Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::from_raw_with_void_ptr: Checks if we’re passing a c_void raw pointer to {Box,Rc,Arc,Weak}::from_raw(_)

What it meansChecks if we’re passing a c_void raw pointer to {Box,Rc,Arc,Weak}::from_raw(_) It is in the suspicious group and warns by default.

Does it matter?

When dealing with c_void raw pointers in FFI, it is easy to run into the pitfall of calling from_raw with the c_void pointer. The type signature of Box::from_raw is fn from_raw(raw: *mut T) -> Box<T>, so if you pass a *mut c_void you will get a Box<c_void> (and similarly for Rc, Arc and Weak). For this to be safe, c_void would need to have the same memory layout as the original type, which is often not the case.

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

let ptr = Box::into_raw(Box::new(42usize)) as *mut c_void;
let _ = unsafe { Box::from_raw(ptr) };

Write this instead

let _ = unsafe { Box::from_raw(ptr as *mut usize) };

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
from_raw_with_void_ptr = "allow"

One run

cargo clippy -- -A clippy::from-raw-with-void-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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.67.0added in Rust

Clippy's own documentation

What it does

Checks if we’re passing a c_void raw pointer to {Box,Rc,Arc,Weak}::from_raw(_)

Why is this bad?

When dealing with c_void raw pointers in FFI, it is easy to run into the pitfall of calling from_raw with the c_void pointer. The type signature of Box::from_raw is fn from_raw(raw: *mut T) -> Box<T>, so if you pass a *mut c_void you will get a Box<c_void> (and similarly for Rc, Arc and Weak). For this to be safe, c_void would need to have the same memory layout as the original type, which is often not the case.

Example

let ptr = Box::into_raw(Box::new(42usize)) as *mut c_void;
let _ = unsafe { Box::from_raw(ptr) };

Use instead:

let _ = unsafe { Box::from_raw(ptr as *mut usize) };

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

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