clippy::transmute_null_to_fn: transmuting a known null pointer into a function pointer

What you see

error: transmuting a known null pointer into a function pointer
 --> main.rs:3:30
  |
3 | let null_fn: fn() = unsafe { std::mem::transmute( std::ptr::null::<()>() ) };
  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this transmute results in undefined behavior
  |
  = help: try wrapping your function pointer type in `Option<T>` instead, and using `None` as a null pointer value
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#transmute_null_to_fn
  = note: `#[deny(clippy::transmute_null_to_fn)]` 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?

Creating a null function pointer is undefined behavior. More info: https://doc.rust-lang.org/nomicon/ffi.html#the-nullable-pointer-optimization

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

Write this instead

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

Turn it off, or on

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

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

[lints.clippy]
transmute_null_to_fn = "allow"

cargo clippy -- -A clippy::transmute-null-to-fn

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.