diff options
Diffstat (limited to 'compiler/rustc_borrowck/src')
6 files changed, 249 insertions, 188 deletions
diff --git a/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs b/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs index fdca6b56540..fda96dde826 100644 --- a/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs +++ b/compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs @@ -565,7 +565,7 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> { let (blame_constraint, path) = self.regioncx.best_blame_constraint( borrow_region, NllRegionVariableOrigin::FreeRegion, - |r| self.regioncx.provides_universal_region(r, borrow_region, outlived_region), + outlived_region, ); let BlameConstraint { category, from_closure, cause, .. } = blame_constraint; diff --git a/compiler/rustc_borrowck/src/diagnostics/opaque_types.rs b/compiler/rustc_borrowck/src/diagnostics/opaque_types.rs index f77f759035b..99913626418 100644 --- a/compiler/rustc_borrowck/src/diagnostics/opaque_types.rs +++ b/compiler/rustc_borrowck/src/diagnostics/opaque_types.rs @@ -243,10 +243,9 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for FindOpaqueRegion<'_, 'tcx> { let opaque_region_vid = self.regioncx.to_region_vid(opaque_region); // Find a path between the borrow region and our opaque capture. - if let Some((path, _)) = - self.regioncx.find_constraint_path_between_regions(self.borrow_region, |r| { - r == opaque_region_vid - }) + if let Some(path) = self + .regioncx + .constraint_path_between_regions(self.borrow_region, opaque_region_vid) { for constraint in path { // If we find a call in this path, then check if it defines the opaque. diff --git a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs index c7d2267e5f7..bec4c934502 100644 --- a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs +++ b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs @@ -61,7 +61,7 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> { | ConstraintCategory::Boring | ConstraintCategory::BoringNoLocation | ConstraintCategory::Internal - | ConstraintCategory::IllegalUniverse => "", + | ConstraintCategory::OutlivesUnnameablePlaceholder(..) => "", } } } @@ -369,11 +369,15 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> { let error_vid = self.regioncx.region_from_element(longer_fr, &error_element); // Find the code to blame for the fact that `longer_fr` outlives `error_fr`. - let (_, cause) = self.regioncx.find_outlives_blame_span( - longer_fr, - NllRegionVariableOrigin::Placeholder(placeholder), - error_vid, - ); + let cause = self + .regioncx + .best_blame_constraint( + longer_fr, + NllRegionVariableOrigin::Placeholder(placeholder), + error_vid, + ) + .0 + .cause; let universe = placeholder.universe; let universe_info = self.regioncx.universe_info(universe); @@ -429,9 +433,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> { ) { debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr); - let (blame_constraint, path) = self.regioncx.best_blame_constraint(fr, fr_origin, |r| { - self.regioncx.provides_universal_region(r, fr, outlived_fr) - }); + let (blame_constraint, path) = + self.regioncx.best_blame_constraint(fr, fr_origin, outlived_fr); let BlameConstraint { category, cause, variance_info, .. } = blame_constraint; debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info); diff --git a/compiler/rustc_borrowck/src/handle_placeholders.rs b/compiler/rustc_borrowck/src/handle_placeholders.rs index 4661906fbeb..b97d43a642b 100644 --- a/compiler/rustc_borrowck/src/handle_placeholders.rs +++ b/compiler/rustc_borrowck/src/handle_placeholders.rs @@ -1,6 +1,7 @@ //! Logic for lowering higher-kinded outlives constraints //! (with placeholders and universes) and turn them into regular //! outlives constraints. +use core::cmp; use rustc_data_structures::frozen::Frozen; use rustc_data_structures::fx::FxIndexMap; @@ -10,7 +11,7 @@ use rustc_index::IndexVec; use rustc_infer::infer::RegionVariableOrigin; use rustc_middle::mir::ConstraintCategory; use rustc_middle::ty::{RegionVid, UniverseIndex}; -use tracing::debug; +use tracing::{debug, trace}; use crate::constraints::{ConstraintSccIndex, OutlivesConstraintSet}; use crate::consumers::OutlivesConstraint; @@ -62,18 +63,71 @@ impl scc::Annotations<RegionVid> for SccAnnotations<'_, '_, RegionTracker> { type SccIdx = ConstraintSccIndex; } +#[derive(Copy, Debug, Clone, PartialEq, Eq)] +enum PlaceholderReachability { + /// This SCC reaches no placeholders. + NoPlaceholders, + /// This SCC reaches at least one placeholder. + Placeholders { + /// The largest-universed placeholder we can reach + max_universe: (UniverseIndex, RegionVid), + + /// The placeholder with the smallest ID + min_placeholder: RegionVid, + + /// The placeholder with the largest ID + max_placeholder: RegionVid, + }, +} + +impl PlaceholderReachability { + /// Merge the reachable placeholders of two graph components. + fn merge(self, other: PlaceholderReachability) -> PlaceholderReachability { + use PlaceholderReachability::*; + match (self, other) { + (NoPlaceholders, NoPlaceholders) => NoPlaceholders, + (NoPlaceholders, p @ Placeholders { .. }) + | (p @ Placeholders { .. }, NoPlaceholders) => p, + ( + Placeholders { + min_placeholder: min_pl, + max_placeholder: max_pl, + max_universe: max_u, + }, + Placeholders { min_placeholder, max_placeholder, max_universe }, + ) => Placeholders { + min_placeholder: cmp::min(min_pl, min_placeholder), + max_placeholder: cmp::max(max_pl, max_placeholder), + max_universe: cmp::max(max_u, max_universe), + }, + } + } + + fn max_universe(&self) -> Option<(UniverseIndex, RegionVid)> { + match self { + Self::NoPlaceholders => None, + Self::Placeholders { max_universe, .. } => Some(*max_universe), + } + } + + /// If we have reached placeholders, determine if they can + /// be named from this universe. + fn can_be_named_by(&self, from: UniverseIndex) -> bool { + self.max_universe() + .is_none_or(|(max_placeholder_universe, _)| from.can_name(max_placeholder_universe)) + } +} + /// An annotation for region graph SCCs that tracks /// the values of its elements. This annotates a single SCC. #[derive(Copy, Debug, Clone)] pub(crate) struct RegionTracker { - /// The largest universe of a placeholder reached from this SCC. - /// This includes placeholders within this SCC. - max_placeholder_universe_reached: UniverseIndex, + reachable_placeholders: PlaceholderReachability, /// The largest universe nameable from this SCC. /// It is the smallest nameable universes of all - /// existential regions reachable from it. - max_nameable_universe: UniverseIndex, + /// existential regions reachable from it. Small Rvids are preferred. + max_nameable_universe: (UniverseIndex, RegionVid), /// The representative Region Variable Id for this SCC. pub(crate) representative: Representative, @@ -81,65 +135,75 @@ pub(crate) struct RegionTracker { impl RegionTracker { pub(crate) fn new(rvid: RegionVid, definition: &RegionDefinition<'_>) -> Self { - let placeholder_universe = + let reachable_placeholders = if matches!(definition.origin, NllRegionVariableOrigin::Placeholder(_)) { - definition.universe + PlaceholderReachability::Placeholders { + max_universe: (definition.universe, rvid), + min_placeholder: rvid, + max_placeholder: rvid, + } } else { - UniverseIndex::ROOT + PlaceholderReachability::NoPlaceholders }; Self { - max_placeholder_universe_reached: placeholder_universe, - max_nameable_universe: definition.universe, + reachable_placeholders, + max_nameable_universe: (definition.universe, rvid), representative: Representative::new(rvid, definition), } } /// The largest universe this SCC can name. It's the smallest - /// largest nameable uninverse of any reachable region. + /// largest nameable universe of any reachable region, or + /// `max_nameable(r) = min (max_nameable(r') for r' reachable from r)` pub(crate) fn max_nameable_universe(self) -> UniverseIndex { - self.max_nameable_universe + self.max_nameable_universe.0 } pub(crate) fn max_placeholder_universe_reached(self) -> UniverseIndex { - self.max_placeholder_universe_reached - } - - fn merge_min_max_seen(&mut self, other: &Self) { - self.max_placeholder_universe_reached = std::cmp::max( - self.max_placeholder_universe_reached, - other.max_placeholder_universe_reached, - ); - - self.max_nameable_universe = - std::cmp::min(self.max_nameable_universe, other.max_nameable_universe); - } - - /// Returns `true` if during the annotated SCC reaches a placeholder - /// with a universe larger than the smallest nameable universe of any - /// reachable existential region. - pub(crate) fn has_incompatible_universes(&self) -> bool { - self.max_nameable_universe().cannot_name(self.max_placeholder_universe_reached) + if let Some((universe, _)) = self.reachable_placeholders.max_universe() { + universe + } else { + UniverseIndex::ROOT + } } /// Determine if the tracked universes of the two SCCs are compatible. pub(crate) fn universe_compatible_with(&self, other: Self) -> bool { + // HACK: We first check whether we can name the highest existential universe + // of `other`. This only exists to avoid errors in case that scc already + // depends on a placeholder it cannot name itself. self.max_nameable_universe().can_name(other.max_nameable_universe()) - || self.max_nameable_universe().can_name(other.max_placeholder_universe_reached) + || other.reachable_placeholders.can_be_named_by(self.max_nameable_universe()) + } + + /// If this SCC reaches a placeholder it can't name, return it. + fn unnameable_placeholder(&self) -> Option<(UniverseIndex, RegionVid)> { + self.reachable_placeholders.max_universe().filter(|&(placeholder_universe, _)| { + !self.max_nameable_universe().can_name(placeholder_universe) + }) } } impl scc::Annotation for RegionTracker { - fn merge_scc(mut self, other: Self) -> Self { - self.representative = self.representative.merge_scc(other.representative); - self.merge_min_max_seen(&other); - self + fn merge_scc(self, other: Self) -> Self { + trace!("{:?} << {:?}", self.representative, other.representative); + + Self { + representative: self.representative.merge_scc(other.representative), + ..self.merge_reached(other) + } } - fn merge_reached(mut self, other: Self) -> Self { - // No update to in-component values, only add seen values. - self.merge_min_max_seen(&other); - self + fn merge_reached(self, other: Self) -> Self { + Self { + max_nameable_universe: cmp::min( + self.max_nameable_universe, + other.max_nameable_universe, + ), + reachable_placeholders: self.reachable_placeholders.merge(other.reachable_placeholders), + representative: self.representative, + } } } @@ -310,28 +374,52 @@ fn rewrite_placeholder_outlives<'tcx>( let annotation = annotations[scc]; - // If this SCC participates in a universe violation, - // e.g. if it reaches a region with a universe smaller than - // the largest region reached, add a requirement that it must - // outlive `'static`. - if annotation.has_incompatible_universes() { - // Optimisation opportunity: this will add more constraints than - // needed for correctness, since an SCC upstream of another with - // a universe violation will "infect" its downstream SCCs to also - // outlive static. - let scc_representative_outlives_static = OutlivesConstraint { - sup: annotation.representative.rvid(), - sub: fr_static, - category: ConstraintCategory::IllegalUniverse, - locations: Locations::All(rustc_span::DUMMY_SP), - span: rustc_span::DUMMY_SP, - variance_info: VarianceDiagInfo::None, - from_closure: false, - }; - outlives_constraints.push(scc_representative_outlives_static); - added_constraints = true; - debug!("Added {:?}: 'static!", annotation.representative.rvid()); - } + let Some((max_u, max_u_rvid)) = annotation.unnameable_placeholder() else { + continue; + }; + + debug!( + "Placeholder universe {max_u:?} is too large for its SCC, represented by {:?}", + annotation.representative + ); + + // We only add one `r: 'static` constraint per SCC, where `r` is the SCC representative. + // 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! + + // 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() + ); + 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 } diff --git a/compiler/rustc_borrowck/src/region_infer/graphviz.rs b/compiler/rustc_borrowck/src/region_infer/graphviz.rs index b2f67c43125..526e1850c2e 100644 --- a/compiler/rustc_borrowck/src/region_infer/graphviz.rs +++ b/compiler/rustc_borrowck/src/region_infer/graphviz.rs @@ -12,9 +12,13 @@ use rustc_middle::ty::UniverseIndex; use super::*; fn render_outlives_constraint(constraint: &OutlivesConstraint<'_>) -> String { - match constraint.locations { - Locations::All(_) => "All(...)".to_string(), - Locations::Single(loc) => format!("{loc:?}"), + if let ConstraintCategory::OutlivesUnnameablePlaceholder(unnameable) = constraint.category { + format!("{unnameable:?} unnameable") + } else { + match constraint.locations { + Locations::All(_) => "All(...)".to_string(), + Locations::Single(loc) => format!("{loc:?}"), + } } } diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs index 3d95eb4663a..f7b838a4638 100644 --- a/compiler/rustc_borrowck/src/region_infer/mod.rs +++ b/compiler/rustc_borrowck/src/region_infer/mod.rs @@ -1285,11 +1285,9 @@ impl<'tcx> RegionInferenceContext<'tcx> { { debug!("try_propagate_universal_region_error: fr_minus={:?}", fr_minus); - let blame_span_category = self.find_outlives_blame_span( - longer_fr, - NllRegionVariableOrigin::FreeRegion, - shorter_fr, - ); + let blame_constraint = self + .best_blame_constraint(longer_fr, NllRegionVariableOrigin::FreeRegion, shorter_fr) + .0; // Grow `shorter_fr` until we find some non-local regions. (We // always will.) We'll call them `shorter_fr+` -- they're ever @@ -1302,8 +1300,8 @@ impl<'tcx> RegionInferenceContext<'tcx> { propagated_outlives_requirements.push(ClosureOutlivesRequirement { subject: ClosureOutlivesSubject::Region(fr_minus), outlived_free_region: fr, - blame_span: blame_span_category.1.span, - category: blame_span_category.0, + blame_span: blame_constraint.cause.span, + category: blame_constraint.category, }); } return RegionRelationCheckResult::Propagated; @@ -1342,66 +1340,15 @@ impl<'tcx> RegionInferenceContext<'tcx> { } } - /// We have a constraint `fr1: fr2` that is not satisfied, where - /// `fr2` represents some universal region. Here, `r` is some - /// region where we know that `fr1: r` and this function has the - /// job of determining whether `r` is "to blame" for the fact that - /// `fr1: fr2` is required. - /// - /// This is true under two conditions: - /// - /// - `r == fr2` - /// - `fr2` is `'static` and `r` is some placeholder in a universe - /// that cannot be named by `fr1`; in that case, we will require - /// that `fr1: 'static` because it is the only way to `fr1: r` to - /// be satisfied. (See `add_incompatible_universe`.) - pub(crate) fn provides_universal_region( + pub(crate) fn constraint_path_between_regions( &self, - r: RegionVid, - fr1: RegionVid, - fr2: RegionVid, - ) -> bool { - debug!("provides_universal_region(r={:?}, fr1={:?}, fr2={:?})", r, fr1, fr2); - let result = { - r == fr2 || { - fr2 == self.universal_regions().fr_static && self.cannot_name_placeholder(fr1, r) - } - }; - debug!("provides_universal_region: result = {:?}", result); - result - } - - /// If `r2` represents a placeholder region, then this returns - /// `true` if `r1` cannot name that placeholder in its - /// value; otherwise, returns `false`. - pub(crate) fn cannot_name_placeholder(&self, r1: RegionVid, r2: RegionVid) -> bool { - match self.definitions[r2].origin { - NllRegionVariableOrigin::Placeholder(placeholder) => { - let r1_universe = self.definitions[r1].universe; - debug!( - "cannot_name_value_of: universe1={r1_universe:?} placeholder={:?}", - placeholder - ); - r1_universe.cannot_name(placeholder.universe) - } - - NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => { - false - } + from_region: RegionVid, + to_region: RegionVid, + ) -> Option<Vec<OutlivesConstraint<'tcx>>> { + if from_region == to_region { + bug!("Tried to find a path between {from_region:?} and itself!"); } - } - - /// Finds a good `ObligationCause` to blame for the fact that `fr1` outlives `fr2`. - pub(crate) fn find_outlives_blame_span( - &self, - fr1: RegionVid, - fr1_origin: NllRegionVariableOrigin, - fr2: RegionVid, - ) -> (ConstraintCategory<'tcx>, ObligationCause<'tcx>) { - let BlameConstraint { category, cause, .. } = self - .best_blame_constraint(fr1, fr1_origin, |r| self.provides_universal_region(r, fr1, fr2)) - .0; - (category, cause) + self.constraint_path_to(from_region, |to| to == to_region, true).map(|o| o.0) } /// Walks the graph of constraints (where `'a: 'b` is considered @@ -1410,15 +1357,30 @@ impl<'tcx> RegionInferenceContext<'tcx> { /// /// Returns: a series of constraints as well as the region `R` /// that passed the target test. + /// If `include_static_outlives_all` is `true`, then the synthetic + /// outlives constraints `'static -> a` for every region `a` are + /// considered in the search, otherwise they are ignored. #[instrument(skip(self, target_test), ret)] - pub(crate) fn find_constraint_path_between_regions( + pub(crate) fn constraint_path_to( &self, from_region: RegionVid, target_test: impl Fn(RegionVid) -> bool, + include_placeholder_static: bool, ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> { - self.find_constraint_path_between_regions_inner(true, from_region, &target_test).or_else( - || self.find_constraint_path_between_regions_inner(false, from_region, &target_test), + self.find_constraint_path_between_regions_inner( + true, + from_region, + &target_test, + include_placeholder_static, ) + .or_else(|| { + self.find_constraint_path_between_regions_inner( + false, + from_region, + &target_test, + include_placeholder_static, + ) + }) } /// The constraints we get from equating the hidden type of each use of an opaque @@ -1438,6 +1400,7 @@ impl<'tcx> RegionInferenceContext<'tcx> { ignore_opaque_type_constraints: bool, from_region: RegionVid, target_test: impl Fn(RegionVid) -> bool, + include_placeholder_static: bool, ) -> Option<(Vec<OutlivesConstraint<'tcx>>, RegionVid)> { let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions); context[from_region] = Trace::StartRegion; @@ -1452,7 +1415,7 @@ impl<'tcx> RegionInferenceContext<'tcx> { while let Some(r) = deque.pop_front() { debug!( - "find_constraint_path_between_regions: from_region={:?} r={:?} value={}", + "constraint_path_to: from_region={:?} r={:?} value={}", from_region, r, self.region_value_str(r), @@ -1525,8 +1488,10 @@ impl<'tcx> RegionInferenceContext<'tcx> { // This loop can be hot. for constraint in edges { match constraint.category { - ConstraintCategory::IllegalUniverse => { - debug!("Ignoring illegal universe constraint: {constraint:?}"); + ConstraintCategory::OutlivesUnnameablePlaceholder(_) + if !include_placeholder_static => + { + debug!("Ignoring illegal placeholder constraint: {constraint:?}"); continue; } ConstraintCategory::OpaqueType if ignore_opaque_type_constraints => { @@ -1535,6 +1500,7 @@ impl<'tcx> RegionInferenceContext<'tcx> { } _ => {} } + debug_assert_eq!(constraint.sup, r); handle_trace(constraint.sub, Trace::FromGraph(constraint)); } @@ -1549,33 +1515,10 @@ impl<'tcx> RegionInferenceContext<'tcx> { pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, location: Location) -> RegionVid { trace!(scc = ?self.constraint_sccs.scc(fr1)); trace!(universe = ?self.max_nameable_universe(self.constraint_sccs.scc(fr1))); - self.find_constraint_path_between_regions(fr1, |r| { - // First look for some `r` such that `fr1: r` and `r` is live at `location` + self.constraint_path_to(fr1, |r| { trace!(?r, liveness_constraints=?self.liveness_constraints.pretty_print_live_points(r)); self.liveness_constraints.is_live_at(r, location) - }) - .or_else(|| { - // If we fail to find that, we may find some `r` such that - // `fr1: r` and `r` is a placeholder from some universe - // `fr1` cannot name. This would force `fr1` to be - // `'static`. - self.find_constraint_path_between_regions(fr1, |r| self.cannot_name_placeholder(fr1, r)) - }) - .or_else(|| { - // If we fail to find THAT, it may be that `fr1` is a - // placeholder that cannot "fit" into its SCC. In that - // case, there should be some `r` where `fr1: r` and `fr1` is a - // placeholder that `r` cannot name. We can blame that - // edge. - // - // Remember that if `R1: R2`, then the universe of R1 - // must be able to name the universe of R2, because R2 will - // be at least `'empty(Universe(R2))`, and `R1` must be at - // larger than that. - self.find_constraint_path_between_regions(fr1, |r| self.cannot_name_placeholder(r, fr1)) - }) - .map(|(_path, r)| r) - .unwrap() + }, true).unwrap().1 } /// Get the region outlived by `longer_fr` and live at `element`. @@ -1619,22 +1562,38 @@ impl<'tcx> RegionInferenceContext<'tcx> { /// creating a constraint path that forces `R` to outlive /// `from_region`, and then finding the best choices within that /// path to blame. - #[instrument(level = "debug", skip(self, target_test))] + #[instrument(level = "debug", skip(self))] pub(crate) fn best_blame_constraint( &self, from_region: RegionVid, from_region_origin: NllRegionVariableOrigin, - target_test: impl Fn(RegionVid) -> bool, + to_region: RegionVid, ) -> (BlameConstraint<'tcx>, Vec<OutlivesConstraint<'tcx>>) { - // Find all paths - let (path, target_region) = self - .find_constraint_path_between_regions(from_region, target_test) - .or_else(|| { - self.find_constraint_path_between_regions(from_region, |r| { - self.cannot_name_placeholder(from_region, r) - }) - }) - .unwrap(); + assert!(from_region != to_region, "Trying to blame a region for itself!"); + + let path = self.constraint_path_between_regions(from_region, to_region).unwrap(); + + // If we are passing through a constraint added because we reached an unnameable placeholder `'unnameable`, + // redirect search towards `'unnameable`. + let due_to_placeholder_outlives = path.iter().find_map(|c| { + if let ConstraintCategory::OutlivesUnnameablePlaceholder(unnameable) = c.category { + Some(unnameable) + } else { + None + } + }); + + // Edge case: it's possible that `'from_region` is an unnameable placeholder. + let path = if let Some(unnameable) = due_to_placeholder_outlives + && unnameable != from_region + { + // We ignore the extra edges due to unnameable placeholders to get + // an explanation that was present in the original constraint graph. + self.constraint_path_to(from_region, |r| r == unnameable, false).unwrap().0 + } else { + path + }; + debug!( "path={:#?}", path.iter() @@ -1742,7 +1701,7 @@ impl<'tcx> RegionInferenceContext<'tcx> { ConstraintCategory::Cast { unsize_to: Some(unsize_ty), is_implicit_coercion: true, - } if target_region == self.universal_regions().fr_static + } if to_region == self.universal_regions().fr_static // Mirror the note's condition, to minimize how often this diverts blame. && let ty::Adt(_, args) = unsize_ty.kind() && args.iter().any(|arg| arg.as_type().is_some_and(|ty| ty.is_trait())) @@ -1780,7 +1739,7 @@ impl<'tcx> RegionInferenceContext<'tcx> { // specific, and are not used for relations that would make sense to blame. ConstraintCategory::BoringNoLocation => 6, // Do not blame internal constraints. - ConstraintCategory::IllegalUniverse => 7, + ConstraintCategory::OutlivesUnnameablePlaceholder(_) => 7, ConstraintCategory::Internal => 8, }; @@ -1817,6 +1776,14 @@ impl<'tcx> RegionInferenceContext<'tcx> { path[best_choice] }; + assert!( + !matches!( + best_constraint.category, + ConstraintCategory::OutlivesUnnameablePlaceholder(_) + ), + "Illegal placeholder constraint blamed; should have redirected to other region relation" + ); + let blame_constraint = BlameConstraint { category: best_constraint.category, from_closure: best_constraint.from_closure, |