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traverse region graph instead of SCCs to compute polonius loan scopes

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By using SCC for better performance, we also have to take into account
SCCs whose representative is an existential region but also contains a
placeholder.

By only checking the representative, we may miss that the loan escapes
the function. This can be fixed by picking a better representative, or
removing placeholders from the main path.

This is the simplest fix: forgo efficiency and traverse the region graph
instead of the SCCs.
This commit is contained in:
Rémy Rakic 2023-11-04 00:55:27 +00:00
parent 5020f7c3b8
commit de7a8305ae
2 changed files with 14 additions and 13 deletions
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6
compiler/rustc_borrowck/src/dataflow.rs
View File

@ -273,11 +273,10 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
) {
let sccs = self.regioncx.constraint_sccs();
let universal_regions = self.regioncx.universal_regions();
let issuing_region_scc = sccs.scc(issuing_region);
// We first handle the cases where the loan doesn't go out of scope, depending on the issuing
// region's successors.
for scc in sccs.depth_first_search(issuing_region_scc) {
for successor in self.regioncx.region_graph().depth_first_search(issuing_region) {
// 1. Via applied member constraints
//
// The issuing region can flow into the choice regions, and they are either:
@ -290,6 +289,7 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
// For additional insurance via fuzzing and crater, we verify that the constraint's min
// choice indeed escapes the function. In the future, we could e.g. turn this check into
// a debug assert and early return as an optimization.
let scc = sccs.scc(successor);
for constraint in self.regioncx.applied_member_constraints(scc) {
if universal_regions.is_universal_region(constraint.min_choice) {
return;
@ -300,7 +300,7 @@ impl<'tcx> PoloniusOutOfScopePrecomputer<'_, 'tcx> {
//
// If the issuing region outlives such a region, its loan escapes the function and
// cannot go out of scope. We can early return.
if self.regioncx.scc_is_live_at_all_points(scc) {
if self.regioncx.is_region_live_at_all_points(successor) {
return;
}
}

19
compiler/rustc_borrowck/src/region_infer/mod.rs
View File

@ -22,11 +22,10 @@ use rustc_middle::traits::ObligationCauseCode;
use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
use rustc_span::Span;
use crate::constraints::graph::{self, NormalConstraintGraph, RegionGraph};
use crate::dataflow::BorrowIndex;
use crate::{
constraints::{
graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet,
},
constraints::{ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet},
diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo},
member_constraints::{MemberConstraintSet, NllMemberConstraintIndex},
nll::PoloniusOutput,
@ -2293,19 +2292,21 @@ impl<'tcx> RegionInferenceContext<'tcx> {
self.constraint_sccs.as_ref()
}
/// Returns whether the given SCC is live at all points: whether the representative is a
/// Access to the region graph, built from the outlives constraints.
pub(crate) fn region_graph(&self) -> RegionGraph<'_, 'tcx, graph::Normal> {
self.constraint_graph.region_graph(&self.constraints, self.universal_regions.fr_static)
}
/// Returns whether the given region is considered live at all points: whether it is a
/// placeholder or a free region.
pub(crate) fn scc_is_live_at_all_points(&self, scc: ConstraintSccIndex) -> bool {
pub(crate) fn is_region_live_at_all_points(&self, region: RegionVid) -> bool {
// FIXME: there must be a cleaner way to find this information. At least, when
// higher-ranked subtyping is abstracted away from the borrowck main path, we'll only
// need to check whether this is a universal region.
let representative = self.scc_representatives[scc];
let origin = self.var_infos[representative].origin;
let origin = self.region_definition(region).origin;
let live_at_all_points = matches!(
origin,
RegionVariableOrigin::Nll(
NllRegionVariableOrigin::Placeholder(_) | NllRegionVariableOrigin::FreeRegion
)
);
live_at_all_points
}