mir-borrowck: Implement end-user output for field of downcast projection

Author: zilbuz

src/librustc_mir/borrow_check.rs

@@ -1125,11 +1125,6 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
                 match proj.elem {
                     ProjectionElem::Deref =>
                         self.describe_field(&proj.base, field_index),
-                    ProjectionElem::Downcast(..) => {
-                        debug!("End-user description not implemented for field of projection {:?}",
-                               proj);
-                        format!("<downcast>{}", field_index)
-                    },
                     ProjectionElem::Field(..) => {
                         debug!("End-user description not implemented for field of projection {:?}",
                                proj);
@@ -1145,6 +1140,8 @@ impl<'c, 'b, 'a: 'b+'c, 'gcx, 'tcx: 'a> MirBorrowckCtxt<'c, 'b, 'a, 'gcx, 'tcx>
                                proj);
                         format!("<constant_index>{}", field_index)
                     },
+                    ProjectionElem::Downcast(def, variant_index) =>
+                        format!("{}", def.variants[variant_index].fields[field_index].name),
                     ProjectionElem::Subslice { .. } => {
                         debug!("End-user description not implemented for field of projection {:?}",
                                proj);

src/test/compile-fail/borrowck/borrowck-describe-lvalue.rs

@@ -233,4 +233,24 @@ fn main() {
             _ => panic!("other case"),
         }
     }
+    // Downcasted field
+    {
+        enum E<X> { A(X), B { x: X } }
+
+        let mut e = E::A(3);
+        let _e = &mut e;
+        match e {
+            E::A(ref ax) =>
+                //[ast]~^ ERROR cannot borrow `e.0` as immutable because `e` is also borrowed as mutable
+                //[mir]~^^ ERROR cannot borrow `e.0` as immutable because `e` is also borrowed as mutable (Ast)
+                //[mir]~| ERROR cannot borrow `e.0` as immutable because it is also borrowed as mutable (Mir)
+                //[mir]~| ERROR cannot use `e` because it was mutably borrowed (Mir)
+                println!("e.ax: {:?}", ax),
+            E::B { x: ref bx } =>
+                //[ast]~^ ERROR cannot borrow `e.x` as immutable because `e` is also borrowed as mutable
+                //[mir]~^^ ERROR cannot borrow `e.x` as immutable because `e` is also borrowed as mutable (Ast)
+                //[mir]~| ERROR cannot borrow `e.x` as immutable because it is also borrowed as mutable (Mir)
+                println!("e.bx: {:?}", bx),
+        }
+    }
 }