Report even duplilcate errors in case the feature gat is not active

src/librustc_typeck/check/mod.rs

@@ -1394,11 +1394,6 @@ fn check_union(tcx: TyCtxt<'_>, id: hir::HirId, span: Span) {
 /// When the `#![feature(untagged_unions)]` gate is active,
 /// check that the fields of the `union` does not contain fields that need dropping.
 fn check_union_fields(tcx: TyCtxt<'_>, span: Span, item_def_id: DefId) -> bool {
-    // Without the feature we check that all fields are `Copy` in our stability checking
-    // infrastructure.
-    if !tcx.features().untagged_unions {
-        return true;
-    }
     let item_type = tcx.type_of(item_def_id);
     if let ty::Adt(def, substs) = item_type.kind {
         assert!(def.is_union());

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

@@ -7,11 +7,11 @@ union U2<T: Copy> { // OK
 }
 
 union U3 { //~ ERROR unions with non-`Copy` fields are unstable
-    a: String,
+    a: String, //~ ERROR unions may not contain fields that need dropping
 }
 
 union U4<T> { //~ ERROR unions with non-`Copy` fields are unstable
-    a: T,
+    a: T, //~ ERROR unions may not contain fields that need dropping
 }
 
 union U5 { //~ ERROR unions with `Drop` implementations are unstable

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

@@ -31,6 +31,31 @@ LL | | }
    = note: for more information, see https://github.com/rust-lang/rust/issues/32836
    = help: add `#![feature(untagged_unions)]` to the crate attributes to enable
 
-error: aborting due to 3 previous errors
+error[E0740]: unions may not contain fields that need dropping
+  --> $DIR/feature-gate-untagged_unions.rs:10:5
+   |
+LL |     a: String,
+   |     ^^^^^^^^^
+   |
+note: `std::mem::ManuallyDrop` can be used to wrap the type
+  --> $DIR/feature-gate-untagged_unions.rs:10:5
+   |
+LL |     a: String,
+   |     ^^^^^^^^^
 
-For more information about this error, try `rustc --explain E0658`.
+error[E0740]: unions may not contain fields that need dropping
+  --> $DIR/feature-gate-untagged_unions.rs:14:5
+   |
+LL |     a: T,
+   |     ^^^^
+   |
+note: `std::mem::ManuallyDrop` can be used to wrap the type
+  --> $DIR/feature-gate-untagged_unions.rs:14:5
+   |
+LL |     a: T,
+   |     ^^^^
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0658, E0740.
+For more information about an error, try `rustc --explain E0658`.