Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::ptr_as_ptr: `as` casting between raw pointers without changing their constness

What it meansChecks for as casts between raw pointers that don’t change their constness, namely *const T to *const U and *mut T to *mut U. It is in the pedantic group and off by default.

What you see

warning: `as` casting between raw pointers without changing their constness
 --> main.rs:5:9
  |
5 | let _ = ptr as *const i32;
  |         ^^^^^^^^^^^^^^^^^ help: try `pointer::cast`, a safer alternative: `ptr.cast::<i32>()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#ptr_as_ptr
  = note: `-W clippy::ptr-as-ptr` 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 is safer because it cannot accidentally change the pointer’s mutability, nor cast the pointer to other types like usize.

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: *mut u32 = &mut 42_u32;
let _ = ptr as *const i32;
let _ = mut_ptr as *mut i32;

Write this instead

let ptr: *const u32 = &42_u32;
let mut_ptr: *mut u32 = &mut 42_u32;
let _ = ptr.cast::<i32>();
let _ = mut_ptr.cast::<i32>();

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_as_ptr = "warn"

In code

#![warn(clippy::ptr_as_ptr)]

One run

cargo clippy -- -W clippy::ptr-as-ptr

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Pedanticgroup

allowdefault level

yesfixed by cargo clippy --fix

1.51.0added in Rust

Clippy's own documentation

What it does

Checks for as casts between raw pointers that don’t change their constness, namely *const T to *const U and *mut T to *mut U.

Why is this bad?

Though as casts between raw pointers are not terrible, pointer::cast is safer because it cannot accidentally change the pointer’s mutability, nor cast the pointer to other types like usize.

Example

let ptr: *const u32 = &42_u32;
let mut_ptr: *mut u32 = &mut 42_u32;
let _ = ptr as *const i32;
let _ = mut_ptr as *mut i32;

Use instead:

let ptr: *const u32 = &42_u32;
let mut_ptr: *mut u32 = &mut 42_u32;
let _ = ptr.cast::<i32>();
let _ = mut_ptr.cast::<i32>();

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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