Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::index_refutable_slice: this binding can be a slice pattern to avoid indexing

What it meansThe lint checks for slice bindings in patterns that are only used to access individual slice values. It is in the pedantic group and off by default.

What you see

warning: this binding can be a slice pattern to avoid indexing
 --> main.rs:5:13
  |
5 | if let Some(slice) = slice {
  |             ^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#index_refutable_slice
  = note: `-W clippy::index-refutable-slice` implied by `-W clippy::pedantic`
help: replace the binding and indexed access with a slice pattern
  |
5 ~ if let Some([slice_0, ..]) = slice {
6 ~     println!("{}", slice_0);
  |

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?

Accessing slice values using indices can lead to panics. Using refutable patterns can avoid these. Binding to individual values also improves the readability as they can be named.

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 (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let slice: Option<&[u32]> = Some(&[1, 2, 3]);

if let Some(slice) = slice {
    println!("{}", slice[0]);
}

Write this instead

let slice: Option<&[u32]> = Some(&[1, 2, 3]);

if let Some(&[first, ..]) = slice {
    println!("{}", first);
}

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]
index_refutable_slice = "warn"

In code

#![warn(clippy::index_refutable_slice)]

One run

cargo clippy -- -W clippy::index-refutable-slice

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

max-suggested-slice-pattern-length: When Clippy suggests using a slice pattern, this is the maximum number of elements allowed in the slice pattern that is suggested. If more elements are necessary, the lint is suppressed. For example, [_, _, _, e, ..] is a slice pattern with 4 elements.
msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.59.0added in Rust

Clippy's own documentation

What it does

The lint checks for slice bindings in patterns that are only used to access individual slice values.

Why is this bad?

Accessing slice values using indices can lead to panics. Using refutable patterns can avoid these. Binding to individual values also improves the readability as they can be named.

Limitations

This lint currently only checks for immutable access inside if let patterns.

Example

let slice: Option<&[u32]> = Some(&[1, 2, 3]);

if let Some(slice) = slice {
    println!("{}", slice[0]);
}

Use instead:

let slice: Option<&[u32]> = Some(&[1, 2, 3]);

if let Some(&[first, ..]) = slice {
    println!("{}", first);
}

Configuration

max-suggested-slice-pattern-length: When Clippy suggests using a slice pattern, this is the maximum number of elements allowed in the slice pattern that is suggested. If more elements are necessary, the lint is suppressed. For example, [_, _, _, e, ..] is a slice pattern with 4 elements.

(default: 3)
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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