Specify that packed types must derive, not implement, Copy

Author: Mark-Simulacrum

src/librustc_mir/transform/check_unsafety.rs

@@ -362,8 +362,8 @@ fn unsafe_derive_on_repr_packed<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: D
         format!("#[derive] can't be used on a #[repr(packed)] struct with \
                  type parameters (error E0133)")
     } else {
-        format!("#[derive] can't be used on a non-Copy #[repr(packed)] struct \
-                 (error E0133)")
+        format!("#[derive] can't be used on a #[repr(packed)] struct that \
+                 does not derive Copy (error E0133)")
     };
     tcx.lint_node(SAFE_PACKED_BORROWS,
                   lint_node_id,

src/test/ui/deriving-with-repr-packed.rs

@@ -33,7 +33,7 @@ pub struct Bar(u32, u32, u32);
 struct Y(usize);
 
 #[derive(PartialEq)]
-//~^ ERROR #[derive] can't be used on a non-Copy #[repr(packed)]
+//~^ ERROR #[derive] can't be used
 //~| hard error
 #[repr(packed)]
 struct X(Y);

src/test/ui/deriving-with-repr-packed.stderr

@@ -21,7 +21,7 @@ LL | #[derive(Copy, Clone, PartialEq, Eq)]
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
 
-error: #[derive] can't be used on a non-Copy #[repr(packed)] struct (error E0133)
+error: #[derive] can't be used on a #[repr(packed)] struct that does not derive Copy (error E0133)
   --> $DIR/deriving-with-repr-packed.rs:26:10
    |
 LL | #[derive(PartialEq, Eq)]
@@ -30,7 +30,7 @@ LL | #[derive(PartialEq, Eq)]
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
 
-error: #[derive] can't be used on a non-Copy #[repr(packed)] struct (error E0133)
+error: #[derive] can't be used on a #[repr(packed)] struct that does not derive Copy (error E0133)
   --> $DIR/deriving-with-repr-packed.rs:35:10
    |
 LL | #[derive(PartialEq)]