Update union-specific tests

- update the comments on the `U6` test case, to make it more clear what
to do when non-`Copy` unions are stabilized
- add a crucial `~` that caused a test to pass that wasn’t supposed to
- update stderr of some tests to reflect changes in the actual tests
and compiler error messages

src/test/ui/feature-gate-untagged_unions.rs

@@ -33,13 +33,13 @@ impl Drop for U5 {
 }
 
 // the non-`Copy` error message has priority
-// replace this with unsized error message if
-// non-`Copy` unions are stabilized before unsized unions
+// replace this with an "unsized unions are unstable" error message
+// if non-`Copy` unions are stabilized before unsized unions
 union U6<T: ?Sized> { //~ ERROR unions with non-`Copy` fields are unstable
     a: T
 }
 
-union U7<T: Copy + ?Sized> { // ERROR unsized unions are unstable
+union U7<T: Copy + ?Sized> { //~ ERROR unsized unions are unstable
     a: T
 }
 

src/test/ui/feature-gate-untagged_unions.stderr

@@ -28,6 +28,16 @@ LL | | }
    |
    = help: add #![feature(untagged_unions)] to the crate attributes to enable
 
-error: aborting due to 3 previous errors
+error[E0658]: unions with non-`Copy` fields are unstable (see issue #32836)
+  --> $DIR/feature-gate-untagged_unions.rs:38:1
+   |
+LL | / union U6<T: ?Sized> { //~ ERROR unions with non-`Copy` fields are unstable
+LL | |     a: T
+LL | | }
+   | |_^
+   |
+   = help: add #![feature(untagged_unions)] to the crate attributes to enable
+
+error: aborting due to 4 previous errors
 
 If you want more information on this error, try using "rustc --explain E0658"

src/test/ui/union/union-sized-field.stderr

@@ -1,15 +1,15 @@
 error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
-  --> $DIR/union-sized-field.rs:14:5
+  --> $DIR/union-sized-field.rs:15:5
    |
 LL |     value: T, //~ ERROR the trait bound `T: std::marker::Sized` is not satisfied
    |     ^^^^^^^^ `T` does not have a constant size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = help: consider adding a `where T: std::marker::Sized` bound
-   = note: no field of a union may have a dynamically sized type
+   = note: only single-field unions may have a dynamically sized type
 
 error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
-  --> $DIR/union-sized-field.rs:18:5
+  --> $DIR/union-sized-field.rs:19:5
    |
 LL |     value: T, //~ ERROR the trait bound `T: std::marker::Sized` is not satisfied
    |     ^^^^^^^^ `T` does not have a constant size known at compile-time
@@ -19,7 +19,7 @@ LL |     value: T, //~ ERROR the trait bound `T: std::marker::Sized` is not sati
    = note: only the last field of a struct may have a dynamically sized type
 
 error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
-  --> $DIR/union-sized-field.rs:23:11
+  --> $DIR/union-sized-field.rs:24:11
    |
 LL |     Value(T), //~ ERROR the trait bound `T: std::marker::Sized` is not satisfied
    |           ^^ `T` does not have a constant size known at compile-time