clippy::toplevel_ref_arg: `ref` directly on a function parameter does not prevent taking ownership of the passed argument. Consider using a reference type instead

What you see

warning: `ref` directly on a function parameter does not prevent taking ownership of the passed argument. Consider using a reference type instead
 --> main.rs:3:8
  |
3 | fn foo(ref _x: u8) {}
  |        ^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#toplevel_ref_arg
  = note: `#[warn(clippy::toplevel_ref_arg)]` 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?

The ref declaration makes the function take an owned value, but turns the argument into a reference (which means that the value is destroyed when exiting the function). This adds not much value: either take a reference type, or take an owned value and create references in the body. For let bindings, let x = &foo; is preferred over let ref x = foo. The type of x is more obvious with the former.

Code that works but is not idiomatic Rust. 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

fn foo(ref _x: u8) {}

Write this instead

fn foo(_x: &u8) {}

Turn it off, or on

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

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

[lints.clippy]
toplevel_ref_arg = "allow"

cargo clippy -- -A clippy::toplevel-ref-arg

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.