RS.CLIPPY.ZERO_SIZED_MAP_VALUES

Usage of map with zero-sized value type

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: zero_sized_map_values. Copyright ©2025 The Rust Team. All rights reserved.

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!();
}