Only flag the error for placeholder-existential failures if there is

actually an existential.

Closes: #146467

Author: amandasystems

compiler/rustc_borrowck/src/handle_placeholders.rs

@@ -20,7 +20,7 @@ use crate::ty::VarianceDiagInfo;
 use crate::type_check::free_region_relations::UniversalRegionRelations;
 use crate::type_check::{Locations, MirTypeckRegionConstraints};
 use crate::universal_regions::UniversalRegions;
-use crate::{BorrowckInferCtxt, NllRegionVariableOrigin, bug};
+use crate::{BorrowckInferCtxt, NllRegionVariableOrigin};
 
 /// A set of outlives constraints after rewriting to remove
 /// higher-kinded constraints.
@@ -188,13 +188,23 @@ impl RegionTracker {
     /// the SCC itself.
     ///
     /// Returns *a* culprit (there may be more than one).
-    fn reaches_existential_that_cannot_name_us(&self) -> Option<RegionVid> {
+    fn reaches_existential_that_cannot_name_us(
+        &self,
+        region_definitions: &IndexVec<RegionVid, RegionDefinition<'_>>,
+    ) -> Option<RegionVid> {
         let Representative::Placeholder(_p) = self.representative else {
             return None;
         };
 
         let (reachable_lowest_max_u, reachable_lowest_max_u_rvid) = self.max_nameable_universe;
 
+        let NllRegionVariableOrigin::Existential { .. } =
+            region_definitions[reachable_lowest_max_u_rvid].origin
+        else {
+            debug!("SCC universe wasn't lowered by an existential; skipping.");
+            return None;
+        };
+
         (!self.reachable_placeholders.can_be_named_by(reachable_lowest_max_u))
             .then_some(reachable_lowest_max_u_rvid)
     }
@@ -442,19 +452,7 @@ fn rewrite_placeholder_outlives<'tcx>(
                 variance_info: VarianceDiagInfo::None,
                 from_closure: false,
             });
-        } else if !(annotation.reaches_existential_that_cannot_name_us().is_some()
-            || annotation.reaches_other_placeholder(annotation.representative.rvid()).is_some())
-        {
-            // The SCC's representative is not nameable from some region
-            // that ends up in the SCC. This means there is nothing for us to do.
-            // However, this is only possible under circumstances that produce
-            // errors, so we make sure that we catch them here. Otherwise,
-            // there might actually be soundness issues!
-            bug!(
-                "Universe of SCC {scc:?} should have been lowered by an existential or at least another placeholder but was lowered by {:?}, which is neither.",
-                annotation.max_nameable_universe
-            );
-        };
+        }
     }
     added_constraints
 }
@@ -486,12 +484,14 @@ fn find_placeholder_mismatch_errors<'tcx>(
         let scc = sccs.scc(rvid);
         let annotation = annotations.scc_to_annotation[scc];
 
-        if let Some(existential_that_cannot_name_rvid) =
-            annotation.reaches_existential_that_cannot_name_us()
+        if let Some(cannot_name_rvid) =
+            annotation.reaches_existential_that_cannot_name_us(definitions)
         {
+            debug!("Existential {cannot_name_rvid:?} lowered our universe...");
+
             errors_buffer.push(RegionErrorKind::PlaceholderOutlivesExistentialThatCannotNameIt {
                 longer_fr: rvid,
-                existential_that_cannot_name_longer: existential_that_cannot_name_rvid,
+                existential_that_cannot_name_longer: cannot_name_rvid,
                 placeholder: origin_a,
             })
         }

compiler/rustc_borrowck/src/nll.rs

@@ -136,6 +136,8 @@ pub(crate) fn compute_regions<'tcx>(
         &mut placeholder_errors,
     );
 
+    debug!("Placeholder errors: {placeholder_errors:?}");
+
     // If requested, emit legacy polonius facts.
     polonius::legacy::emit_facts(
         &mut polonius_facts,
@@ -188,6 +190,7 @@ pub(crate) fn compute_regions<'tcx>(
 
     let nll_errors = if region_inference_errors.is_empty() {
         // Only flag the higher-kinded bounds errors if there are no borrowck errors.
+        debug!("No region inference errors, using placeholder errors: {placeholder_errors:?}");
         placeholder_errors
     } else {
         debug!("Errors already reported, skipping these: {placeholder_errors:?}");

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1566,7 +1566,9 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         from_region_origin: NllRegionVariableOrigin,
         to_region: RegionVid,
     ) -> (BlameConstraint<'tcx>, Vec<OutlivesConstraint<'tcx>>) {
-        assert!(from_region != to_region, "Trying to blame a region for itself!");
+        if from_region == to_region {
+            bug!("Trying to blame {from_region:?} for itself!");
+        }
 
         let path = self.constraint_path_between_regions(from_region, to_region).unwrap();