Disclaimer: This chapter is a work-in-progress. What's contained here represents the consensus from issue #72. The statements in here are not (yet) "guaranteed" not to change until an RFC ratifies them.
A function pointer is "valid" (in the sense that it can be produced without causing immediate UB) if and only if it is non-null.
That makes this code UB:
fn bad() {
let x: fn() = unsafe { std::mem::transmute(0usize) }; // This is UB!
}However, any integer value other than NULL is allowed for function pointers:
fn good() {
let x: fn() = unsafe { std::mem::transmute(1usize) }; // This is not UB.
}