Avoid adding edges to static where we already flag other errors

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};
+use crate::{BorrowckInferCtxt, NllRegionVariableOrigin, bug};
 
 /// A set of outlives constraints after rewriting to remove
 /// higher-kinded constraints.
@@ -428,39 +428,33 @@ fn rewrite_placeholder_outlives<'tcx>(
  // That constraint is annotated with some placeholder `unnameable` where
  // `unnameable` is unnameable from `r` and there is a path in the constraint graph
  // between them.
- //
- // There is one exception; if some other region in this SCC can't name `'r`, then
- // we pick the region with the smallest universe in the SCC, so that a path can
- // always start in `'r` to find a motivation that isn't cyclic.
- let blame_to = if annotation.representative.rvid() == max_u_rvid {
- // Assertion: the region that lowered our universe is an existential one and we are a placeholder!
-
+ if annotation.representative.rvid() != max_u_rvid {
+ // FIXME: if we can extract a useful blame span here, future error
+ // reporting and constraint search can be simplified.
+
+ added_constraints = true;
+ outlives_constraints.push(OutlivesConstraint {
+ sup: annotation.representative.rvid(),
+ sub: fr_static,
+ category: ConstraintCategory::OutlivesUnnameablePlaceholder(max_u_rvid),
+ locations: Locations::All(rustc_span::DUMMY_SP),
+ span: rustc_span::DUMMY_SP,
+ 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.
- let small_universed_rvid = annotation.max_nameable_universe.1;
- debug!(
- "{small_universed_rvid:?} lowered our universe to {:?}",
- annotation.max_nameable_universe()
+ // 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
  );
- small_universed_rvid
- } else {
- // `max_u_rvid` is not nameable by the SCC's representative.
- max_u_rvid
  };
-
- // FIXME: if we can extract a useful blame span here, future error
- // reporting and constraint search can be simplified.
-
- added_constraints = true;
- outlives_constraints.push(OutlivesConstraint {
- sup: annotation.representative.rvid(),
- sub: fr_static,
- category: ConstraintCategory::OutlivesUnnameablePlaceholder(blame_to),
- locations: Locations::All(rustc_span::DUMMY_SP),
- span: rustc_span::DUMMY_SP,
- variance_info: VarianceDiagInfo::None,
- from_closure: false,
- });
  }
  added_constraints
 }