Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::as_ptr_cast_mut: casting the result of `as_ptr` to *mut u8

What it meansChecks for the result of a &self-taking as_ptr being cast to a mutable pointer. It is in the nursery group and off by default.

What you see

warning: casting the result of `as_ptr` to *mut u8
 --> main.rs:4:11
  |
4 | let ptr = vec.as_ptr() as *mut u8;
  |           ^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `vec.as_mut_ptr()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#as_ptr_cast_mut
  = note: `-W clippy::as-ptr-cast-mut` implied by `-W clippy::nursery`

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?

Since as_ptr takes a &self, the pointer won’t have write permissions unless interior mutability is used, making it unlikely that having it as a mutable pointer is correct.

New lints still being finished. Off by default and may have false positives.

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 mut vec = Vec::<u8>::with_capacity(1);
let ptr = vec.as_ptr() as *mut u8;
unsafe { ptr.write(4) }; // UNDEFINED BEHAVIOUR

Write this instead

let mut vec = Vec::<u8>::with_capacity(1);
let ptr = vec.as_mut_ptr();
unsafe { ptr.write(4) };

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]
as_ptr_cast_mut = "warn"

In code

#![warn(clippy::as_ptr_cast_mut)]

One run

cargo clippy -- -W clippy::as-ptr-cast-mut

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.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.66.0added in Rust

Clippy's own documentation

What it does

Checks for the result of a &self-taking as_ptr being cast to a mutable pointer.

Why is this bad?

Since as_ptr takes a &self, the pointer won’t have write permissions unless interior mutability is used, making it unlikely that having it as a mutable pointer is correct.

Example

let mut vec = Vec::<u8>::with_capacity(1);
let ptr = vec.as_ptr() as *mut u8;
unsafe { ptr.write(4) }; // UNDEFINED BEHAVIOUR

Use instead:

let mut vec = Vec::<u8>::with_capacity(1);
let ptr = vec.as_mut_ptr();
unsafe { ptr.write(4) };

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

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