factor variances into a proper query

There are now two queries: crate and item. The crate one computes the
variance of all items in the crate; it is sort of an implementation
detail, and not meant to be used. The item one reads from the crate one,
synthesizing correct deps in lieu of the red-green algorithm.

At the same time, remove the `variance_computed` flag, which was a
horrible hack used to force invariance early on (e.g. when type-checking
constants). This is only needed because of trait applications, and
traits are always invariant anyway. Therefore, we now change to take
advantage of the query system:

- When asked to compute variances for a trait, just return a vector
  saying 'all invariant'.
- Remove the corresponding "inferreds" from traits, and tweak the
  constraint generation code to understand that traits are always
  inferred.

src/librustc/dep_graph/dep_node.rs

@@ -81,6 +81,7 @@ pub enum DepNode<D: Clone + Debug> {
     TransCrateItem(D),
     TransInlinedItem(D),
     TransWriteMetadata,
+    CrateVariances,
 
     // Nodes representing bits of computed IR in the tcx. Each shared
     // table in the tcx (or elsewhere) maps to one of these
@@ -89,6 +90,8 @@ pub enum DepNode<D: Clone + Debug> {
     // predicates for an item wind up in `ItemSignature`).
     AssociatedItems(D),
     ItemSignature(D),
+    ItemVarianceConstraints(D),
+    ItemVariances(D),
     IsForeignItem(D),
     TypeParamPredicates((D, D)),
     SizedConstraint(D),
@@ -199,6 +202,7 @@ pub fn map_def<E, OP>(&self, mut op: OP) -> Option<DepNode<E>>
             MirKrate => Some(MirKrate),
             TypeckBodiesKrate => Some(TypeckBodiesKrate),
             Coherence => Some(Coherence),
+            CrateVariances => Some(CrateVariances),
             Resolve => Some(Resolve),
             Variance => Some(Variance),
             PrivacyAccessLevels(k) => Some(PrivacyAccessLevels(k)),
@@ -230,6 +234,8 @@ pub fn map_def<E, OP>(&self, mut op: OP) -> Option<DepNode<E>>
             TransInlinedItem(ref d) => op(d).map(TransInlinedItem),
             AssociatedItems(ref d) => op(d).map(AssociatedItems),
             ItemSignature(ref d) => op(d).map(ItemSignature),
+            ItemVariances(ref d) => op(d).map(ItemVariances),
+            ItemVarianceConstraints(ref d) => op(d).map(ItemVarianceConstraints),
             IsForeignItem(ref d) => op(d).map(IsForeignItem),
             TypeParamPredicates((ref item, ref param)) => {
                 Some(TypeParamPredicates((try_opt!(op(item)), try_opt!(op(param)))))

src/librustc/ty/context.rs

@@ -468,9 +468,6 @@ pub struct GlobalCtxt<'tcx> {
 
     pub lang_items: middle::lang_items::LanguageItems,
 
-    /// True if the variance has been computed yet; false otherwise.
-    pub variance_computed: Cell<bool>,
-
     /// Set of used unsafe nodes (functions or blocks). Unsafe nodes not
     /// present in this set can be warned about.
     pub used_unsafe: RefCell<NodeSet>,
@@ -753,7 +750,6 @@ pub fn create_and_enter<F, R>(s: &'tcx Session,
             dep_graph: dep_graph.clone(),
             types: common_types,
             named_region_map: named_region_map,
-            variance_computed: Cell::new(false),
             trait_map: resolutions.trait_map,
             export_map: resolutions.export_map,
             fulfilled_predicates: RefCell::new(fulfilled_predicates),

src/librustc/ty/maps.rs

@@ -265,6 +265,12 @@ fn describe(_: TyCtxt, _: CrateNum) -> String {
     }
 }
 
+impl<'tcx> QueryDescription for queries::crate_variances<'tcx> {
+    fn describe(_tcx: TyCtxt, _: CrateNum) -> String {
+        format!("computing the variances for items in this crate")
+    }
+}
+
 impl<'tcx> QueryDescription for queries::mir_shims<'tcx> {
     fn describe(tcx: TyCtxt, def: ty::InstanceDef<'tcx>) -> String {
         format!("generating MIR shim for `{}`",
@@ -673,9 +679,13 @@ fn default() -> Self {
     /// True if this is a foreign item (i.e., linked via `extern { ... }`).
     [] is_foreign_item: IsForeignItem(DefId) -> bool,
 
+    /// Get a map with the variance of every item; use `item_variance`
+    /// instead.
+    [] crate_variances: crate_variances(CrateNum) -> Rc<ty::CrateVariancesMap>,
+
     /// Maps from def-id of a type or region parameter to its
     /// (inferred) variance.
-    [pub] variances_of: ItemSignature(DefId) -> Rc<Vec<ty::Variance>>,
+    [pub] variances_of: ItemVariances(DefId) -> Rc<Vec<ty::Variance>>,
 
     /// Maps from an impl/trait def-id to a list of the def-ids of its items
     [] associated_item_def_ids: AssociatedItemDefIds(DefId) -> Rc<Vec<DefId>>,
@@ -810,3 +820,7 @@ fn const_eval_dep_node((def_id, _): (DefId, &Substs)) -> DepNode<DefId> {
 fn mir_keys(_: CrateNum) -> DepNode<DefId> {
     DepNode::MirKeys
 }
+
+fn crate_variances(_: CrateNum) -> DepNode<DefId> {
+    DepNode::CrateVariances
+}

src/librustc/ty/mod.rs

@@ -55,6 +55,7 @@
 use rustc_data_structures::accumulate_vec::IntoIter as AccIntoIter;
 use rustc_data_structures::stable_hasher::{StableHasher, StableHasherResult,
                                            HashStable};
+use rustc_data_structures::transitive_relation::TransitiveRelation;
 
 use hir;
 use hir::itemlikevisit::ItemLikeVisitor;
@@ -309,6 +310,27 @@ pub enum Variance {
     Bivariant,      // T<A> <: T<B>            -- e.g., unused type parameter
 }
 
+/// The crate variances map is computed during typeck and contains the
+/// variance of every item in the local crate. You should not use it
+/// directly, because to do so will make your pass dependent on the
+/// HIR of every item in the local crate. Instead, use
+/// `tcx.item_variances()` to get the variance for a *particular*
+/// item.
+pub struct CrateVariancesMap {
+    /// This relation tracks the dependencies between the variance of
+    /// various items. In particular, if `a < b`, then the variance of
+    /// `a` depends on the sources of `b`.
+    pub dependencies: TransitiveRelation<DefId>,
+
+    /// For each item with generics, maps to a vector of the variance
+    /// of its generics.  If an item has no generics, it will have no
+    /// entry.
+    pub variances: FxHashMap<DefId, Rc<Vec<ty::Variance>>>,
+
+    /// An empty vector, useful for cloning.
+    pub empty_variance: Rc<Vec<ty::Variance>>,
+}
+
 #[derive(Clone, Copy, Debug, RustcDecodable, RustcEncodable)]
 pub struct MethodCallee<'tcx> {
     /// Impl method ID, for inherent methods, or trait method ID, otherwise.

src/librustc/ty/relate.rs

@@ -124,14 +124,8 @@ fn relate_item_substs<'a, 'gcx, 'tcx, R>(relation: &mut R,
            a_subst,
            b_subst);
 
-    let variances;
-    let opt_variances = if relation.tcx().variance_computed.get() {
-        variances = relation.tcx().variances_of(item_def_id);
-        Some(&*variances)
-    } else {
-        None
-    };
-    relate_substs(relation, opt_variances, a_subst, b_subst)
+    let opt_variances = relation.tcx().variances_of(item_def_id);
+    relate_substs(relation, Some(&opt_variances), a_subst, b_subst)
 }
 
 pub fn relate_substs<'a, 'gcx, 'tcx, R>(relation: &mut R,

src/librustc_data_structures/transitive_relation.rs

@@ -134,6 +134,20 @@ pub fn contains(&self, a: &T, b: &T) -> bool {
         }
     }
 
+    /// Returns a vector of all things less than `a`.
+    ///
+    /// Really this probably ought to be `impl Iterator<Item=&T>`, but
+    /// I'm too lazy to make that work, and -- given the caching
+    /// strategy -- it'd be a touch tricky anyhow.
+    pub fn less_than(&self, a: &T) -> Vec<&T> {
+        match self.index(a) {
+            Some(a) => self.with_closure(|closure| {
+                closure.iter(a.0).map(|i| &self.elements[i]).collect()
+            }),
+            None => vec![],
+        }
+    }
+
     /// Picks what I am referring to as the "postdominating"
     /// upper-bound for `a` and `b`. This is usually the least upper
     /// bound, but in cases where there is no single least upper

src/librustc_typeck/lib.rs

@@ -293,6 +293,7 @@ pub fn provide(providers: &mut Providers) {
     collect::provide(providers);
     coherence::provide(providers);
     check::provide(providers);
+    variance::provide(providers);
 }
 
 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>)
@@ -307,9 +308,6 @@ pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>)
 
     })?;
 
-    time(time_passes, "variance inference", ||
-         variance::infer_variance(tcx));
-
     tcx.sess.track_errors(|| {
         time(time_passes, "impl wf inference", ||
              impl_wf_check::impl_wf_check(tcx));

src/librustc_typeck/variance/README.md

@@ -97,51 +97,29 @@ types involved before considering variance.
 
 #### Dependency graph management
 
-Because variance works in two phases, if we are not careful, we wind
-up with a muddled mess of a dep-graph. Basically, when gathering up
-the constraints, things are fairly well-structured, but then we do a
-fixed-point iteration and write the results back where they
-belong. You can't give this fixed-point iteration a single task
-because it reads from (and writes to) the variance of all types in the
-crate. In principle, we *could* switch the "current task" in a very
-fine-grained way while propagating constraints in the fixed-point
-iteration and everything would be automatically tracked, but that
-would add some overhead and isn't really necessary anyway.
-
-Instead what we do is to add edges into the dependency graph as we
-construct the constraint set: so, if computing the constraints for
-node `X` requires loading the inference variables from node `Y`, then
-we can add an edge `Y -> X`, since the variance we ultimately infer
-for `Y` will affect the variance we ultimately infer for `X`.
-
-At this point, we've basically mirrored the inference graph in the
-dependency graph. This means we can just completely ignore the
-fixed-point iteration, since it is just shuffling values along this
-graph. In other words, if we added the fine-grained switching of tasks
-I described earlier, all it would show is that we repeatedly read the
-values described by the constraints, but those edges were already
-added when building the constraints in the first place.
-
-Here is how this is implemented (at least as of the time of this
-writing). The associated `DepNode` for the variance map is (at least
-presently) `Signature(DefId)`. This means that, in `constraints.rs`,
-when we visit an item to load up its constraints, we set
-`Signature(DefId)` as the current task (the "memoization" pattern
-described in the `dep-graph` README). Then whenever we find an
-embedded type or trait, we add a synthetic read of `Signature(DefId)`,
-which covers the variances we will compute for all of its
-parameters. This read is synthetic (i.e., we call
-`variance_map.read()`) because, in fact, the final variance is not yet
-computed -- the read *will* occur (repeatedly) during the fixed-point
-iteration phase.
-
-In fact, we don't really *need* this synthetic read. That's because we
-do wind up looking up the `TypeScheme` or `TraitDef` for all
-references types/traits, and those reads add an edge from
-`Signature(DefId)` (that is, they share the same dep node as
-variance). However, I've kept the synthetic reads in place anyway,
-just for future-proofing (in case we change the dep-nodes in the
-future), and because it makes the intention a bit clearer I think.
+Because variance is a whole-crate inference, its dependency graph
+can become quite muddled if we are not careful. To resolve this, we refactor
+into two queries:
+
+- `crate_variances` computes the variance for all items in the current crate.
+- `item_variances` accesses the variance for an individual reading; it
+  works by requesting `crate_variances` and extracting the relevant data.
+  
+If you limit yourself to reading `item_variances`, your code will only
+depend then on the inference inferred for that particular item.
+
+Eventually, the goal is to rely on the red-green dependency management
+algorithm. At the moment, however, we rely instead on a hack, where
+`item_variances` ignores the dependencies of accessing
+`crate_variances` and instead computes the *correct* dependencies
+itself. To this end, when we build up the constraints in the system,
+we also built up a transitive `dependencies` relation as part of the
+crate map. A `(X, Y)` pair is added to the map each time we have a
+constraint that the variance of some inferred for the item `X` depends
+on the variance of some element of `Y`. This is to some extent a
+mirroring of the inference graph in the dependency graph. This means
+we can just completely ignore the fixed-point iteration, since it is
+just shuffling values along this graph.
 
 ### Addendum: Variance on traits
 

src/librustc_typeck/variance/constraints.rs

@@ -15,6 +15,7 @@
 
 use hir::def_id::DefId;
 use middle::resolve_lifetime as rl;
+use rustc::dep_graph::{AssertDepGraphSafe, DepNode};
 use rustc::ty::subst::Substs;
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::hir::map as hir_map;
@@ -22,12 +23,12 @@
 use rustc::hir;
 use rustc::hir::itemlikevisit::ItemLikeVisitor;
 
+use rustc_data_structures::transitive_relation::TransitiveRelation;
+
 use super::terms::*;
 use super::terms::VarianceTerm::*;
 use super::xform::*;
 
-use dep_graph::DepNode::ItemSignature as VarianceDepNode;
-
 pub struct ConstraintContext<'a, 'tcx: 'a> {
     pub terms_cx: TermsContext<'a, 'tcx>,
 
@@ -38,6 +39,11 @@ pub struct ConstraintContext<'a, 'tcx: 'a> {
     bivariant: VarianceTermPtr<'a>,
 
     pub constraints: Vec<Constraint<'a>>,
+
+    /// This relation tracks the dependencies between the variance of
+    /// various items. In particular, if `a < b`, then the variance of
+    /// `a` depends on the sources of `b`.
+    pub dependencies: TransitiveRelation<DefId>,
 }
 
 /// Declares that the variable `decl_id` appears in a location with
@@ -57,7 +63,6 @@ pub struct Constraint<'a> {
 ///
 /// then while we are visiting `Bar<T>`, the `CurrentItem` would have
 /// the def-id and generics of `Foo`.
-#[allow(dead_code)] // TODO -- `def_id` field not used yet
 pub struct CurrentItem<'a> {
     def_id: DefId,
     generics: &'a ty::Generics,
@@ -77,10 +82,10 @@ pub fn add_constraints_from_crate<'a, 'tcx>(terms_cx: TermsContext<'a, 'tcx>)
         invariant: invariant,
         bivariant: bivariant,
         constraints: Vec::new(),
+        dependencies: TransitiveRelation::new(),
     };
 
-    // See README.md for a discussion on dep-graph management.
-    tcx.visit_all_item_likes_in_krate(VarianceDepNode, &mut constraint_cx);
+    tcx.hir.krate().visit_all_item_likes(&mut constraint_cx);
 
     constraint_cx
 }
@@ -90,13 +95,64 @@ fn visit_item(&mut self, item: &hir::Item) {
         let tcx = self.terms_cx.tcx;
         let def_id = tcx.hir.local_def_id(item.id);
 
+        // Encapsulate constructing the constraints into a task we can
+        // reference later. This can go away once the red-green
+        // algorithm is in place.
+        //
+        // See README.md for a detailed discussion
+        // on dep-graph management.
+        match item.node {
+            hir::ItemEnum(..) |
+            hir::ItemStruct(..) |
+            hir::ItemUnion(..) => {
+                tcx.dep_graph.with_task(DepNode::ItemVarianceConstraints(def_id),
+                                        AssertDepGraphSafe(self),
+                                        def_id,
+                                        visit_item_task);
+            }
+            _ => {
+                // Nothing to do here, skip the task.
+            }
+        }
+
+        fn visit_item_task<'a, 'tcx>(ccx: AssertDepGraphSafe<&mut ConstraintContext<'a, 'tcx>>,
+                                     def_id: DefId)
+        {
+            ccx.0.build_constraints_for_item(def_id);
+        }
+    }
+
+    fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {
+    }
+
+    fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {
+    }
+}
+
+/// Is `param_id` a lifetime according to `map`?
+fn is_lifetime(map: &hir_map::Map, param_id: ast::NodeId) -> bool {
+    match map.find(param_id) {
+        Some(hir_map::NodeLifetime(..)) => true,
+        _ => false,
+    }
+}
+
+impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
+    fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> {
+        self.terms_cx.tcx
+    }
+
+    fn build_constraints_for_item(&mut self, def_id: DefId) {
+        let tcx = self.tcx();
+        let id = self.tcx().hir.as_local_node_id(def_id).unwrap();
+        let item = tcx.hir.expect_item(id);
         debug!("visit_item item={}", tcx.hir.node_to_string(item.id));
 
         match item.node {
             hir::ItemEnum(..) |
             hir::ItemStruct(..) |
             hir::ItemUnion(..) => {
-                let generics = tcx.generics_of(did);
+                let generics = tcx.generics_of(def_id);
                 let current_item = &CurrentItem { def_id, generics };
 
                 // Not entirely obvious: constraints on structs/enums do not
@@ -105,24 +161,14 @@ fn visit_item(&mut self, item: &hir::Item) {
                 //
                 // self.add_constraints_from_generics(generics);
 
-                for field in tcx.adt_def(did).all_fields() {
+                for field in tcx.adt_def(def_id).all_fields() {
                     self.add_constraints_from_ty(current_item,
-                                                 tcx.item_type(field.did),
+                                                 tcx.type_of(field.did),
                                                  self.covariant);
                 }
             }
-            hir::ItemTrait(..) => {
-                let generics = tcx.generics_of(did);
-                let current_item = &CurrentItem { def_id, generics };
-                let trait_ref = ty::TraitRef {
-                    def_id: def_id,
-                    substs: Substs::identity_for_item(tcx, def_id)
-                };
-                self.add_constraints_from_trait_ref(current_item,
-                                                    trait_ref,
-                                                    self.invariant);
-            }
 
+            hir::ItemTrait(..) |
             hir::ItemExternCrate(_) |
             hir::ItemUse(..) |
             hir::ItemStatic(..) |
@@ -133,38 +179,25 @@ fn visit_item(&mut self, item: &hir::Item) {
             hir::ItemGlobalAsm(..) |
             hir::ItemTy(..) |
             hir::ItemImpl(..) |
-            hir::ItemDefaultImpl(..) => {}
+            hir::ItemDefaultImpl(..) => {
+                span_bug!(item.span, "`build_constraints_for_item` invoked for non-type-def");
             }
         }
-
-    fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {
     }
 
-    fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {
-    }
-}
-
-/// Is `param_id` a lifetime according to `map`?
-fn is_lifetime(map: &hir_map::Map, param_id: ast::NodeId) -> bool {
-    match map.find(param_id) {
-        Some(hir_map::NodeLifetime(..)) => true,
-        _ => false,
+    /// Load the generics for another item, adding a corresponding
+    /// relation into the dependencies to indicate that the variance
+    /// for `current` relies on `def_id`.
+    fn read_generics(&mut self, current: &CurrentItem, def_id: DefId) -> &'tcx ty::Generics {
+        let generics = self.tcx().generics_of(def_id);
+        if self.tcx().dep_graph.is_fully_enabled() {
+            self.dependencies.add(current.def_id, def_id);
         }
-}
-
-impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
-    fn tcx(&self) -> TyCtxt<'a, 'tcx, 'tcx> {
-        self.terms_cx.tcx
+        generics
     }
 
-    fn inferred_index(&self, param_id: ast::NodeId) -> InferredIndex {
-        match self.terms_cx.inferred_map.get(&param_id) {
-            Some(&index) => index,
-            None => {
-                bug!("no inferred index entry for {}",
-                     self.tcx().hir.node_to_string(param_id));
-            }
-        }
+    fn opt_inferred_index(&self, param_id: ast::NodeId) -> Option<&InferredIndex> {
+        self.terms_cx.inferred_map.get(&param_id)
     }
 
     fn find_binding_for_lifetime(&self, param_id: ast::NodeId) -> ast::NodeId {
@@ -245,8 +278,27 @@ fn declared_variance(&self,
             // Parameter on an item defined within current crate:
             // variance not yet inferred, so return a symbolic
             // variance.
-            let InferredIndex(index) = self.inferred_index(param_node_id);
+            if let Some(&InferredIndex(index)) = self.opt_inferred_index(param_node_id) {
                 self.terms_cx.inferred_infos[index].term
+            } else {
+                // If there is no inferred entry for a type parameter,
+                // it must be declared on a (locally defiend) trait -- they don't
+                // get inferreds because they are always invariant.
+                if cfg!(debug_assertions) {
+                    let item_node_id = self.tcx().hir.as_local_node_id(item_def_id).unwrap();
+                    let item = self.tcx().hir.expect_item(item_node_id);
+                    let success = match item.node {
+                        hir::ItemTrait(..) => true,
+                        _ => false,
+                    };
+                    if !success {
+                        bug!("parameter {:?} has no inferred, but declared on non-trait: {:?}",
+                             item_def_id,
+                             item);
+                    }
+                }
+                self.invariant
+            }
         } else {
             // Parameter on an item defined within another crate:
             // variance already inferred, just look it up.
@@ -305,11 +357,6 @@ fn add_constraints_from_trait_ref(&mut self,
 
         let trait_generics = self.tcx().generics_of(trait_ref.def_id);
 
-        // This edge is actually implied by the call to
-        // `trait_def`, but I'm trying to be future-proof. See
-        // README.md for a discussion on dep-graph management.
-        self.tcx().dep_graph.read(VarianceDepNode(trait_ref.def_id));
-
         self.add_constraints_from_substs(current,
                                          trait_ref.def_id,
                                          &trait_generics.types,
@@ -362,12 +409,7 @@ fn add_constraints_from_ty(&mut self,
             }
 
             ty::TyAdt(def, substs) => {
-                let adt_generics = self.tcx().generics_of(def.did);
-
-                // This edge is actually implied by the call to
-                // `trait_def`, but I'm trying to be future-proof. See
-                // README.md for a discussion on dep-graph management.
-                self.tcx().dep_graph.read(VarianceDepNode(def.did));
+                let adt_generics = self.read_generics(current, def.did);
 
                 self.add_constraints_from_substs(current,
                                                  def.did,
@@ -381,11 +423,6 @@ fn add_constraints_from_ty(&mut self,
                 let trait_ref = &data.trait_ref;
                 let trait_generics = self.tcx().generics_of(trait_ref.def_id);
 
-                // This edge is actually implied by the call to
-                // `trait_def`, but I'm trying to be future-proof. See
-                // README.md for a discussion on dep-graph management.
-                self.tcx().dep_graph.read(VarianceDepNode(trait_ref.def_id));
-
                 self.add_constraints_from_substs(current,
                                                  trait_ref.def_id,
                                                  &trait_generics.types,
@@ -505,7 +542,7 @@ fn add_constraints_from_region(&mut self,
                 let def_id = current.generics.regions[i].def_id;
                 let node_id = self.tcx().hir.as_local_node_id(def_id).unwrap();
                 if self.is_to_be_inferred(node_id) {
-                    let index = self.inferred_index(node_id);
+                    let &index = self.opt_inferred_index(node_id).unwrap();
                     self.add_constraint(index, variance);
                 }
             }

src/librustc_typeck/variance/mod.rs

@@ -12,7 +12,12 @@
 //! parameters. See README.md for details.
 
 use arena;
-use rustc::ty::TyCtxt;
+use rustc::dep_graph::DepNode;
+use rustc::hir;
+use rustc::hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
+use rustc::ty::{self, CrateVariancesMap, TyCtxt};
+use rustc::ty::maps::Providers;
+use std::rc::Rc;
 
 /// Defines the `TermsContext` basically houses an arena where we can
 /// allocate terms.
@@ -27,10 +32,60 @@
 /// Code for transforming variances.
 mod xform;
 
-pub fn infer_variance<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
+pub fn provide(providers: &mut Providers) {
+    *providers = Providers {
+        variances_of,
+        crate_variances,
+        ..*providers
+    };
+}
+
+fn crate_variances<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, crate_num: CrateNum)
+                             -> Rc<CrateVariancesMap> {
+    assert_eq!(crate_num, LOCAL_CRATE);
     let mut arena = arena::TypedArena::new();
     let terms_cx = terms::determine_parameters_to_be_inferred(tcx, &mut arena);
     let constraints_cx = constraints::add_constraints_from_crate(terms_cx);
-    solve::solve_constraints(constraints_cx);
-    tcx.variance_computed.set(true);
+    Rc::new(solve::solve_constraints(constraints_cx))
+}
+
+fn variances_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item_def_id: DefId)
+                            -> Rc<Vec<ty::Variance>> {
+    let item_id = tcx.hir.as_local_node_id(item_def_id).expect("expected local def-id");
+    let item = tcx.hir.expect_item(item_id);
+    match item.node {
+        hir::ItemTrait(..) => {
+            // Traits are always invariant.
+            let generics = tcx.generics_of(item_def_id);
+            assert!(generics.parent.is_none());
+            Rc::new(vec![ty::Variance::Invariant; generics.count()])
+        }
+
+        hir::ItemEnum(..) |
+        hir::ItemStruct(..) |
+        hir::ItemUnion(..) => {
+            // Everything else must be inferred.
+
+            // Lacking red/green, we read the variances for all items here
+            // but ignore the dependencies, then re-synthesize the ones we need.
+            let crate_map = tcx.dep_graph.with_ignore(|| tcx.crate_variances(LOCAL_CRATE));
+            tcx.dep_graph.read(DepNode::ItemVarianceConstraints(item_def_id));
+            for &dep_def_id in crate_map.dependencies.less_than(&item_def_id) {
+                if dep_def_id.is_local() {
+                    tcx.dep_graph.read(DepNode::ItemVarianceConstraints(dep_def_id));
+                } else {
+                    tcx.dep_graph.read(DepNode::ItemVariances(dep_def_id));
+                }
+            }
+
+            crate_map.variances.get(&item_def_id)
+                               .unwrap_or(&crate_map.empty_variance)
+                               .clone()
+        }
+
+        _ => {
+            // Variance not relevant.
+            span_bug!(item.span, "asked to compute variance for wrong kind of item")
+        }
+    }
 }

src/librustc_typeck/variance/solve.rs

@@ -15,7 +15,9 @@
 //! optimal solution to the constraints. The final variance for each
 //! inferred is then written into the `variance_map` in the tcx.
 
+use rustc::hir::def_id::DefId;
 use rustc::ty;
+use rustc_data_structures::fx::FxHashMap;
 use std::rc::Rc;
 
 use super::constraints::*;
@@ -31,8 +33,8 @@ struct SolveContext<'a, 'tcx: 'a> {
     solutions: Vec<ty::Variance>,
 }
 
-pub fn solve_constraints(constraints_cx: ConstraintContext) {
-    let ConstraintContext { terms_cx, constraints, .. } = constraints_cx;
+pub fn solve_constraints(constraints_cx: ConstraintContext) -> ty::CrateVariancesMap {
+    let ConstraintContext { terms_cx, dependencies, constraints, .. } = constraints_cx;
 
     let solutions = terms_cx.inferred_infos
         .iter()
@@ -45,7 +47,10 @@ pub fn solve_constraints(constraints_cx: ConstraintContext) {
         solutions: solutions,
     };
     solutions_cx.solve();
-    solutions_cx.write();
+    let variances = solutions_cx.create_map();
+    let empty_variance = Rc::new(Vec::new());
+
+    ty::CrateVariancesMap { dependencies, variances, empty_variance }
 }
 
 impl<'a, 'tcx> SolveContext<'a, 'tcx> {
@@ -83,7 +88,7 @@ fn solve(&mut self) {
         }
     }
 
-    fn write(&self) {
+    fn create_map(&self) -> FxHashMap<DefId, Rc<Vec<ty::Variance>>> {
         // Collect all the variances for a particular item and stick
         // them into the variance map. We rely on the fact that we
         // generate all the inferreds for a particular item
@@ -95,11 +100,7 @@ fn write(&self) {
 
         let tcx = self.terms_cx.tcx;
 
-        // Ignore the writes here because the relevant edges were
-        // already accounted for in `constraints.rs`. See the section
-        // on dependency graph management in README.md for more
-        // information.
-        let _ignore = tcx.dep_graph.in_ignore();
+        let mut map = FxHashMap();
 
         let solutions = &self.solutions;
         let inferred_infos = &self.terms_cx.inferred_infos;
@@ -137,9 +138,10 @@ fn write(&self) {
                           item_variances);
             }
 
-            tcx.maps.variances_of.borrow_mut()
-               .insert(item_def_id, Rc::new(item_variances));
+            map.insert(item_def_id, Rc::new(item_variances));
         }
+
+        map
     }
 
     fn evaluate(&self, term: VarianceTermPtr<'a>) -> ty::Variance {

src/librustc_typeck/variance/terms.rs

@@ -139,7 +139,6 @@ fn lang_items(tcx: TyCtxt) -> Vec<(ast::NodeId, Vec<ty::Variance>)> {
 impl<'a, 'tcx> TermsContext<'a, 'tcx> {
     fn add_inferreds_for_item(&mut self,
                               item_id: ast::NodeId,
-                              has_self: bool,
                               generics: &hir::Generics) {
         //! Add "inferreds" for the generic parameters declared on this
         //! item. This has a lot of annoying parameters because we are
@@ -149,38 +148,17 @@ fn add_inferreds_for_item(&mut self,
         //!
 
         // NB: In the code below for writing the results back into the
-        // tcx, we rely on the fact that all inferreds for a particular
-        // item are assigned continuous indices.
+        // `CrateVariancesMap`, we rely on the fact that all inferreds
+        // for a particular item are assigned continuous indices.
 
-        let inferreds_on_entry = self.num_inferred();
-
-        if has_self {
-            self.add_inferred(item_id, 0, item_id);
-        }
-
-        for (i, p) in generics.lifetimes.iter().enumerate() {
+        for (p, i) in generics.lifetimes.iter().zip(0..) {
             let id = p.lifetime.id;
-            let i = has_self as usize + i;
             self.add_inferred(item_id, i, id);
         }
 
-        for (i, p) in generics.ty_params.iter().enumerate() {
-            let i = has_self as usize + generics.lifetimes.len() + i;
+        for (p, i) in generics.ty_params.iter().zip(generics.lifetimes.len()..) {
             self.add_inferred(item_id, i, p.id);
         }
-
-        // If this item has no type or lifetime parameters,
-        // then there are no variances to infer, so just
-        // insert an empty entry into the variance map.
-        // Arguably we could just leave the map empty in this
-        // case but it seems cleaner to be able to distinguish
-        // "invalid item id" from "item id with no
-        // parameters".
-        if self.num_inferred() == inferreds_on_entry {
-            let item_def_id = self.tcx.hir.local_def_id(item_id);
-            self.tcx.maps.variances_of.borrow_mut()
-                .insert(item_def_id, self.empty_variances.clone());
-        }
     }
 
     fn add_inferred(&mut self, item_id: ast::NodeId, index: usize, param_id: ast::NodeId) {
@@ -232,15 +210,10 @@ fn visit_item(&mut self, item: &hir::Item) {
             hir::ItemEnum(_, ref generics) |
             hir::ItemStruct(_, ref generics) |
             hir::ItemUnion(_, ref generics) => {
-                self.add_inferreds_for_item(item.id, false, generics);
-            }
-            hir::ItemTrait(_, ref generics, ..) => {
-                // Note: all inputs for traits are ultimately
-                // constrained to be invariant. See `visit_item` in
-                // the impl for `ConstraintContext` in `constraints.rs`.
-                self.add_inferreds_for_item(item.id, true, generics);
+                self.add_inferreds_for_item(item.id, generics);
             }
 
+            hir::ItemTrait(..) |
             hir::ItemExternCrate(_) |
             hir::ItemUse(..) |
             hir::ItemDefaultImpl(..) |

src/test/compile-fail/dep-graph-variance-alias.rs

+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that changing what a `type` points to does not go unnoticed
+// by the variance analysis.
+
+// compile-flags: -Z query-dep-graph
+
+#![feature(rustc_attrs)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+fn main() { }
+
+struct Foo<T> {
+    f: T
+}
+
+#[rustc_if_this_changed]
+type TypeAlias<T> = Foo<T>;
+
+#[rustc_then_this_would_need(ItemVariances)] //~ ERROR OK
+struct Use<T> {
+    x: TypeAlias<T>
+}