do not apply DerefMut on union field

compiler/rustc_typeck/src/check/place_op.rs

@@ -193,7 +193,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Convert auto-derefs, indices, etc of an expression from `Deref` and `Index`
     /// into `DerefMut` and `IndexMut` respectively.
     ///
-    /// This is a second pass of typechecking derefs/indices. We need this we do not
+    /// This is a second pass of typechecking derefs/indices. We need this because we do not
     /// always know whether a place needs to be mutable or not in the first pass.
     /// This happens whether there is an implicit mutable reborrow, e.g. when the type
     /// is used as the receiver of a method call.
@@ -236,6 +236,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             if let ty::Ref(region, _, mutbl) = method.sig.output().kind {
                                 *deref = OverloadedDeref { region, mutbl };
                             }
+                            // If this is a union field, also throw an error.
+                            // Union fields should not get mutable auto-deref'd (see RFC 2514).
+                            if let hir::ExprKind::Field(ref outer_expr, _) = expr.kind {
+                                let ty = self.node_ty(outer_expr.hir_id);
+                                if ty.ty_adt_def().map_or(false, |adt| adt.is_union()) {
+                                    let mut err = self.tcx.sess.struct_span_err(
+                                        expr.span,
+                                        "not automatically applying `DerefMut` on union field",
+                                    );
+                                    err.help("writing to this field calls the destructor for the old value");
+                                    err.help("add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor");
+                                    err.emit();
+                                }
+                            }
                         }
                     }
                     source = adjustment.target;

src/test/ui/union/union-deref.rs

@@ -0,0 +1,13 @@
+//! Test the part of RFC 2514 that is about not applying `DerefMut` coercions
+//! of union fields.
+#![feature(untagged_unions)]
+
+use std::mem::ManuallyDrop;
+
+union U<T> { x:(), f: ManuallyDrop<(T,)> }
+
+fn main() {
+    let mut u : U<Vec<i32>> = U { x: () };
+    unsafe { (*u.f).0 = Vec::new() }; // explicit deref, this compiles
+    unsafe { u.f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on union field
+}

src/test/ui/union/union-deref.stderr

@@ -0,0 +1,11 @@
+error: not automatically applying `DerefMut` on union field
+  --> $DIR/union-deref.rs:12:14
+   |
+LL |     unsafe { u.f.0 = Vec::new() };
+   |              ^^^
+   |
+   = help: writing to this field calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: aborting due to previous error
+