Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::derivable_impls: this `impl` can be derived

What it meansDetects manual std::default::Default implementations that are identical to a derived implementation. It is in the complexity group and warns by default.

What you see

warning: this `impl` can be derived
  --> main.rs:7:1
   |
 7 | / impl Default for Foo {
 8 | |     fn default() -> Self {
 9 | |         Self {
10 | |             bar: false
...  |
13 | | }
   | |_^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#derivable_impls
   = note: `#[warn(clippy::derivable_impls)]` on by default
help: replace the manual implementation with a derive attribute
   |
 3 + #[derive(Default)]
 4 | struct Foo {
   |

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?

It is less concise.

Code that does something simple in a complicated way. Warns by default.

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

struct Foo {
    bar: bool
}

impl Default for Foo {
    fn default() -> Self {
        Self {
            bar: false
        }
    }
}

Write this instead

#[derive(Default)]
struct Foo {
    bar: bool
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
derivable_impls = "allow"

One run

cargo clippy -- -A clippy::derivable-impls

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.57.0added in Rust

Clippy's own documentation

What it does

Detects manual std::default::Default implementations that are identical to a derived implementation.

Why is this bad?

It is less concise.

Example

struct Foo {
    bar: bool
}

impl Default for Foo {
    fn default() -> Self {
        Self {
            bar: false
        }
    }
}

Use instead:

#[derive(Default)]
struct Foo {
    bar: bool
}

Known problems

Derive macros sometimes use incorrect bounds in generic types and the user defined impl may be more generalized or specialized than what derive will produce. This lint can’t detect the manual impl has exactly equal bounds, and therefore this lint is disabled for types with generic parameters.

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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