Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::transmute_ptr_to_ptr: transmute from a pointer to a pointer

What it meansChecks for transmutes from a pointer to a pointer, or from a reference to a reference. It is in the pedantic group and off by default.

What you see

warning: transmute from a pointer to a pointer
 --> main.rs:6:25
  |
6 |     let _: *const f32 = std::mem::transmute(ptr);
  |                         ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#transmute_ptr_to_ptr
  = note: `-W clippy::transmute-ptr-to-ptr` implied by `-W clippy::pedantic`
help: use `pointer::cast` instead
  |
6 -     let _: *const f32 = std::mem::transmute(ptr);
6 +     let _: *const f32 = ptr.cast::<f32>();
  |

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?

Transmutes are dangerous, and these can instead be written as casts.

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

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 ptr = &1u32 as *const u32;
unsafe {
    // pointer-to-pointer transmute
    let _: *const f32 = std::mem::transmute(ptr);
    // ref-ref transmute
    let _: &f32 = std::mem::transmute(&1u32);
}
// These can be respectively written:
let _ = ptr as *const f32;
let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };

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]
transmute_ptr_to_ptr = "warn"

In code

#![warn(clippy::transmute_ptr_to_ptr)]

One run

cargo clippy -- -W clippy::transmute-ptr-to-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.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for transmutes from a pointer to a pointer, or from a reference to a reference.

Why is this bad?

Transmutes are dangerous, and these can instead be written as casts.

Example

let ptr = &1u32 as *const u32;
unsafe {
    // pointer-to-pointer transmute
    let _: *const f32 = std::mem::transmute(ptr);
    // ref-ref transmute
    let _: &f32 = std::mem::transmute(&1u32);
}
// These can be respectively written:
let _ = ptr as *const f32;
let _ = unsafe{ &*(&1u32 as *const u32 as *const f32) };

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

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