Move placeholder handling to a proper preprocessing step

This commit breaks out the logic of placheolder rewriting
into its own preprocessing step. It's one of the more boring
parts of #130227.

The only functional change from this is that the preprocessing
step (where extra `r: 'static` constraints are added) is
performed upstream of Polonius legacy, finally affecting
Polonius. That is mostly a by-product, though.

Author: amandasystems

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -5,11 +5,9 @@ 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 tracing::debug;
 
-use crate::region_infer::{AnnotatedSccs, ConstraintSccs, RegionDefinition, SccAnnotations};
 use crate::type_check::Locations;
-use crate::universal_regions::UniversalRegions;
 
 pub(crate) mod graph;
 
@@ -53,112 +51,6 @@ impl<'tcx> OutlivesConstraintSet<'tcx> {
     ) -> &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>>,
-    ) -> AnnotatedSccs {
-        let constraint_graph = self.graph(definitions.len());
-        let region_graph = &constraint_graph.region_graph(self, static_region);
-        let mut annotation_visitor = SccAnnotations::new(definitions);
-        (
-            ConstraintSccs::new_with_annotation(&region_graph, &mut annotation_visitor),
-            annotation_visitor.scc_to_annotation,
-        )
-    }
-
-    /// 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
-    /// [`crate::region_infer::RegionTracker`].
-    ///
-    /// 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 'static 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>>,
-    ) -> AnnotatedSccs {
-        let fr_static = universal_regions.fr_static;
-        let (sccs, annotations) = 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 = 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.
-                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, annotations)
-        }
-    }
 }
 
 impl<'tcx> Index<OutlivesConstraintIndex> for OutlivesConstraintSet<'tcx> {