RS.CLIPPY.CAST_SLICE_DIFFERENT_SIZES

Casting using `as` between raw pointers to slices of types with different sizes

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: cast_slice_different_sizes. Copyright ©2025 The Rust Team. All rights reserved.

What it does

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

Why is this bad?

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.

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);
}