Auto merge of rust-lang#45473 - SimonSapin:variance-red-green, r=nikomatsakis

Remove dependency tracking for variance computation

This custom tracking is now replaced by the red/green algorithm.

Fix rust-lang#45471

Author: bors

src/librustc/ich/impls_ty.rs

@@ -755,13 +755,11 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for ty::CrateVariancesMap {
                                           hcx: &mut StableHashingContext<'gcx>,
                                           hasher: &mut StableHasher<W>) {
         let ty::CrateVariancesMap {
-            ref dependencies,
             ref variances,
             // This is just an irrelevant helper value.
             empty_variance: _,
         } = *self;
 
-        dependencies.hash_stable(hcx, hasher);
         variances.hash_stable(hcx, hasher);
     }
 }

src/librustc/ty/mod.rs

@@ -55,7 +55,6 @@ use rustc_const_math::ConstInt;
 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;
 
@@ -313,11 +312,6 @@ pub enum Variance {
 /// `tcx.variances_of()` 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.

src/librustc_typeck/variance/README.md

@@ -104,22 +104,16 @@ into two queries:
 - `crate_variances` computes the variance for all items in the current crate.
 - `variances_of` accesses the variance for an individual reading; it
   works by requesting `crate_variances` and extracting the relevant data.
-  
+
 If you limit yourself to reading `variances_of`, 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
-`variances_of` 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.
+Ultimately, this setup relies on the red-green algorithm.
+In particular, every variance query ultimately depends on -- effectively --
+all type definitions in the entire crate (through `crate_variances`),
+but since most changes will not result in a change
+to the actual results from variance inference,
+the `variances_of` query will wind up being considered green after it is re-evaluated.
 
 ### Addendum: Variance on traits
 

src/librustc_typeck/variance/constraints.rs

@@ -22,7 +22,6 @@ use syntax::ast;
 use rustc::hir;
 use rustc::hir::itemlikevisit::ItemLikeVisitor;
 
-use rustc_data_structures::transitive_relation::TransitiveRelation;
 use rustc_data_structures::stable_hasher::StableHashingContextProvider;
 
 use super::terms::*;
@@ -38,11 +37,6 @@ 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
@@ -63,7 +57,6 @@ pub struct Constraint<'a> {
 /// then while we are visiting `Bar<T>`, the `CurrentItem` would have
 /// the def-id and the start of `Foo`'s inferreds.
 pub struct CurrentItem {
-    def_id: DefId,
     inferred_start: InferredIndex,
 }
 
@@ -81,7 +74,6 @@ pub fn add_constraints_from_crate<'a, 'tcx>(terms_cx: TermsContext<'a, 'tcx>)
         invariant,
         bivariant,
         constraints: Vec::new(),
-        dependencies: TransitiveRelation::new(),
     };
 
     tcx.hir.krate().visit_all_item_likes(&mut constraint_cx);
@@ -201,7 +193,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
 
         let id = tcx.hir.as_local_node_id(def_id).unwrap();
         let inferred_start = self.terms_cx.inferred_starts[&id];
-        let current_item = &CurrentItem { def_id, inferred_start };
+        let current_item = &CurrentItem { inferred_start };
         match tcx.type_of(def_id).sty {
             ty::TyAdt(def, _) => {
                 // Not entirely obvious: constraints on structs/enums do not
@@ -410,12 +402,6 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
             return;
         }
 
-        // Add a corresponding relation into the dependencies to
-        // indicate that the variance for `current` relies on `def_id`.
-        if self.tcx().dep_graph.is_fully_enabled() {
-            self.dependencies.add(current.def_id, def_id);
-        }
-
         let (local, remote) = if let Some(id) = self.tcx().hir.as_local_node_id(def_id) {
             (Some(self.terms_cx.inferred_starts[&id]), None)
         } else {

src/librustc_typeck/variance/mod.rs

@@ -94,20 +94,9 @@ fn variances_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item_def_id: DefId)
 
     // 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));
+    let crate_map = tcx.crate_variances(LOCAL_CRATE);
     let dep_node = item_def_id.to_dep_node(tcx, DepKind::ItemVarianceConstraints);
     tcx.dep_graph.read(dep_node);
-    for &dep_def_id in crate_map.dependencies.less_than(&item_def_id) {
-        if dep_def_id.is_local() {
-            let dep_node = dep_def_id.to_dep_node(tcx, DepKind::ItemVarianceConstraints);
-            tcx.dep_graph.read(dep_node);
-        } else {
-            let dep_node = dep_def_id.to_dep_node(tcx, DepKind::ItemVariances);
-            tcx.dep_graph.read(dep_node);
-        }
-    }
 
     crate_map.variances.get(&item_def_id)
                        .unwrap_or(&crate_map.empty_variance)

src/librustc_typeck/variance/solve.rs

@@ -34,7 +34,7 @@ struct SolveContext<'a, 'tcx: 'a> {
 }
 
 pub fn solve_constraints(constraints_cx: ConstraintContext) -> ty::CrateVariancesMap {
-    let ConstraintContext { terms_cx, dependencies, constraints, .. } = constraints_cx;
+    let ConstraintContext { terms_cx, constraints, .. } = constraints_cx;
 
     let mut solutions = vec![ty::Bivariant; terms_cx.inferred_terms.len()];
     for &(id, ref variances) in &terms_cx.lang_items {
@@ -53,7 +53,7 @@ pub fn solve_constraints(constraints_cx: ConstraintContext) -> ty::CrateVariance
     let variances = solutions_cx.create_map();
     let empty_variance = Rc::new(Vec::new());
 
-    ty::CrateVariancesMap { dependencies, variances, empty_variance }
+    ty::CrateVariancesMap { variances, empty_variance }
 }
 
 impl<'a, 'tcx> SolveContext<'a, 'tcx> {