RS.CLIPPY.MAP_ENTRY

Use of `contains_key` followed by `insert` on a `HashMap` or `BTreeMap`

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: map_entry. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks for usage of contains_key + insert on HashMap or BTreeMap.

Why is this bad?

Using entry is more efficient.

Known problems

The suggestion may have type inference errors in some cases. e.g.

let mut map = std::collections::HashMap::new();
let _ = if !map.contains_key(&0) {
    map.insert(0, 0)
} else {
    None
};

Example

if !map.contains_key(&k) {
    map.insert(k, v);
}

Use instead:

map.entry(k).or_insert(v);