Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::return_self_not_must_use: missing `#[must_use]` attribute on a method returning `Self`

What it meansThis lint warns when a method returning Self doesn’t have the #[must_use] attribute. It is in the pedantic group and off by default.

What you see

warning: missing `#[must_use]` attribute on a method returning `Self`
 --> main.rs:4:5
  |
4 | /     pub fn bar(&self) -> Self {
5 | |         Self
6 | |     }
  | |_____^
  |
  = help: consider adding the `#[must_use]` attribute to the method or directly to the `Self` type
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#return_self_not_must_use
  = note: `-W clippy::return-self-not-must-use` 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?

Methods returning Self often create new values, having the #[must_use] attribute prevents users from “forgetting” to use the newly created value. The #[must_use] attribute can be added to the type itself to ensure that instances are never forgotten. Functions returning a type marked with #[must_use] will not be linted, as the usage is already enforced by the type attribute.

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

How to fix it

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

Flagged

pub struct Bar;
impl Bar {
    // Missing attribute
    pub fn bar(&self) -> Self {
        Self
    }
}

Write this instead

// It's better to have the `#[must_use]` attribute on the method like this:
pub struct Bar;
impl Bar {
    #[must_use]
    pub fn bar(&self) -> Self {
        Self
    }
}

// Or on the type definition like this:
#[must_use]
pub struct Bar;
impl Bar {
    pub fn bar(&self) -> Self {
        Self
    }
}

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]
return_self_not_must_use = "warn"

In code

#![warn(clippy::return_self_not_must_use)]

One run

cargo clippy -- -W clippy::return-self-not-must-use

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

nofixed by cargo clippy --fix

1.59.0added in Rust

Clippy's own documentation

What it does

This lint warns when a method returning Self doesn’t have the #[must_use] attribute.

Why is this bad?

Methods returning Self often create new values, having the #[must_use] attribute prevents users from “forgetting” to use the newly created value.

The #[must_use] attribute can be added to the type itself to ensure that instances are never forgotten. Functions returning a type marked with #[must_use] will not be linted, as the usage is already enforced by the type attribute.

Limitations

This lint is only applied on methods taking a self argument. It would be mostly noise if it was added on constructors for example.

Example

pub struct Bar;
impl Bar {
    // Missing attribute
    pub fn bar(&self) -> Self {
        Self
    }
}

Use instead:

// It's better to have the `#[must_use]` attribute on the method like this:
pub struct Bar;
impl Bar {
    #[must_use]
    pub fn bar(&self) -> Self {
        Self
    }
}

// Or on the type definition like this:
#[must_use]
pub struct Bar;
impl Bar {
    pub fn bar(&self) -> Self {
        Self
    }
}

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

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