Clippy lint · Performance · checked with Clippy 0.1.99

clippy::map_entry: usage of `contains_key` followed by `insert` on a `HashMap`

What it meansChecks for usage of contains_key + insert on HashMap or BTreeMap. It is in the performance group and warns by default.

What you see

warning: usage of `contains_key` followed by `insert` on a `HashMap`
 --> main.rs:4:9
  |
4 |   let _ = if !map.contains_key(&0) {
  |  _________^
5 | |     map.insert(0, 0)
6 | | } else {
7 | |     None
8 | | };
  | |_^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#map_entry
  = note: `#[warn(clippy::map_entry)]` on by default
help: try
  |
4 ~ let _ = if let std::collections::hash_map::Entry::Vacant(e) = map.entry(0) {
5 +     e.insert(0);
6 +     None
7 + } else {
8 +     None
9 ~ };
  |

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Using entry is more efficient.

Code that could be written to run faster. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

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

Write this instead

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

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::map_entry)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
map_entry = "allow"

One run

cargo clippy -- -A clippy::map-entry

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Performancegroup

warndefault level

yesfixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

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);

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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