module main

// Regression test for https://github.com/vlang/v/issues/27354
// An anon fn passed to a C callback inside a method that is reached only via
// interface dispatch on a `mut` receiver must be marked used by markused,
// otherwise -skip-unused (the default) drops its definition and the generated
// C fails with `use of undeclared identifier 'anon_fn_...'`.

fn C.atexit(cb voidptr) int

interface Handler {
    setup()
}

struct Impl {
mut:
    n int
}

fn (mut i Impl) setup() {
    C.atexit(fn () {})
}

fn run(mut h Handler) {
    h.setup()
}

fn main() {
    mut impl := Impl{
        n: 99
    }
    run(mut impl)
    println('done')
}