Clippy lint · Performance · checked with Clippy 0.1.99

clippy::sliced_string_as_bytes: calling `as_bytes` after slicing a string

What it meansChecks for string slices immediately followed by as_bytes. It is in the performance group and warns by default.

What you see

warning: calling `as_bytes` after slicing a string
 --> main.rs:4:1
  |
4 | s[1..5].as_bytes();
  | ^^^^^^^^^^^^^^^^^^ help: try: `&s.as_bytes()[1..5]`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#sliced_string_as_bytes
  = note: `#[warn(clippy::sliced_string_as_bytes)]` on by default

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 involves doing an unnecessary UTF-8 alignment check which is less efficient, and can cause a panic.

Code that could be written to run faster. Warns by default.

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 s = "Lorem ipsum";
s[1..5].as_bytes();

Write this instead

let s = "Lorem ipsum";
&s.as_bytes()[1..5];

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
sliced_string_as_bytes = "allow"

One run

cargo clippy -- -A clippy::sliced-string-as-bytes

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.

Performancegroup

warndefault level

nofixed by cargo clippy --fix

1.86.0added in Rust

Clippy's own documentation

What it does

Checks for string slices immediately followed by as_bytes.

Why is this bad?

It involves doing an unnecessary UTF-8 alignment check which is less efficient, and can cause a panic.

Known problems

In some cases, the UTF-8 validation and potential panic from string slicing may be required for the code’s correctness. If you need to ensure the slice boundaries fall on valid UTF-8 character boundaries, the original form (s[1..5].as_bytes()) should be preferred.

Example

let s = "Lorem ipsum";
s[1..5].as_bytes();

Use instead:

let s = "Lorem ipsum";
&s.as_bytes()[1..5];

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

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