Remove precise placeholder tracking from region inference

Author: amandasystems

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -224,64 +224,38 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         &self,
         diag: &mut Diag<'_>,
         lower_bound: RegionVid,
-    ) {
+    ) -> Option<()> {
         let tcx = self.infcx.tcx;
 
         // find generic associated types in the given region 'lower_bound'
-        let gat_id_and_generics = self
-            .regioncx
-            .placeholders_contained_in(lower_bound)
-            .map(|placeholder| {
-                if let Some(id) = placeholder.bound.kind.get_id()
-                    && let Some(placeholder_id) = id.as_local()
-                    && let gat_hir_id = tcx.local_def_id_to_hir_id(placeholder_id)
-                    && let Some(generics_impl) =
-                        tcx.parent_hir_node(tcx.parent_hir_id(gat_hir_id)).generics()
-                {
-                    Some((gat_hir_id, generics_impl))
-                } else {
-                    None
-                }
-            })
-            .collect::<Vec<_>>();
-        debug!(?gat_id_and_generics);
+        let scc = self.regioncx.constraint_sccs().scc(lower_bound);
+        let placeholder: ty::PlaceholderRegion = self.regioncx.placeholder_representative(scc)?;
+        let placeholder_id = placeholder.bound.kind.get_id()?.as_local()?;
+        let gat_hir_id = self.infcx.tcx.local_def_id_to_hir_id(placeholder_id);
+        let generics_impl =
+            self.infcx.tcx.parent_hir_node(self.infcx.tcx.parent_hir_id(gat_hir_id)).generics()?;
 
         // Look for the where-bound which introduces the placeholder.
         // As we're using the HIR, we need to handle both `for<'a> T: Trait<'a>`
         // and `T: for<'a> Trait`<'a>.
         let mut hrtb_bounds = vec![];
-        gat_id_and_generics.iter().flatten().for_each(|&(gat_hir_id, generics)| {
-            for pred in generics.predicates {
-                let BoundPredicate(WhereBoundPredicate { bound_generic_params, bounds, .. }) =
-                    pred.kind
-                else {
-                    continue;
-                };
-                if bound_generic_params
-                    .iter()
-                    .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
-                    .is_some()
-                {
-                    for bound in *bounds {
-                        hrtb_bounds.push(bound);
-                    }
-                } else {
-                    for bound in *bounds {
-                        if let Trait(trait_bound) = bound {
-                            if trait_bound
-                                .bound_generic_params
-                                .iter()
-                                .rfind(|bgp| tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
-                                .is_some()
-                            {
-                                hrtb_bounds.push(bound);
-                                return;
-                            }
-                        }
-                    }
+
+        for pred in generics_impl.predicates {
+            let BoundPredicate(WhereBoundPredicate { bound_generic_params, bounds, .. }) =
+                pred.kind
+            else {
+                continue;
+            };
+            if bound_generic_params
+                .iter()
+                .rfind(|bgp| self.infcx.tcx.local_def_id_to_hir_id(bgp.def_id) == gat_hir_id)
+                .is_some()
+            {
+                for bound in *bounds {
+                    hrtb_bounds.push(bound);
                 }
             }
-        });
+        }
         debug!(?hrtb_bounds);
 
         let mut suggestions = vec![];
@@ -327,6 +301,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 Applicability::MaybeIncorrect,
             );
         }
+        Some(())
     }
 
     /// Produces nice borrowck error diagnostics for all the errors collected in `nll_errors`.

compiler/rustc_borrowck/src/handle_placeholders.rs

@@ -14,7 +14,7 @@ use tracing::{debug, instrument, trace};
 use crate::constraints::{ConstraintSccIndex, OutlivesConstraintSet};
 use crate::consumers::OutlivesConstraint;
 use crate::diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo};
-use crate::region_infer::values::{LivenessValues, PlaceholderIndices};
+use crate::region_infer::values::LivenessValues;
 use crate::region_infer::{ConstraintSccs, RegionDefinition, Representative, TypeTest};
 use crate::ty::VarianceDiagInfo;
 use crate::type_check::free_region_relations::UniversalRegionRelations;
@@ -32,7 +32,6 @@ pub(crate) struct LoweredConstraints<'tcx> {
     pub(crate) type_tests: Vec<TypeTest<'tcx>>,
     pub(crate) liveness_constraints: LivenessValues,
     pub(crate) universe_causes: FxIndexMap<UniverseIndex, UniverseInfo<'tcx>>,
-    pub(crate) placeholder_indices: PlaceholderIndices,
 }
 
 impl<'d, 'tcx, A: scc::Annotation> SccAnnotations<'d, 'tcx, A> {
@@ -62,7 +61,7 @@ impl scc::Annotations<RegionVid> for SccAnnotations<'_, '_, RegionTracker> {
 }
 
 #[derive(Copy, Debug, Clone, PartialEq, Eq)]
-enum PlaceholderReachability {
+pub(crate) enum PlaceholderReachability {
     /// This SCC reaches no placeholders.
     NoPlaceholders,
     /// This SCC reaches at least one placeholder.
@@ -120,7 +119,7 @@ impl PlaceholderReachability {
 /// the values of its elements. This annotates a single SCC.
 #[derive(Copy, Debug, Clone)]
 pub(crate) struct RegionTracker {
-    reachable_placeholders: PlaceholderReachability,
+    pub(crate) reachable_placeholders: PlaceholderReachability,
 
     /// The smallest max nameable universe of all
     /// regions reachable from this SCC.
@@ -245,6 +244,16 @@ impl RegionTracker {
             PlaceholderReachability::Placeholders { min_placeholder, .. } => Some(min_placeholder),
         }
     }
+
+    /// If this SCC reaches at least one placeholder, return
+    /// its region vid. If there's more than one, return the one
+    /// with the smallest vid.
+    pub(crate) fn reached_placeholder(&self) -> Option<RegionVid> {
+        match self.reachable_placeholders {
+            PlaceholderReachability::NoPlaceholders => None,
+            PlaceholderReachability::Placeholders { min_placeholder, .. } => Some(min_placeholder),
+        }
+    }
 }
 
 impl scc::Annotation for RegionTracker {
@@ -350,12 +359,11 @@ pub(crate) fn compute_sccs_applying_placeholder_outlives_constraints<'tcx>(
     let (definitions, has_placeholders) = region_definitions(infcx, universal_regions);
 
     let MirTypeckRegionConstraints {
-        placeholder_indices,
-        placeholder_index_to_region: _,
         liveness_constraints,
         mut outlives_constraints,
         universe_causes,
         type_tests,
+        placeholder_to_region: _
     } = constraints;
 
     let fr_static = universal_regions.fr_static;
@@ -385,7 +393,6 @@ pub(crate) fn compute_sccs_applying_placeholder_outlives_constraints<'tcx>(
             outlives_constraints: Frozen::freeze(outlives_constraints),
             liveness_constraints,
             universe_causes,
-            placeholder_indices,
         };
     }
     debug!("Placeholders present; activating placeholder handling logic!");
@@ -426,7 +433,6 @@ pub(crate) fn compute_sccs_applying_placeholder_outlives_constraints<'tcx>(
         type_tests,
         liveness_constraints,
         universe_causes,
-        placeholder_indices,
     }
 }