Change untagged_unions to not allow union fields with drop

Union fields may now never have a type with attached destructor.
This for example allows unions to use arbitrary field types only by wrapping
them in ManuallyDrop.

The stable rule remains, that union fields must be Copy. We use the new
rule for the `untagged_union` feature.

See RFC 2514.

Author: bluss

src/librustc_typeck/check/mod.rs

@@ -1302,9 +1302,38 @@ fn check_union<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     def.destructor(tcx); // force the destructor to be evaluated
     check_representable(tcx, span, def_id);
 
+    check_dropless(tcx, span, def_id);
     check_packed(tcx, span, def_id);
 }
 
+fn check_dropless<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                sp: Span,
+                                item_def_id: DefId)
+                                -> bool {
+    // Without the feature we check Copy types only later
+    if !tcx.features().untagged_unions {
+        return true;
+    }
+    let t = tcx.type_of(item_def_id);
+    if let ty::Adt(def, substs) = t.sty {
+        if def.is_union() {
+            let fields = &def.non_enum_variant().fields;
+            for field in fields {
+                let field_ty = field.ty(tcx, substs);
+                let param_env = tcx.param_env(field.did);
+                if field_ty.needs_drop(tcx, param_env) {
+                    struct_span_err!(tcx.sess, sp, E0730,
+                                     "unions may not contain fields that need dropping")
+                                .span_label(sp, "ManuallyDrop can be used to wrap such fields")
+                                .emit();
+                    return false;
+                }
+            }
+        }
+    }
+    return true;
+}
+
 fn check_opaque<'a, 'tcx>(
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     def_id: DefId,

src/librustc_typeck/diagnostics.rs

@@ -4831,6 +4831,10 @@ fn make_recursive_type() -> impl Sized {
 ```
 "##,
 
+E0730: r##"
+A `union` can not have fields with destructors.
+"##,
+
 }
 
 register_diagnostics! {