clippy::unnecessary_unwrap_unchecked: usage of `unwrap_unchecked` when an `_unchecked` variant of the function exists

What you see

warning: usage of `unwrap_unchecked` when an `_unchecked` variant of the function exists
 --> main.rs:3:18
  |
3 | let s = unsafe { std::str::from_utf8(&[]).unwrap_unchecked() };
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_unwrap_unchecked
  = note: `#[warn(clippy::unnecessary_unwrap_unchecked)]` on by default
help: use `std::str::from_utf8_unchecked` instead, and remove the call to `.unwrap_unchecked()`
  |
3 - let s = unsafe { std::str::from_utf8(&[]).unwrap_unchecked() };
3 + let s = unsafe { std::str::from_utf8_unchecked(&[]) };
  |

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?

Calling the non-unchecked variant may result in checking that is then discarded if unwrap_unchecked is called directly afterwards, whereas the unchecked variant most likely avoids performing the check completely.

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 s = unsafe { std::str::from_utf8(&[]).unwrap_unchecked() };

Write this instead

let s = unsafe { std::str::from_utf8_unchecked(&[]) };

Turn it off, or on

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

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

[lints.clippy]
unnecessary_unwrap_unchecked = "allow"

cargo clippy -- -A clippy::unnecessary-unwrap-unchecked

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.