1 files changed, 135 insertions, 0 deletions
diff --git a/compiler/rustc_typeck/src/variance/solve.rs b/compiler/rustc_typeck/src/variance/solve.rs
new file mode 100644
index 000000000..97aca621a
--- /dev/null
+++ b/compiler/rustc_typeck/src/variance/solve.rs
@@ -0,0 +1,135 @@
+//! Constraint solving
+//!
+//! The final phase iterates over the constraints, refining the variance
+//! for each inferred until a fixed point is reached. This will be the
+//! optimal solution to the constraints. The final variance for each
+//! inferred is then written into the `variance_map` in the tcx.
+
+use rustc_data_structures::fx::FxHashMap;
+use rustc_hir::def_id::DefId;
+use rustc_middle::ty;
+
+use super::constraints::*;
+use super::terms::VarianceTerm::*;
+use super::terms::*;
+use super::xform::*;
+
+struct SolveContext<'a, 'tcx> {
+    terms_cx: TermsContext<'a, 'tcx>,
+    constraints: Vec<Constraint<'a>>,
+
+    // Maps from an InferredIndex to the inferred value for that variable.
+    solutions: Vec<ty::Variance>,
+}
+
+pub fn solve_constraints<'tcx>(
+    constraints_cx: ConstraintContext<'_, 'tcx>,
+) -> ty::CrateVariancesMap<'tcx> {
+    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 {
+        let InferredIndex(start) = terms_cx.inferred_starts[&id];
+        for (i, &variance) in variances.iter().enumerate() {
+            solutions[start + i] = variance;
+        }
+    }
+
+    let mut solutions_cx = SolveContext { terms_cx, constraints, solutions };
+    solutions_cx.solve();
+    let variances = solutions_cx.create_map();
+
+    ty::CrateVariancesMap { variances }
+}
+
+impl<'a, 'tcx> SolveContext<'a, 'tcx> {
+    fn solve(&mut self) {
+        // Propagate constraints until a fixed point is reached.  Note
+        // that the maximum number of iterations is 2C where C is the
+        // number of constraints (each variable can change values at most
+        // twice). Since number of constraints is linear in size of the
+        // input, so is the inference process.
+        let mut changed = true;
+        while changed {
+            changed = false;
+
+            for constraint in &self.constraints {
+                let Constraint { inferred, variance: term } = *constraint;
+                let InferredIndex(inferred) = inferred;
+                let variance = self.evaluate(term);
+                let old_value = self.solutions[inferred];
+                let new_value = glb(variance, old_value);
+                if old_value != new_value {
+                    debug!(
+                        "updating inferred {} \
+                            from {:?} to {:?} due to {:?}",
+                        inferred, old_value, new_value, term
+                    );
+
+                    self.solutions[inferred] = new_value;
+                    changed = true;
+                }
+            }
+        }
+    }
+
+    fn enforce_const_invariance(&self, generics: &ty::Generics, variances: &mut [ty::Variance]) {
+        let tcx = self.terms_cx.tcx;
+
+        // Make all const parameters invariant.
+        for param in generics.params.iter() {
+            if let ty::GenericParamDefKind::Const { .. } = param.kind {
+                variances[param.index as usize] = ty::Invariant;
+            }
+        }
+
+        // Make all the const parameters in the parent invariant (recursively).
+        if let Some(def_id) = generics.parent {
+            self.enforce_const_invariance(tcx.generics_of(def_id), variances);
+        }
+    }
+
+    fn create_map(&self) -> FxHashMap<DefId, &'tcx [ty::Variance]> {
+        let tcx = self.terms_cx.tcx;
+
+        let solutions = &self.solutions;
+        self.terms_cx
+            .inferred_starts
+            .iter()
+            .map(|(&def_id, &InferredIndex(start))| {
+                let generics = tcx.generics_of(def_id);
+                let count = generics.count();
+
+                let variances = tcx.arena.alloc_slice(&solutions[start..(start + count)]);
+
+                // Const parameters are always invariant.
+                self.enforce_const_invariance(generics, variances);
+
+                // Functions are permitted to have unused generic parameters: make those invariant.
+                if let ty::FnDef(..) = tcx.type_of(def_id).kind() {
+                    for variance in variances.iter_mut() {
+                        if *variance == ty::Bivariant {
+                            *variance = ty::Invariant;
+                        }
+                    }
+                }
+
+                (def_id.to_def_id(), &*variances)
+            })
+            .collect()
+    }
+
+    fn evaluate(&self, term: VarianceTermPtr<'a>) -> ty::Variance {
+        match *term {
+            ConstantTerm(v) => v,
+
+            TransformTerm(t1, t2) => {
+                let v1 = self.evaluate(t1);
+                let v2 = self.evaluate(t2);
+                v1.xform(v2)
+            }
+
+            InferredTerm(InferredIndex(index)) => self.solutions[index],
+        }
+    }
+}