Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::manual_hash_one: manual implementation of `BuildHasher::hash_one`

What it meansChecks for cases where BuildHasher::hash_one can be used. It is in the complexity group and warns by default.

What you see

warning: manual implementation of `BuildHasher::hash_one`
  --> main.rs:11:12
   |
11 | let hash = hasher.finish();
   |            ^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_hash_one
   = note: `#[warn(clippy::manual_hash_one)]` on by default
help: try
   |
 9 + 
10 + 
11 ~ let hash = s.hash_one(&value);
   |

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?

It is more concise to use the hash_one method.

Code that does something simple in a complicated way. 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

use std::hash::{BuildHasher, Hash, Hasher};
use std::collections::hash_map::RandomState;

let s = RandomState::new();
let value = vec![1, 2, 3];

let mut hasher = s.build_hasher();
value.hash(&mut hasher);
let hash = hasher.finish();

Write this instead

use std::hash::BuildHasher;
use std::collections::hash_map::RandomState;

let s = RandomState::new();
let value = vec![1, 2, 3];

let hash = s.hash_one(&value);

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
manual_hash_one = "allow"

One run

cargo clippy -- -A clippy::manual-hash-one

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.75.0added in Rust

Clippy's own documentation

What it does

Checks for cases where BuildHasher::hash_one can be used.

Why is this bad?

It is more concise to use the hash_one method.

Example

use std::hash::{BuildHasher, Hash, Hasher};
use std::collections::hash_map::RandomState;

let s = RandomState::new();
let value = vec![1, 2, 3];

let mut hasher = s.build_hasher();
value.hash(&mut hasher);
let hash = hasher.finish();

Use instead:

use std::hash::BuildHasher;
use std::collections::hash_map::RandomState;

let s = RandomState::new();
let value = vec![1, 2, 3];

let hash = s.hash_one(&value);

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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