Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::ptr_cast_constness: `as` casting between raw pointers while changing only its constness

What it meansChecks for as casts between raw pointers that change their constness, namely *const T to *mut T and *mut T to *const T. It is in the pedantic group and off by default.

What you see

warning: `as` casting between raw pointers while changing only its constness
 --> main.rs:4:15
  |
4 | let mut_ptr = ptr as *mut u32;
  |               ^^^^^^^^^^^^^^^ help: try `pointer::cast_mut`, a safer alternative: `ptr.cast_mut()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#ptr_cast_constness
  = note: `-W clippy::ptr-cast-constness` implied by `-W clippy::pedantic`

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?

Though as casts between raw pointers are not terrible, pointer::cast_mut and pointer::cast_const are safer because they cannot accidentally cast the pointer to another type. Or, when null pointers are involved, null() and null_mut() can be used directly.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

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 ptr: *const u32 = &42_u32;
let mut_ptr = ptr as *mut u32;
let ptr = mut_ptr as *const u32;
let ptr1 = std::ptr::null::<u32>() as *mut u32;
let ptr2 = std::ptr::null_mut::<u32>() as *const u32;
let ptr3 = std::ptr::null::<u32>().cast_mut();
let ptr4 = std::ptr::null_mut::<u32>().cast_const();

Write this instead

let ptr: *const u32 = &42_u32;
let mut_ptr = ptr.cast_mut();
let ptr = mut_ptr.cast_const();
let ptr1 = std::ptr::null_mut::<u32>();
let ptr2 = std::ptr::null::<u32>();
let ptr3 = std::ptr::null_mut::<u32>();
let ptr4 = std::ptr::null::<u32>();

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
ptr_cast_constness = "warn"

In code

#![warn(clippy::ptr_cast_constness)]

One run

cargo clippy -- -W clippy::ptr-cast-constness

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.

Pedanticgroup

allowdefault level

yesfixed by cargo clippy --fix

1.72.0added in Rust

Clippy's own documentation

What it does

Checks for as casts between raw pointers that change their constness, namely *const T to *mut T and *mut T to *const T.

Why is this bad?

Though as casts between raw pointers are not terrible, pointer::cast_mut and pointer::cast_const are safer because they cannot accidentally cast the pointer to another type. Or, when null pointers are involved, null() and null_mut() can be used directly.

Example

let ptr: *const u32 = &42_u32;
let mut_ptr = ptr as *mut u32;
let ptr = mut_ptr as *const u32;
let ptr1 = std::ptr::null::<u32>() as *mut u32;
let ptr2 = std::ptr::null_mut::<u32>() as *const u32;
let ptr3 = std::ptr::null::<u32>().cast_mut();
let ptr4 = std::ptr::null_mut::<u32>().cast_const();

Use instead:

let ptr: *const u32 = &42_u32;
let mut_ptr = ptr.cast_mut();
let ptr = mut_ptr.cast_const();
let ptr1 = std::ptr::null_mut::<u32>();
let ptr2 = std::ptr::null::<u32>();
let ptr3 = std::ptr::null_mut::<u32>();
let ptr4 = std::ptr::null::<u32>();

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

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