Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::ignored_unit_patterns: matching over `()` is more explicit

What it meansChecks for usage of _ in patterns of type (). It is in the pedantic group and off by default.

What you see

warning: matching over `()` is more explicit
 --> main.rs:4:8
  |
4 |     Ok(_) => println!("Working directory created"),
  |        ^ help: use `()` instead of `_`: `()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#ignored_unit_patterns
  = note: `-W clippy::ignored-unit-patterns` implied by `-W clippy::pedantic`

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?

Matching with () explicitly instead of _ outlines the fact that the pattern contains no data. Also it would detect a type change that _ would ignore.

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

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

match std::fs::create_dir("tmp-work-dir") {
    Ok(_) => println!("Working directory created"),
    Err(s) => eprintln!("Could not create directory: {s}"),
}

Write this instead

match std::fs::create_dir("tmp-work-dir") {
    Ok(()) => println!("Working directory created"),
    Err(s) => eprintln!("Could not create directory: {s}"),
}

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]
ignored_unit_patterns = "warn"

In code

#![warn(clippy::ignored_unit_patterns)]

One run

cargo clippy -- -W clippy::ignored-unit-patterns

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

yesfixed by cargo clippy --fix

1.73.0added in Rust

Clippy's own documentation

What it does

Checks for usage of _ in patterns of type ().

Why is this bad?

Matching with () explicitly instead of _ outlines the fact that the pattern contains no data. Also it would detect a type change that _ would ignore.

Example

match std::fs::create_dir("tmp-work-dir") {
    Ok(_) => println!("Working directory created"),
    Err(s) => eprintln!("Could not create directory: {s}"),
}

Use instead:

match std::fs::create_dir("tmp-work-dir") {
    Ok(()) => println!("Working directory created"),
    Err(s) => eprintln!("Could not create directory: {s}"),
}

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

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