Do not require that unsafe fields lack drop glue

Instead, we adopt the position that introducing an `unsafe` field
itself carries a safety invariant: that if you assign an invariant
to that field weaker than what the field's destructor requires,
you must ensure that field is in a droppable state in your
destructor.

See:
- rust-lang/rfcs#3458 (comment)
- https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/unsafe.20fields.20RFC/near/502113897

Author: jswrenn

core/src/marker.rs

@@ -453,8 +453,8 @@ impl Copy for ! {}
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Copy for &T {}
 
-/// Marker trait for the types that are allowed in union fields, unsafe fields,
-/// and unsafe binder types.
+/// Marker trait for the types that are allowed in union fields and unsafe
+/// binder types.
 ///
 /// Implemented for:
 /// * `&T`, `&mut T` for all `T`,