_key` followed by `insert` on a `BTreeMap`
  --> $DIR/entry_btree.rs:12:5
   |
LL | /     if !m.contains_key(&k) {
LL | |         m.insert(k, v);
LL | |         foo();
LL | |     }
   | |_____^
   |
   = note: `-D clippy::map-entry` implied by `-D warnings`
help: try this
   |
LL ~     if let std::collections::btree_map::Entry::Vacant(e) = m.entry(k) {
LL +         e.insert(v);
LL +         foo();
LL +     }
   |

error: aborting due to previous error