Remove explicit tracking of placeholders from borrowcheck

Author: amandasystems

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -29,7 +29,6 @@ use tracing::{debug, instrument, trace};
 
 use super::{OutlivesSuggestionBuilder, RegionName, RegionNameSource};
 use crate::nll::ConstraintDescription;
-use crate::region_infer::values::RegionElement;
 use crate::region_infer::{BlameConstraint, TypeTest};
 use crate::session_diagnostics::{
     FnMutError, FnMutReturnTypeErr, GenericDoesNotLiveLongEnough, LifetimeOutliveErr,
@@ -104,38 +103,9 @@ pub(crate) enum RegionErrorKind<'tcx> {
     /// A generic bound failure for a type test (`T: 'a`).
     TypeTestError { type_test: TypeTest<'tcx> },
 
-    /// 'a outlives 'b, and both are placeholders.
-    PlaceholderOutlivesPlaceholder {
-        rvid_a: RegionVid,
-        rvid_b: RegionVid,
-        origin_a: ty::PlaceholderRegion<'tcx>,
-        origin_b: ty::PlaceholderRegion<'tcx>,
-    },
-
-    /// Indicates that a placeholder has a universe too large for one
-    /// of its member existentials, or, equivalently, that there is
-    /// a path through the outlives constraint graph from a placeholder
-    /// to an existential region that cannot name it.
-    PlaceholderOutlivesExistentialThatCannotNameIt {
-        /// the placeholder that transitively outlives an
-        /// existential that shouldn't leak into it
-        longer_fr: RegionVid,
-        /// The existential leaking into `longer_fr`.
-        existential_that_cannot_name_longer: RegionVid,
-        // `longer_fr`'s originating placeholder region.
-        placeholder: ty::PlaceholderRegion<'tcx>,
-    },
-
-    /// Higher-ranked subtyping error. A placeholder outlives
-    /// either a location or a universal region.
-    PlaceholderOutlivesLocationOrUniversal {
-        /// The placeholder free region.
-        longer_fr: RegionVid,
-        /// The region element that erroneously must be outlived by `longer_fr`.
-        error_element: RegionElement<'tcx>,
-        /// The placeholder region.
-        placeholder: ty::PlaceholderRegion<'tcx>,
-    },
+    /// 'p outlives 'r, which does not hold. 'p is always a placeholder
+    /// and 'r is some other region.
+    PlaceholderOutlivesIllegalRegion { longer_fr: RegionVid, illegally_outlived_r: RegionVid },
 
     /// Any other lifetime error.
     RegionError {
@@ -228,60 +198,47 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         let tcx = self.infcx.tcx;
 
         // find generic associated types in the given region 'lower_bound'
-        let gat_id_and_generics = self
+        let scc = self.regioncx.constraint_sccs().scc(lower_bound);
+        let Some(gat_hir_id) = 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);
+            .placeholder_representative(scc)
+            .and_then(|placeholder| placeholder.bound.kind.get_id())
+            .and_then(|id| id.as_local())
+            .map(|local| self.infcx.tcx.local_def_id_to_hir_id(local))
+        else {
+            return;
+        };
 
         // 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;
-                            }
-                        }
-                    }
+
+        // FIXME(amandasystems) we can probably flatten this.
+        for pred in self
+            .infcx
+            .tcx
+            .parent_hir_node(self.infcx.tcx.parent_hir_id(gat_hir_id))
+            .generics()
+            .map(|gen_impl| gen_impl.predicates)
+            .into_iter()
+            .flatten()
+        {
+            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![];
@@ -384,56 +341,13 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     }
                 }
 
-                RegionErrorKind::PlaceholderOutlivesLocationOrUniversal {
+                RegionErrorKind::PlaceholderOutlivesIllegalRegion {
                     longer_fr,
-                    placeholder,
-                    error_element,
-                } => self.report_erroneous_rvid_reaches_placeholder(
-                    longer_fr,
-                    placeholder,
-                    self.regioncx.region_from_element(longer_fr, &error_element),
-                ),
-                RegionErrorKind::PlaceholderOutlivesPlaceholder {
-                    rvid_a,
-                    rvid_b,
-                    origin_a,
-                    origin_b,
+                    illegally_outlived_r,
                 } => {
-                    debug!(
-                        "Placeholder mismatch: {rvid_a:?} ({origin_a:?}) reaches {rvid_b:?} ({origin_b:?})"
-                    );
-
-                    let cause = self
-                        .regioncx
-                        .best_blame_constraint(
-                            rvid_a,
-                            NllRegionVariableOrigin::Placeholder(origin_a),
-                            rvid_b,
-                        )
-                        .0
-                        .cause;
-
-                    // FIXME We may be able to shorten the code path here, and immediately
-                    // report a `RegionResolutionError::UpperBoundUniverseConflict`, but
-                    // that's left for a future refactoring.
-                    self.regioncx.universe_info(origin_a.universe).report_erroneous_element(
-                        self,
-                        origin_a,
-                        Some(origin_b),
-                        cause,
-                    );
+                    self.report_erroneous_rvid_reaches_placeholder(longer_fr, illegally_outlived_r)
                 }
 
-                RegionErrorKind::PlaceholderOutlivesExistentialThatCannotNameIt {
-                    longer_fr,
-                    existential_that_cannot_name_longer,
-                    placeholder,
-                } => self.report_erroneous_rvid_reaches_placeholder(
-                    longer_fr,
-                    placeholder,
-                    existential_that_cannot_name_longer,
-                ),
-
                 RegionErrorKind::RegionError { fr_origin, longer_fr, shorter_fr, is_reported } => {
                     if is_reported {
                         self.report_region_error(