Commits on May 22, 2019

1. Allow null-pointer-optimized enums in FFI if their underlying represe… …

   …ntation is FFI safe
   
   This allows types like Option<NonZeroU8> to be used in FFI without triggering the improper_ctypes lint. This works by changing the is_repr_nullable_ptr function to consider an enum E to be FFI-safe if:
   
   - E has no explicit #[repr(...)].
   - It only has two variants.
   - One of those variants is empty (meaning it has no fields).
   - The other variant has only one field.
   - That field is one of the following:
     - &T
     - &mut T
     - extern "C" fn
     - core::num::NonZero*
     - core::ptr::NonNull<T>
     - #[repr(transparent)] struct wrapper around one of the types in this list.
   - The size of E and its field are both known and are both the same size (implying E is participating in the nonnull optimization).

   

   mjbshaw committed May 22, 2019
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