RS.CLIPPY.RESULT_UNIT_ERR

Public function returning `Result` with an `Err` type of `()`

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

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.