Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::unit_hash: Detects ().hash(_)

What it meansDetects ().hash(_). It is in the correctness group and denied by default (the build fails).

Does it matter?

Hashing a unit value doesn’t do anything as the implementation of Hash for () is a no-op.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

match my_enum {
	Empty => ().hash(&mut state),
	WithValue(x) => x.hash(&mut state),
}

Write this instead

match my_enum {
	Empty => 0_u8.hash(&mut state),
	WithValue(x) => x.hash(&mut state),
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unit_hash = "allow"

One run

cargo clippy -- -A clippy::unit-hash

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.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

1.58.0added in Rust

Clippy's own documentation

What it does

Detects ().hash(_).

Why is this bad?

Hashing a unit value doesn’t do anything as the implementation of Hash for () is a no-op.

Example

match my_enum {
	Empty => ().hash(&mut state),
	WithValue(x) => x.hash(&mut state),
}

Use instead:

match my_enum {
	Empty => 0_u8.hash(&mut state),
	WithValue(x) => x.hash(&mut state),
}

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?