Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::transmute_bytes_to_str: transmute from a `&[u8]` to a `&str`

What it meansChecks for transmutes from a &[u8] to a &str. It is in the complexity group and warns by default.

What you see

warning: transmute from a `&[u8]` to a `&str`
 --> main.rs:5:19
  |
5 |     let _: &str = std::mem::transmute(b); // where b: &[u8]
  |                   ^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::str::from_utf8(b).unwrap()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#transmute_bytes_to_str
  = note: `#[warn(clippy::transmute_bytes_to_str)]` 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?

Not every byte slice is a valid UTF-8 string.

Code that does something simple in a complicated way. 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 b: &[u8] = &[1_u8, 2_u8];
unsafe {
    let _: &str = std::mem::transmute(b); // where b: &[u8]
}

// should be:
let _ = std::str::from_utf8(b).unwrap();

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
transmute_bytes_to_str = "allow"

One run

cargo clippy -- -A clippy::transmute-bytes-to-str

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

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for transmutes from a &[u8] to a &str.

Why is this bad?

Not every byte slice is a valid UTF-8 string.

Known problems

from_utf8 which this lint suggests using is slower than transmute as it needs to validate the input. If you are certain that the input is always a valid UTF-8, use from_utf8_unchecked which is as fast as transmute but has a semantically meaningful name.
You might want to handle errors returned from from_utf8 instead of calling unwrap.

Example

let b: &[u8] = &[1_u8, 2_u8];
unsafe {
    let _: &str = std::mem::transmute(b); // where b: &[u8]
}

// should be:
let _ = std::str::from_utf8(b).unwrap();

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

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