Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::indexing_slicing: indexing may panic

What it meansChecks for usage of indexing or slicing that may panic at runtime. This lint does not report on indexing or slicing operations that always panic, out_of_bounds_indexing already handles those cases. It is in the restriction group and off by default.

What you see

warning: indexing may panic
 --> main.rs:6:1
  |
6 | x[2];
  | ^^^^
  |
  = help: consider using `.get(n)` or `.get_mut(n)` instead
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#indexing_slicing
  = note: requested on the command line with `-W clippy::indexing-slicing`

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?

To avoid implicit panics from indexing and slicing. There are “checked” alternatives which do not panic, and can be used with unwrap() to make an explicit panic when it is desired.

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

// Vector
let x = vec![0, 1, 2, 3];

x[2];
x[100];
&x[2..100];

// Array
let y = [0, 1, 2, 3];

let i = 10; // Could be a runtime value
let j = 20;
&y[i..j];

Write this instead

x.get(2);
x.get(100);
x.get(2..100);

let i = 10;
let j = 20;
y.get(i..j);

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]
indexing_slicing = "warn"

In code

#![warn(clippy::indexing_slicing)]

One run

cargo clippy -- -W clippy::indexing-slicing

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

allow-indexing-slicing-in-tests: Whether indexing_slicing should be allowed in test functions or #[cfg(test)]
suppress-restriction-lint-in-const: Whether to suppress a restriction lint in constant code. In same cases the restructured operation might not be unavoidable, as the suggested counterparts are unavailable in constant code. This configuration will cause restriction lints to trigger even if no suggestion can be made.

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for usage of indexing or slicing that may panic at runtime.

This lint does not report on indexing or slicing operations that always panic, out_of_bounds_indexing already handles those cases.

Why restrict this?

To avoid implicit panics from indexing and slicing.

There are “checked” alternatives which do not panic, and can be used with unwrap() to make an explicit panic when it is desired.

Limitations

This lint does not check for the usage of indexing or slicing on strings. These are covered by the more specific string_slice lint.

Example

// Vector
let x = vec![0, 1, 2, 3];

x[2];
x[100];
&x[2..100];

// Array
let y = [0, 1, 2, 3];

let i = 10; // Could be a runtime value
let j = 20;
&y[i..j];

Use instead:

x.get(2);
x.get(100);
x.get(2..100);

let i = 10;
let j = 20;
y.get(i..j);

Configuration

allow-indexing-slicing-in-tests: Whether indexing_slicing should be allowed in test functions or #[cfg(test)]

(default: false)
suppress-restriction-lint-in-const: Whether to suppress a restriction lint in constant code. In same cases the restructured operation might not be unavoidable, as the suggested counterparts are unavailable in constant code. This configuration will cause restriction lints to trigger even if no suggestion can be made.

(default: false)

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

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