add universes to type inference variables

This gives each type inference variable a notion of universe but doesn't
do anything with it. We can always get the "current universe" from
infer_ctxt. This relies on the property of type variables that they can
never interact with siblings.

Author: sgrif

src/librustc/infer/combine.rs

@@ -407,7 +407,7 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                             drop(variables);
                             self.relate(&u, &u)
                         }
-                        TypeVariableValue::Unknown { .. } => {
+                        TypeVariableValue::Unknown { universe } => {
                             match self.ambient_variance {
                                 // Invariant: no need to make a fresh type variable.
                                 ty::Invariant => return Ok(t),
@@ -424,7 +424,7 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                             }
 
                             let origin = *variables.var_origin(vid);
-                            let new_var_id = variables.new_var(false, origin);
+                            let new_var_id = variables.new_var(universe, false, origin);
                             let u = self.tcx().mk_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}",
                                    vid, u);

src/librustc/infer/mod.rs

@@ -852,7 +852,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     pub fn next_ty_var_id(&self, diverging: bool, origin: TypeVariableOrigin) -> TyVid {
         self.type_variables
             .borrow_mut()
-            .new_var(diverging, origin)
+            .new_var(self.universe, diverging, origin)
     }
 
     pub fn next_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
@@ -920,7 +920,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             -> Ty<'tcx> {
         let ty_var_id = self.type_variables
                             .borrow_mut()
-                            .new_var(false,
+                            .new_var(self.universe,
+                                     false,
                                      TypeVariableOrigin::TypeParameterDefinition(span, def.name));
 
         self.tcx.mk_var(ty_var_id)

src/librustc/infer/type_variable.rs

@@ -78,10 +78,12 @@ struct TypeVariableData {
 #[derive(Copy, Clone, Debug)]
 pub enum TypeVariableValue<'tcx> {
     Known { value: Ty<'tcx> },
-    Unknown,
+    Unknown { universe: ty::UniverseIndex },
 }
 
 impl<'tcx> TypeVariableValue<'tcx> {
+    /// If this value is known, returns the type it is known to be.
+    /// Otherwise, `None`.
     pub fn known(&self) -> Option<Ty<'tcx>> {
         match *self {
             TypeVariableValue::Unknown { .. } => None,
@@ -181,10 +183,11 @@ impl<'tcx> TypeVariableTable<'tcx> {
     ///   The code in this module doesn't care, but it can be useful
     ///   for improving error messages.
     pub fn new_var(&mut self,
+                   universe: ty::UniverseIndex,
                    diverging: bool,
                    origin: TypeVariableOrigin)
                    -> ty::TyVid {
-        let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown);
+        let eq_key = self.eq_relations.new_key(TypeVariableValue::Unknown { universe });
 
         let sub_key = self.sub_relations.new_key(());
         assert_eq!(eq_key.vid, sub_key);
@@ -437,7 +440,11 @@ impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
             (&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
 
             // If both sides are *unknown*, it hardly matters, does it?
-            (&TypeVariableValue::Unknown, &TypeVariableValue::Unknown) => Ok(*value1),
+            (&TypeVariableValue::Unknown { universe: universe1 },
+             &TypeVariableValue::Unknown { universe: universe2 }) =>  {
+                let universe = cmp::min(universe1, universe2);
+                Ok(TypeVariableValue::Unknown { universe })
+            }
         }
     }
 }