Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::unwrap_in_result: `expect` used in a function that returns a `Result`

What it meansChecks for functions of type Result that contain expect() or unwrap() It is in the restriction group and off by default.

What you see

warning: `expect` used in a function that returns a `Result`
 --> main.rs:4:13
  |
4 |       let i = i_str
  |  _____________^
5 | |         .parse::<i32>()
6 | |         .expect("cannot divide the input by three");
  | |___________________________________________________^
  |
note: in this function signature
 --> main.rs:3:37
  |
3 | fn divisible_by_3(i_str: String) -> Result<(), String> {
  |                                     ^^^^^^^^^^^^^^^^^^
  = help: consider using the `?` operator or calling the `.map_err()` method
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unwrap_in_result
  = note: requested on the command line with `-W clippy::unwrap-in-result`

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?

These functions promote recoverable errors to non-recoverable errors, which may be undesirable in code bases which wish to avoid panics, or be a bug in the specific function.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

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

Flagged

fn divisible_by_3(i_str: String) -> Result<(), String> {
    let i = i_str
        .parse::<i32>()
        .expect("cannot divide the input by three");

    if i % 3 != 0 {
        Err("Number is not divisible by 3")?
    }

    Ok(())
}

Write this instead

fn divisible_by_3(i_str: String) -> Result<(), String> {
    let i = i_str
        .parse::<i32>()
        .map_err(|e| format!("cannot divide the input by three: {}", e))?;

    if i % 3 != 0 {
        Err("Number is not divisible by 3")?
    }

    Ok(())
}

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]
unwrap_in_result = "warn"

In code

#![warn(clippy::unwrap_in_result)]

One run

cargo clippy -- -W clippy::unwrap-in-result

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.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.48.0added in Rust

Clippy's own documentation

What it does

Checks for functions of type Result that contain expect() or unwrap()

Why restrict this?

These functions promote recoverable errors to non-recoverable errors, which may be undesirable in code bases which wish to avoid panics, or be a bug in the specific function.

Known problems

This can cause false positives in functions that handle both recoverable and non recoverable errors.

Example

Before:

fn divisible_by_3(i_str: String) -> Result<(), String> {
    let i = i_str
        .parse::<i32>()
        .expect("cannot divide the input by three");

    if i % 3 != 0 {
        Err("Number is not divisible by 3")?
    }

    Ok(())
}

After:

fn divisible_by_3(i_str: String) -> Result<(), String> {
    let i = i_str
        .parse::<i32>()
        .map_err(|e| format!("cannot divide the input by three: {}", e))?;

    if i % 3 != 0 {
        Err("Number is not divisible by 3")?
    }

    Ok(())
}

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

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