Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::size_of_ref: argument to `size_of_val()` is a reference to a reference

What it meansChecks for calls to size_of_val() where the argument is a reference to a reference. It is in the suspicious group and warns by default.

What you see

warning: argument to `size_of_val()` is a reference to a reference
  --> main.rs:13:9
   |
13 |         size_of_val(&self)
   |         ^^^^^^^^^^^^^^^^^^
   |
   = help: dereference the argument to `size_of_val()` to get the size of the value instead of the size of the reference-type
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#size_of_ref
   = note: `#[warn(clippy::size_of_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?

Calling size_of_val() with a reference to a reference as the argument yields the size of the reference-type, not the size of the value behind the reference.

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

struct Foo {
    buffer: [u8],
}

impl Foo {
    fn size(&self) -> usize {
        // Note that `&self` as an argument is a `&&Foo`: Because `self`
        // is already a reference, `&self` is a double-reference.
        // The return value of `size_of_val()` therefore is the
        // size of the reference-type, not the size of `self`.
        size_of_val(&self)
    }
}

Write this instead

struct Foo {
    buffer: [u8],
}

impl Foo {
    fn size(&self) -> usize {
        // Correct
        size_of_val(self)
    }
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
size_of_ref = "allow"

One run

cargo clippy -- -A clippy::size-of-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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.68.0added in Rust

Clippy's own documentation

What it does

Checks for calls to size_of_val() where the argument is a reference to a reference.

Why is this bad?

Calling size_of_val() with a reference to a reference as the argument yields the size of the reference-type, not the size of the value behind the reference.

Example

struct Foo {
    buffer: [u8],
}

impl Foo {
    fn size(&self) -> usize {
        // Note that `&self` as an argument is a `&&Foo`: Because `self`
        // is already a reference, `&self` is a double-reference.
        // The return value of `size_of_val()` therefore is the
        // size of the reference-type, not the size of `self`.
        size_of_val(&self)
    }
}

Use instead:

struct Foo {
    buffer: [u8],
}

impl Foo {
    fn size(&self) -> usize {
        // Correct
        size_of_val(self)
    }
}

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

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