Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::borrow_deref_ref: deref on an immutable reference

What it meansChecks for &*(&T). It is in the complexity group and warns by default.

What you see

warning: deref on an immutable reference
 --> main.rs:5:15
  |
5 | let addr_y = &&*x as *const _ as usize; // assert ok now, and lint triggered.
  |               ^^^ help: if you would like to reborrow, try removing `&*`: `x`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#borrow_deref_ref
  = note: `#[warn(clippy::borrow_deref_ref)]` 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?

Dereferencing and then borrowing a reference value has no effect in most cases.

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

let x = &12;
let addr_x = &x as *const _ as usize;
let addr_y = &&*x as *const _ as usize; // assert ok now, and lint triggered.
                                        // But if we fix it, assert will fail.
assert_ne!(addr_x, addr_y);

Write this instead

let s = &String::new();

let a: &String = &* s;

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
borrow_deref_ref = "allow"

One run

cargo clippy -- -A clippy::borrow-deref-ref

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

1.63.0added in Rust

Clippy's own documentation

What it does

Checks for &*(&T).

Why is this bad?

Dereferencing and then borrowing a reference value has no effect in most cases.

Known problems

False negative on such code:

let x = &12;
let addr_x = &x as *const _ as usize;
let addr_y = &&*x as *const _ as usize; // assert ok now, and lint triggered.
                                        // But if we fix it, assert will fail.
assert_ne!(addr_x, addr_y);

Example

let s = &String::new();

let a: &String = &* s;

Use instead:

let a: &String = s;

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

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