Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::iter_over_hash_type: iteration over unordered hash-based type

What it meansThis is a restriction lint which prevents the use of hash types (i.e., HashSet and HashMap) in for loops. It is in the restriction group and off by default.

What you see

warning: iteration over unordered hash-based type
 --> main.rs:4:5
  |
4 |     for (key, value) in my_map { /* ... */ }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#iter_over_hash_type
  = note: requested on the command line with `-W clippy::iter-over-hash-type`

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?

Because hash types are unordered, when iterated through such as in a for loop, the values are returned in an undefined order. As a result, on redundant systems this may cause inconsistencies and anomalies. In addition, the unknown order of the elements may reduce readability or introduce other undesired side effects.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

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

Flagged

    let my_map = std::collections::HashMap::<i32, String>::new();
    for (key, value) in my_map { /* ... */ }

Write this instead

    let my_map = std::collections::HashMap::<i32, String>::new();
    let mut keys = my_map.keys().clone().collect::<Vec<_>>();
    keys.sort();
    for key in keys {
        let value = &my_map[key];
    }

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]
iter_over_hash_type = "warn"

In code

#![warn(clippy::iter_over_hash_type)]

One run

cargo clippy -- -W clippy::iter-over-hash-type

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.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.76.0added in Rust

Clippy's own documentation

What it does

This is a restriction lint which prevents the use of hash types (i.e., HashSet and HashMap) in for loops.

Why restrict this?

Because hash types are unordered, when iterated through such as in a for loop, the values are returned in an undefined order. As a result, on redundant systems this may cause inconsistencies and anomalies. In addition, the unknown order of the elements may reduce readability or introduce other undesired side effects.

Example

    let my_map = std::collections::HashMap::<i32, String>::new();
    for (key, value) in my_map { /* ... */ }

Use instead:

    let my_map = std::collections::HashMap::<i32, String>::new();
    let mut keys = my_map.keys().clone().collect::<Vec<_>>();
    keys.sort();
    for key in keys {
        let value = &my_map[key];
    }

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

Up next

Also worth your time

Did this answer your question?