[derive] Support deriving NoCell

[joshlf]

Makes progress on #251

[joshlf]

TODO: Add tests for the types described in #656 (comment):

Can you expand on a few edge cases here? I'm not totally clear on what "contains" means. Is this NoCell?

struct R<'a> {
    field: &'a UnsafeCell<u8>,
}

...and this?

struct ZLA {
    field: [UnsafeCell<u8>; 0];
}

...and this?

struct CPC {
    field: PhantomData<UnsafeCell<u8>>;
}

...and this?

trait Trait {
    type Assoc;
}

impl<T> Trait for UnsafeCell<T> {
    type Assoc = T;
}

struct CAT<T>
where
    T: NoCell
{
    field: <UnsafeCell<T> as Trait>::Assoc,
}

...and this?

enum Enum {
    Uninhabited(!, UnsafeCell<u8>),
    Inhabited(u8),
}

[jswrenn]

For testing (and potentially the derive expansion), you might find this macro nifty:

macro_rules! assert_unfreeze {
    ($ty:ty) => {
        const _: () = {
            static _assert_unfreeze: Option<$ty> = None;
        };
    }
}

assert_unfreeze!(u8); // accepted!

assert_unfreeze!(core::cell::UnsafeCell<u8>); // rejected! compilation error!

[joshlf]

For testing (and potentially the derive expansion), you might find this macro nifty:

macro_rules! assert_unfreeze {
    ($ty:ty) => {
        const _: () = {
            static _assert_unfreeze: Option<$ty> = None;
        };
    }
}

assert_unfreeze!(u8); // accepted!

assert_unfreeze!(core::cell::UnsafeCell<u8>); // rejected! compilation error!

Didn't end up needing this, but it's very nifty!