Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::string_from_utf8_as_bytes: calling a slice of `as_bytes()` with `from_utf8` should be not necessary

What it meansCheck if the string is transformed to byte array and cast back to string. It is in the complexity group and warns by default.

What you see

warning: calling a slice of `as_bytes()` with `from_utf8` should be not necessary
 --> main.rs:3:1
  |
3 | std::str::from_utf8(&"Hello World!".as_bytes()[6..11]).unwrap();
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Some(&"Hello World!"[6..11])`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#string_from_utf8_as_bytes
  = note: `#[warn(clippy::string_from_utf8_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’s unnecessary, the string can be used directly.

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

std::str::from_utf8(&"Hello World!".as_bytes()[6..11]).unwrap();

Write this instead

&"Hello World!"[6..11];

Turn it off, or on

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

On one item

#[allow(clippy::string_from_utf8_as_bytes)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
string_from_utf8_as_bytes = "allow"

One run

cargo clippy -- -A clippy::string-from-utf8-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.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.50.0added in Rust

Clippy's own documentation

What it does

Check if the string is transformed to byte array and cast back to string.

Why is this bad?

It’s unnecessary, the string can be used directly.

Example

std::str::from_utf8(&"Hello World!".as_bytes()[6..11]).unwrap();

Use instead:

&"Hello World!"[6..11];

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

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