| use std::fmt; |
| use std::ops::Index; |
| |
| use rustc_index::{IndexSlice, IndexVec}; |
| use rustc_middle::mir::ConstraintCategory; |
| use rustc_middle::ty::{RegionVid, TyCtxt, VarianceDiagInfo}; |
| use rustc_span::Span; |
| use tracing::{debug, instrument}; |
| |
| use crate::region_infer::{ConstraintSccs, RegionDefinition, RegionTracker}; |
| use crate::type_check::Locations; |
| use crate::universal_regions::UniversalRegions; |
| |
| pub(crate) mod graph; |
| |
| /// A set of NLL region constraints. These include "outlives" |
| /// constraints of the form `R1: R2`. Each constraint is identified by |
| /// a unique `OutlivesConstraintIndex` and you can index into the set |
| /// (`constraint_set[i]`) to access the constraint details. |
| #[derive(Clone, Debug, Default)] |
| pub(crate) struct OutlivesConstraintSet<'tcx> { |
| outlives: IndexVec<OutlivesConstraintIndex, OutlivesConstraint<'tcx>>, |
| } |
| |
| impl<'tcx> OutlivesConstraintSet<'tcx> { |
| pub(crate) fn push(&mut self, constraint: OutlivesConstraint<'tcx>) { |
| debug!("OutlivesConstraintSet::push({:?})", constraint); |
| if constraint.sup == constraint.sub { |
| // 'a: 'a is pretty uninteresting |
| return; |
| } |
| self.outlives.push(constraint); |
| } |
| |
| /// Constructs a "normal" graph from the constraint set; the graph makes it |
| /// easy to find the constraints affecting a particular region. |
| /// |
| /// N.B., this graph contains a "frozen" view of the current |
| /// constraints. Any new constraints added to the `OutlivesConstraintSet` |
| /// after the graph is built will not be present in the graph. |
| pub(crate) fn graph(&self, num_region_vars: usize) -> graph::NormalConstraintGraph { |
| graph::ConstraintGraph::new(graph::Normal, self, num_region_vars) |
| } |
| |
| /// Like `graph`, but constraints a reverse graph where `R1: R2` |
| /// represents an edge `R2 -> R1`. |
| pub(crate) fn reverse_graph(&self, num_region_vars: usize) -> graph::ReverseConstraintGraph { |
| graph::ConstraintGraph::new(graph::Reverse, self, num_region_vars) |
| } |
| |
| pub(crate) fn outlives( |
| &self, |
| ) -> &IndexSlice<OutlivesConstraintIndex, OutlivesConstraint<'tcx>> { |
| &self.outlives |
| } |
| |
| /// Computes cycles (SCCs) in the graph of regions. In particular, |
| /// find all regions R1, R2 such that R1: R2 and R2: R1 and group |
| /// them into an SCC, and find the relationships between SCCs. |
| pub(crate) fn compute_sccs( |
| &self, |
| static_region: RegionVid, |
| definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>, |
| ) -> ConstraintSccs { |
| let constraint_graph = self.graph(definitions.len()); |
| let region_graph = &constraint_graph.region_graph(self, static_region); |
| ConstraintSccs::new_with_annotation(®ion_graph, |r| { |
| RegionTracker::new(r, &definitions[r]) |
| }) |
| } |
| |
| /// This method handles Universe errors by rewriting the constraint |
| /// graph. For each strongly connected component in the constraint |
| /// graph such that there is a series of constraints |
| /// A: B: C: ... : X where |
| /// A's universe is smaller than X's and A is a placeholder, |
| /// add a constraint that A: 'static. This is a safe upper bound |
| /// in the face of borrow checker/trait solver limitations that will |
| /// eventually go away. |
| /// |
| /// For a more precise definition, see the documentation for |
| /// [`RegionTracker::has_incompatible_universes()`]. |
| /// |
| /// This edge case used to be handled during constraint propagation |
| /// by iterating over the strongly connected components in the constraint |
| /// graph while maintaining a set of bookkeeping mappings similar |
| /// to what is stored in `RegionTracker` and manually adding 'sttaic as |
| /// needed. |
| /// |
| /// It was rewritten as part of the Polonius project with the goal of moving |
| /// higher-kindedness concerns out of the path of the borrow checker, |
| /// for two reasons: |
| /// |
| /// 1. Implementing Polonius is difficult enough without also |
| /// handling them. |
| /// 2. The long-term goal is to handle higher-kinded concerns |
| /// in the trait solver, where they belong. This avoids |
| /// logic duplication and allows future trait solvers |
| /// to compute better bounds than for example our |
| /// "must outlive 'static" here. |
| /// |
| /// This code is a stop-gap measure in preparation for the future trait solver. |
| /// |
| /// Every constraint added by this method is an |
| /// internal `IllegalUniverse` constraint. |
| #[instrument(skip(self, universal_regions, definitions))] |
| pub(crate) fn add_outlives_static( |
| &mut self, |
| universal_regions: &UniversalRegions<'tcx>, |
| definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>, |
| ) -> ConstraintSccs { |
| let fr_static = universal_regions.fr_static; |
| let sccs = self.compute_sccs(fr_static, definitions); |
| |
| // Changed to `true` if we added any constraints to `self` and need to |
| // recompute SCCs. |
| let mut added_constraints = false; |
| |
| for scc in sccs.all_sccs() { |
| // No point in adding 'static: 'static! |
| // This micro-optimisation makes somewhat sense |
| // because static outlives *everything*. |
| if scc == sccs.scc(fr_static) { |
| continue; |
| } |
| |
| let annotation = sccs.annotation(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. |
| added_constraints = true; |
| let scc_representative_outlives_static = OutlivesConstraint { |
| sup: annotation.representative, |
| 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, |
| }; |
| self.push(scc_representative_outlives_static); |
| } |
| } |
| |
| if added_constraints { |
| // We changed the constraint set and so must recompute SCCs. |
| self.compute_sccs(fr_static, definitions) |
| } else { |
| // If we didn't add any back-edges; no more work needs doing |
| sccs |
| } |
| } |
| } |
| |
| impl<'tcx> Index<OutlivesConstraintIndex> for OutlivesConstraintSet<'tcx> { |
| type Output = OutlivesConstraint<'tcx>; |
| |
| fn index(&self, i: OutlivesConstraintIndex) -> &Self::Output { |
| &self.outlives[i] |
| } |
| } |
| |
| #[derive(Copy, Clone, PartialEq, Eq)] |
| pub struct OutlivesConstraint<'tcx> { |
| // NB. The ordering here is not significant for correctness, but |
| // it is for convenience. Before we dump the constraints in the |
| // debugging logs, we sort them, and we'd like the "super region" |
| // to be first, etc. (In particular, span should remain last.) |
| /// The region SUP must outlive SUB... |
| pub sup: RegionVid, |
| |
| /// Region that must be outlived. |
| pub sub: RegionVid, |
| |
| /// Where did this constraint arise? |
| pub locations: Locations, |
| |
| /// The `Span` associated with the creation of this constraint. |
| /// This should be used in preference to obtaining the span from |
| /// `locations`, since the `locations` may give a poor span |
| /// in some cases (e.g. converting a constraint from a promoted). |
| pub span: Span, |
| |
| /// What caused this constraint? |
| pub category: ConstraintCategory<'tcx>, |
| |
| /// Variance diagnostic information |
| pub variance_info: VarianceDiagInfo<TyCtxt<'tcx>>, |
| |
| /// If this constraint is promoted from closure requirements. |
| pub from_closure: bool, |
| } |
| |
| impl<'tcx> fmt::Debug for OutlivesConstraint<'tcx> { |
| fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result { |
| write!( |
| formatter, |
| "({:?}: {:?}) due to {:?} ({:?}) ({:?})", |
| self.sup, self.sub, self.locations, self.variance_info, self.category, |
| ) |
| } |
| } |
| |
| rustc_index::newtype_index! { |
| #[debug_format = "OutlivesConstraintIndex({})"] |
| pub(crate) struct OutlivesConstraintIndex {} |
| } |
| |
| rustc_index::newtype_index! { |
| #[orderable] |
| #[debug_format = "ConstraintSccIndex({})"] |
| pub struct ConstraintSccIndex {} |
| } |
