Make Copy unsafe to implement for ADTs with unsafe fields

As a rule, the application of `unsafe` to a declaration requires that use-sites
of that declaration also require `unsafe`. For example, a field declared
`unsafe` may only be read in the lexical context of an `unsafe` block.

For nearly all safe traits, the safety obligations of fields are explicitly
discharged when they are mentioned in method definitions. For example,
idiomatically implementing `Clone` (a safe trait) for a type with unsafe fields
will require `unsafe` to clone those fields.

Prior to this commit, `Copy` violated this rule. The trait is marked safe, and
although it has no explicit methods, its implementation permits reads of `Self`.

This commit resolves this by making `Copy` conditionally safe to implement. It
remains safe to implement for ADTs without unsafe fields, but unsafe to
implement for ADTs with unsafe fields.

Tracking: rust-lang#132922

Author: jswrenn

example/mini_core.rs

@@ -52,24 +52,24 @@ impl<T: ?Sized> LegacyReceiver for &mut T {}
 impl<T: ?Sized, A: Allocator> LegacyReceiver for Box<T, A> {}
 
 #[lang = "copy"]
-pub unsafe trait Copy {}
-
-unsafe impl Copy for bool {}
-unsafe impl Copy for u8 {}
-unsafe impl Copy for u16 {}
-unsafe impl Copy for u32 {}
-unsafe impl Copy for u64 {}
-unsafe impl Copy for usize {}
-unsafe impl Copy for i8 {}
-unsafe impl Copy for i16 {}
-unsafe impl Copy for i32 {}
-unsafe impl Copy for isize {}
-unsafe impl Copy for f32 {}
-unsafe impl Copy for f64 {}
-unsafe impl Copy for char {}
-unsafe impl<'a, T: ?Sized> Copy for &'a T {}
-unsafe impl<T: ?Sized> Copy for *const T {}
-unsafe impl<T: ?Sized> Copy for *mut T {}
+pub trait Copy {}
+
+impl Copy for bool {}
+impl Copy for u8 {}
+impl Copy for u16 {}
+impl Copy for u32 {}
+impl Copy for u64 {}
+impl Copy for usize {}
+impl Copy for i8 {}
+impl Copy for i16 {}
+impl Copy for i32 {}
+impl Copy for isize {}
+impl Copy for f32 {}
+impl Copy for f64 {}
+impl Copy for char {}
+impl<'a, T: ?Sized> Copy for &'a T {}
+impl<T: ?Sized> Copy for *const T {}
+impl<T: ?Sized> Copy for *mut T {}
 
 #[lang = "sync"]
 pub unsafe trait Sync {}