Update implied_outlives_bounds to properly register implied bounds behind normalization

Author: jackh726

Diff for: compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -226,7 +226,6 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
         let param_env = self.param_env;
         self.add_outlives_bounds(outlives::explicit_outlives_bounds(param_env));
 
-        // Finally:
         // - outlives is reflexive, so `'r: 'r` for every region `'r`
         // - `'static: 'r` for every region `'r`
         // - `'r: 'fn_body` for every (other) universally quantified
@@ -263,7 +262,9 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
             // We add implied bounds from both the unnormalized and normalized ty.
             // See issue #87748
             let constraints_unnorm = self.add_implied_bounds(ty);
-            constraints_unnorm.map(|c| constraints.push(c));
+            if let Some(c) = constraints_unnorm {
+                constraints.push(c)
+            }
             let TypeOpOutput { output: norm_ty, constraints: constraints_normalize, .. } = self
                 .param_env
                 .and(type_op::normalize::Normalize::new(ty))
@@ -279,7 +280,9 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
                         error_info: None,
                     }
                 });
-            constraints_normalize.map(|c| constraints.push(c));
+            if let Some(c) = constraints_normalize {
+                constraints.push(c)
+            }
 
             // Note: we need this in examples like
             // ```
@@ -295,7 +298,9 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
             // Both &Self::Bar and &() are WF
             if ty != norm_ty {
                 let constraints_norm = self.add_implied_bounds(norm_ty);
-                constraints_norm.map(|c| constraints.push(c));
+                if let Some(c) = constraints_norm {
+                    constraints.push(c)
+                }
             }
 
             normalized_inputs_and_output.push(norm_ty);

Diff for: compiler/rustc_borrowck/src/type_check/mod.rs

@@ -910,7 +910,7 @@ pub(crate) struct MirTypeckRegionConstraints<'tcx> {
 }
 
 impl<'tcx> MirTypeckRegionConstraints<'tcx> {
-    /// Creates a `Region` that for a given `PlaceholderRegion`, or returns the
+    /// Creates a `Region` for a given `PlaceholderRegion`, or returns the
     /// region that corresponds to a previously created one.
     fn placeholder_region(
         &mut self,

Diff for: compiler/rustc_traits/src/implied_outlives_bounds.rs

@@ -70,48 +70,61 @@ fn compute_implied_outlives_bounds<'tcx>(
         let obligations = wf::obligations(ocx.infcx, param_env, CRATE_DEF_ID, 0, arg, DUMMY_SP)
             .unwrap_or_default();
 
-        // While these predicates should all be implied by other parts of
-        // the program, they are still relevant as they may constrain
-        // inference variables, which is necessary to add the correct
-        // implied bounds in some cases, mostly when dealing with projections.
-        ocx.register_obligations(
-            obligations.iter().filter(|o| o.predicate.has_non_region_infer()).cloned(),
-        );
-
-        // From the full set of obligations, just filter down to the
-        // region relationships.
-        outlives_bounds.extend(obligations.into_iter().filter_map(|obligation| {
+        for obligation in obligations {
             debug!(?obligation);
             assert!(!obligation.has_escaping_bound_vars());
-            match obligation.predicate.kind().no_bound_vars() {
-                None => None,
-                Some(pred) => match pred {
-                    ty::PredicateKind::Clause(ty::Clause::Trait(..))
-                    | ty::PredicateKind::Subtype(..)
-                    | ty::PredicateKind::Coerce(..)
-                    | ty::PredicateKind::Clause(ty::Clause::Projection(..))
-                    | ty::PredicateKind::ClosureKind(..)
-                    | ty::PredicateKind::ObjectSafe(..)
-                    | ty::PredicateKind::ConstEvaluatable(..)
-                    | ty::PredicateKind::ConstEquate(..)
-                    | ty::PredicateKind::Ambiguous
-                    | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
-                    ty::PredicateKind::WellFormed(arg) => {
-                        wf_args.push(arg);
-                        None
+
+            // While these predicates should all be implied by other parts of
+            // the program, they are still relevant as they may constrain
+            // inference variables, which is necessary to add the correct
+            // implied bounds in some cases, mostly when dealing with projections.
+            //
+            // Another important point here: we only register `Projection`
+            // predicates, since otherwise we might register outlives
+            // predicates containing inference variables, and we don't
+            // learn anything new from those.
+            if obligation.predicate.has_non_region_infer() {
+                match obligation.predicate.kind().skip_binder() {
+                    ty::PredicateKind::Clause(ty::Clause::Projection(..)) => {
+                        ocx.register_obligation(obligation.clone());
                     }
+                    _ => {}
+                }
+            }
 
-                    ty::PredicateKind::Clause(ty::Clause::RegionOutlives(
-                        ty::OutlivesPredicate(r_a, r_b),
-                    )) => Some(ty::OutlivesPredicate(r_a.into(), r_b)),
+            let pred = match obligation.predicate.kind().no_bound_vars() {
+                None => continue,
+                Some(pred) => pred,
+            };
+            match pred {
+                ty::PredicateKind::Clause(ty::Clause::Trait(..))
+                | ty::PredicateKind::Subtype(..)
+                | ty::PredicateKind::Coerce(..)
+                | ty::PredicateKind::Clause(ty::Clause::Projection(..))
+                | ty::PredicateKind::ClosureKind(..)
+                | ty::PredicateKind::ObjectSafe(..)
+                | ty::PredicateKind::ConstEvaluatable(..)
+                | ty::PredicateKind::ConstEquate(..)
+                | ty::PredicateKind::Ambiguous
+                | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
+
+                // We need to search through *all* WellFormed predicates
+                ty::PredicateKind::WellFormed(arg) => {
+                    wf_args.push(arg);
+                }
+
+                // We need to register region relationships
+                ty::PredicateKind::Clause(ty::Clause::RegionOutlives(ty::OutlivesPredicate(
+                    r_a,
+                    r_b,
+                ))) => outlives_bounds.push(ty::OutlivesPredicate(r_a.into(), r_b)),
 
-                    ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
-                        ty_a,
-                        r_b,
-                    ))) => Some(ty::OutlivesPredicate(ty_a.into(), r_b)),
-                },
+                ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
+                    ty_a,
+                    r_b,
+                ))) => outlives_bounds.push(ty::OutlivesPredicate(ty_a.into(), r_b)),
             }
-        }));
+        }
     }
 
     // This call to `select_all_or_error` is necessary to constrain inference variables, which we

Diff for: compiler/rustc_traits/src/lib.rs

@@ -4,6 +4,7 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 #![feature(let_chains)]
+#![feature(drain_filter)]
 #![recursion_limit = "256"]
 
 #[macro_use]