Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::zero_sized_map_values: Checks for maps with zero-sized value types anywhere in the code

What it meansChecks for maps with zero-sized value types anywhere in the code. It is in the pedantic group and off by default.

Does it matter?

Since there is only a single value for a zero-sized type, a map containing zero sized values is effectively a set. Using a set in that case improves readability and communicates intent more clearly.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

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

Flagged

fn unique_words(text: &str) -> HashMap<&str, ()> {
    todo!();
}

Write this instead

fn unique_words(text: &str) -> HashSet<&str> {
    todo!();
}

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]
zero_sized_map_values = "warn"

In code

#![warn(clippy::zero_sized_map_values)]

One run

cargo clippy -- -W clippy::zero-sized-map-values

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.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.50.0added in Rust

Clippy's own documentation

What it does

Checks for maps with zero-sized value types anywhere in the code.

Why is this bad?

Since there is only a single value for a zero-sized type, a map containing zero sized values is effectively a set. Using a set in that case improves readability and communicates intent more clearly.

Known problems

A zero-sized type cannot be recovered later if it contains private fields.
This lints the signature of public items

Example

fn unique_words(text: &str) -> HashMap<&str, ()> {
    todo!();
}

Use instead:

fn unique_words(text: &str) -> HashSet<&str> {
    todo!();
}

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

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