RS.CLIPPY.TRANSMUTE_NULL_TO_FN

Transmute results in a null function pointer, which is undefined behavior

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: transmute_null_to_fn. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks for null function pointer creation through transmute.

Why is this bad?

Creating a null function pointer is undefined behavior.

More info: https://doc.rust-lang.org/nomicon/ffi.html#the-nullable-pointer-optimization

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_fn: fn() = unsafe { std::mem::transmute( std::ptr::null::<()>() ) };

Use instead:

let null_fn: Option<fn()> = None;