Clippy lint · Style · checked with Clippy 0.1.99

clippy::result_unit_err: this returns a `Result<_, ()>`

What it meansChecks for public functions that return a Result with an Err type of (). It suggests using a custom type that implements std::error::Error. It is in the style group and warns by default.

What you see

warning: this returns a `Result<_, ()>`
 --> main.rs:1:1
  |
1 | pub fn read_u8() -> Result<u8, ()> { Err(()) }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: use a custom `Error` type instead
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#result_unit_err
  = note: `#[warn(clippy::result_unit_err)]` on by default

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?

Unit does not implement Error and carries no further information about what went wrong.

Code that works but is not idiomatic Rust. Warns by default.

How to fix it

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

Flagged

pub fn read_u8() -> Result<u8, ()> { Err(()) }

Write this instead

use std::fmt;

#[derive(Debug)]
pub struct EndOfStream;

impl fmt::Display for EndOfStream {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "End of Stream")
    }
}

impl std::error::Error for EndOfStream { }

pub fn read_u8() -> Result<u8, EndOfStream> { Err(EndOfStream) }

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
result_unit_err = "allow"

One run

cargo clippy -- -A clippy::result-unit-err

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.

Stylegroup

warndefault level

nofixed by cargo clippy --fix

1.49.0added in Rust

Clippy's own documentation

What it does

Checks for public functions that return a Result with an Err type of (). It suggests using a custom type that implements std::error::Error.

Why is this bad?

Unit does not implement Error and carries no further information about what went wrong.

Known problems

Of course, this lint assumes that Result is used for a fallible operation (which is after all the intended use). However code may opt to (mis)use it as a basic two-variant-enum. In that case, the suggestion is misguided, and the code should use a custom enum instead.

Examples

pub fn read_u8() -> Result<u8, ()> { Err(()) }

should become

use std::fmt;

#[derive(Debug)]
pub struct EndOfStream;

impl fmt::Display for EndOfStream {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "End of Stream")
    }
}

impl std::error::Error for EndOfStream { }

pub fn read_u8() -> Result<u8, EndOfStream> { Err(EndOfStream) }

Note that there are crates that simplify creating the error type, e.g. thiserror.

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

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