Rust has only-const callable functions thanks to PMEs

[lcnr]

#[expect(unconditional_recursion)]
const fn codegen_diverge<T>() {
    codegen_diverge::<(T,)>();
}

const fn only_const_callable() -> usize {
    if false {
        codegen_diverge::<()>();
    }
    
    2
}

const OK: usize = only_const_callable();

fn main() {
    println!("{}", OK);
    println!("{}", const { only_const_callable() });
    // println!("{}", only_const_callable()); // error
}

We only actually recursively instantiate codegen_diverge during mono item collection. This means using only_const_callable at runtime results in a post monomophization error. CTFE lazily instantiates functions on demand so it never overflows.