Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::set_contains_or_insert: usage of `HashSet::insert` after `HashSet::contains`

What it meansChecks for usage of contains to see if a value is not present in a set like HashSet or BTreeSet, followed by an insert. It is in the nursery group and off by default.

What you see

warning: usage of `HashSet::insert` after `HashSet::contains`
 --> main.rs:6:9
  |
6 | if !set.contains(&value) {
  |         ^^^^^^^^^^^^^^^^
7 |     set.insert(value);
  |         ^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#set_contains_or_insert
  = note: `-W clippy::set-contains-or-insert` implied by `-W clippy::nursery`

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 just insert and checking the returned bool is more efficient.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

use std::collections::HashSet;
let mut set = HashSet::new();
let value = 5;
if !set.contains(&value) {
    set.insert(value);
    println!("inserted {value:?}");
}

Write this instead

use std::collections::HashSet;
let mut set = HashSet::new();
let value = 5;
if set.insert(&value) {
    println!("inserted {value:?}");
}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
set_contains_or_insert = "warn"

In code

#![warn(clippy::set_contains_or_insert)]

One run

cargo clippy -- -W clippy::set-contains-or-insert

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.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.81.0added in Rust

Clippy's own documentation

What it does

Checks for usage of contains to see if a value is not present in a set like HashSet or BTreeSet, followed by an insert.

Why is this bad?

Using just insert and checking the returned bool is more efficient.

Known problems

In case the value that wants to be inserted is borrowed and also expensive or impossible to clone. In such a scenario, the developer might want to check with contains before inserting, to avoid the clone. In this case, it will report a false positive.

Example

use std::collections::HashSet;
let mut set = HashSet::new();
let value = 5;
if !set.contains(&value) {
    set.insert(value);
    println!("inserted {value:?}");
}

Use instead:

use std::collections::HashSet;
let mut set = HashSet::new();
let value = 5;
if set.insert(&value) {
    println!("inserted {value:?}");
}

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

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