Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::transmuting_null: transmuting a known null pointer into a reference

What it meansChecks for transmute calls which would receive a null pointer. It is in the correctness group and denied by default (the build fails).

What you see

error: transmuting a known null pointer into a reference
 --> main.rs:3:31
  |
3 | let null_ref: &u64 = unsafe { std::mem::transmute(0 as *const u64) };
  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#transmuting_null
  = note: `#[deny(clippy::transmuting_null)]` on by default

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?

Transmuting a null pointer is undefined behavior.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

let null_ref: &u64 = unsafe { std::mem::transmute(0 as *const u64) };

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
transmuting_null = "allow"

One run

cargo clippy -- -A clippy::transmuting-null

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.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

1.35.0added in Rust

Clippy's own documentation

What it does

Checks for transmute calls which would receive a null pointer.

Why is this bad?

Transmuting a null pointer is undefined behavior.

Known problems

Not all cases can be detected at the moment of this writing. For example, variables which hold a null pointer and are then fed to a transmute call, aren’t detectable yet.

Example

let null_ref: &u64 = unsafe { std::mem::transmute(0 as *const u64) };

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

Up next

Also worth your time

Did this answer your question?