clippy::mutable_key_type: Checks for sets/maps with mutable key types

use std::cmp::{PartialEq, Eq};
use std::collections::HashSet;
use std::hash::{Hash, Hasher};
use std::sync::atomic::AtomicUsize;

struct Bad(AtomicUsize);
impl PartialEq for Bad {
    fn eq(&self, rhs: &Self) -> bool {
         ..
    }
}

impl Eq for Bad {}

impl Hash for Bad {
    fn hash<H: Hasher>(&self, h: &mut H) {
        ..
    }
}

fn main() {
    let _: HashSet<Bad> = HashSet::new();
}

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

[lints.clippy]
mutable_key_type = "allow"

cargo clippy -- -A clippy::mutable-key-type

Clippy's own documentation

What it does

Checks for sets/maps with mutable key types.

Why is this bad?

All of HashMap, HashSet, BTreeMap and BtreeSet rely on either the hash or the order of keys be unchanging, so having types with interior mutability is a bad idea.

Known problems

False Positives

It’s correct to use a struct that contains interior mutability as a key when its implementation of Hash or Ord doesn’t access any of the interior mutable types. However, this lint is unable to recognize this, so it will often cause false positives in these cases.

False Negatives

This lint does not follow raw pointers (*const T or *mut T) as Hash and Ord apply only to the address of the contained value. This can cause false negatives for custom collections that use raw pointers internally.

Example

use std::cmp::{PartialEq, Eq};
use std::collections::HashSet;
use std::hash::{Hash, Hasher};
use std::sync::atomic::AtomicUsize;

struct Bad(AtomicUsize);
impl PartialEq for Bad {
    fn eq(&self, rhs: &Self) -> bool {
         ..
    }
}

impl Eq for Bad {}

impl Hash for Bad {
    fn hash<H: Hasher>(&self, h: &mut H) {
        ..
    }
}

fn main() {
    let _: HashSet<Bad> = HashSet::new();
}

Configuration

• ignore-interior-mutability: A list of paths to types that should be treated as if they do not contain interior mutability

  (default: ["bytes::Bytes"])

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

Related lints

• missing_transmute_annotationsChecks if transmute calls have all generics specified
• multi_assignmentsassignments don't nest intuitively
• mut_range_boundattempt to mutate range bound within loop
• needless_character_iterationchecking if a string is ascii using iterators
• needless_maybe_sized`?Sized` bound is ignored because of a `Sized` requirement
• no_effect_replacereplacing text with itself

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker