pub use default_boxed_derive::DefaultBoxed;

/// Helper trait to create a boxed instance of the given type with a default value for each field.
///
/// This trait can be derived.
///
/// To derive this trait, each field needs to also implement this trait, but all types which
/// implements `Default` implements this trait via the blanket `impl` already.
///
/// In addition, if a field is an array, only the item type needs to implement this trait, and each
/// item would be initialized separately.
pub trait DefaultBoxed: Sized {
    fn default_boxed() -> Box<Self> {
        let mut v = Vec::with_capacity(1);
        let raw = v.as_mut_ptr();
        std::mem::forget(v);
        unsafe {
            Self::default_in_place(raw);
            Box::from_raw(raw)
        }
    }

    unsafe fn default_in_place(ptr: *mut Self);
}

impl<T: Default> DefaultBoxed for T {
    unsafe fn default_in_place(ptr: *mut Self) {
        std::ptr::write(ptr, Default::default());
    }
}