Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::cast_slice_different_sizes: casting between raw pointers to `[i32]` (element size 4) and `[u8]` (element size 1) does not adjust the count

What it meansChecks for as casts between raw pointers to slices with differently sized elements. It is in the correctness group and denied by default (the build fails).

What you see

error: casting between raw pointers to `[i32]` (element size 4) and `[u8]` (element size 1) does not adjust the count
 --> main.rs:4:9
  |
4 | let p = &a as *const [i32] as *const [u8];
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with `ptr::slice_from_raw_parts`: `core::ptr::slice_from_raw_parts(&a as *const [i32] as *const u8, ..)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#cast_slice_different_sizes
  = note: `#[deny(clippy::cast_slice_different_sizes)]` 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 produced raw pointer to a slice does not update its length metadata. The produced pointer will point to a different number of bytes than the original pointer because the length metadata of a raw slice pointer is in elements rather than bytes. Producing a slice reference from the raw pointer will either create a slice with less data (which can be surprising) or create a slice with more data and cause Undefined Behavior.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

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 a = [1_i32, 2, 3, 4];
let p = &a as *const [i32] as *const [u8];
unsafe {
    println!("{:?}", &*p);
}

Write this instead

let a = [1_u8, 2, 3, 4];
let p = &a as *const [u8] as *const [u32];
unsafe {
    println!("{:?}", &*p);
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
cast_slice_different_sizes = "allow"

One run

cargo clippy -- -A clippy::cast-slice-different-sizes

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.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

1.61.0added in Rust

Clippy's own documentation

What it does

Checks for as casts between raw pointers to slices with differently sized elements.

Why is this bad?

Example

// Missing data

let a = [1_i32, 2, 3, 4];
let p = &a as *const [i32] as *const [u8];
unsafe {
    println!("{:?}", &*p);
}

// Undefined Behavior (note: also potential alignment issues)

let a = [1_u8, 2, 3, 4];
let p = &a as *const [u8] as *const [u32];
unsafe {
    println!("{:?}", &*p);
}

Instead use ptr::slice_from_raw_parts to construct a slice from a data pointer and the correct length

let a = [1_i32, 2, 3, 4];
let old_ptr = &a as *const [i32];
// The data pointer is cast to a pointer to the target `u8` not `[u8]`
// The length comes from the known length of 4 i32s times the 4 bytes per i32
let new_ptr = core::ptr::slice_from_raw_parts(old_ptr as *const u8, 16);
unsafe {
    println!("{:?}", &*new_ptr);
}

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

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