handle the active field index in unions

Author: nikomatsakis

src/librustc/mir/mod.rs

@@ -1375,10 +1375,14 @@ pub enum AggregateKind<'tcx> {
     /// The type is of the element
     Array(Ty<'tcx>),
     Tuple,
-    /// The second field is variant number (discriminant), it's equal to 0
-    /// for struct and union expressions. The fourth field is active field
-    /// number and is present only for union expressions.
+
+    /// The second field is variant number (discriminant), it's equal
+    /// to 0 for struct and union expressions. The fourth field is
+    /// active field number and is present only for union expressions
+    /// -- e.g. for a union expression `SomeUnion { c: .. }`, the
+    /// active field index would identity the field `c`
     Adt(&'tcx AdtDef, usize, &'tcx Substs<'tcx>, Option<usize>),
+
     Closure(DefId, ClosureSubsts<'tcx>),
     Generator(DefId, ClosureSubsts<'tcx>, GeneratorInterior<'tcx>),
 }

src/librustc_mir/transform/type_check.rs

@@ -1031,14 +1031,16 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
     fn aggregate_field_ty(
         &mut self,
         ak: &Box<AggregateKind<'tcx>>,
-        field: usize,
+        field_index: usize,
         location: Location,
     ) -> Result<Ty<'tcx>, FieldAccessError> {
         let tcx = self.tcx();
 
         match **ak {
-            AggregateKind::Adt(def, variant, substs, _) => {
-                if let Some(field) = def.variants[variant].fields.get(field) {
+            AggregateKind::Adt(def, variant_index, substs, active_field_index) => {
+                let variant = &def.variants[variant_index];
+                let adj_field_index = active_field_index.unwrap_or(field_index);
+                if let Some(field) = variant.fields.get(adj_field_index) {
                     Ok(self.normalize(&field.ty(tcx, substs), location))
                 } else {
                     Err(FieldAccessError::OutOfRange {
@@ -1047,18 +1049,18 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                 }
             }
             AggregateKind::Closure(def_id, substs) => {
-                match substs.upvar_tys(def_id, tcx).nth(field) {
+                match substs.upvar_tys(def_id, tcx).nth(field_index) {
                     Some(ty) => Ok(ty),
                     None => Err(FieldAccessError::OutOfRange {
                         field_count: substs.upvar_tys(def_id, tcx).count(),
                     }),
                 }
             }
             AggregateKind::Generator(def_id, substs, _) => {
-                if let Some(ty) = substs.upvar_tys(def_id, tcx).nth(field) {
-                    Ok(ty);
+                if let Some(ty) = substs.upvar_tys(def_id, tcx).nth(field_index) {
+                    Ok(ty)
                 } else {
-                    match substs.field_tys(def_id, tcx).nth(field) {
+                    match substs.field_tys(def_id, tcx).nth(field_index) {
                         Some(ty) => Ok(ty),
                         None => Err(FieldAccessError::OutOfRange {
                             field_count: substs.field_tys(def_id, tcx).count() + 1,