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Auto merge of #123720 - amandasystems:dyn-enable-refactor, r=nikomatsakis

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Rewrite handling of universe-leaking placeholder regions into outlives constraints

This commit prepares for Polonius by moving handling of leak check/universe errors out of the inference step by rewriting any universe error into an outlives-static constraint.

This variant is a work in progress but seems to pass most tests.

Note that a few debug assertions no longer hold; a few extra eyes on those changes are appreciated!
This commit is contained in:
bors 2024-07-03 01:24:07 +00:00
commit 67f0d43890
4 changed files with 140 additions and 73 deletions
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106
compiler/rustc_borrowck/src/constraints/mod.rs
View File

@ -1,4 +1,6 @@
use crate::region_infer::{ConstraintSccs, RegionDefinition, RegionTracker};
use crate::type_check::Locations;
use crate::universal_regions::UniversalRegions;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::mir::ConstraintCategory;
use rustc_middle::ty::{RegionVid, TyCtxt, VarianceDiagInfo};
@ -48,6 +50,110 @@ 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>>,
) -> ConstraintSccs {
let constraint_graph = self.graph(definitions.len());
let region_graph = &constraint_graph.region_graph(self, static_region);
ConstraintSccs::new_with_annotation(&region_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> {

3
compiler/rustc_borrowck/src/diagnostics/region_errors.rs
View File

@ -66,7 +66,8 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
ConstraintCategory::Predicate(_)
| ConstraintCategory::Boring
| ConstraintCategory::BoringNoLocation
| ConstraintCategory::Internal => "",
| ConstraintCategory::Internal
| ConstraintCategory::IllegalUniverse => "",
}
}
}

101
compiler/rustc_borrowck/src/region_infer/mod.rs
View File

@ -62,7 +62,7 @@ pub struct RegionTracker {
/// The representative Region Variable Id for this SCC. We prefer
/// placeholders over existentially quantified variables, otherwise
/// it's the one with the smallest Region Variable ID.
representative: RegionVid,
pub(crate) representative: RegionVid,
/// Is the current representative a placeholder?
representative_is_placeholder: bool,
@ -97,7 +97,7 @@ impl scc::Annotation for RegionTracker {
}
impl RegionTracker {
fn new(rvid: RegionVid, definition: &RegionDefinition<'_>) -> Self {
pub(crate) fn new(rvid: RegionVid, definition: &RegionDefinition<'_>) -> Self {
let (representative_is_placeholder, representative_is_existential) = match definition.origin
{
rustc_infer::infer::NllRegionVariableOrigin::FreeRegion => (false, false),
@ -116,7 +116,9 @@ impl RegionTracker {
representative_is_existential,
}
}
fn universe(self) -> UniverseIndex {
/// The smallest-indexed universe reachable from and/or in this SCC.
fn min_universe(self) -> UniverseIndex {
self.min_reachable_universe
}
@ -132,8 +134,8 @@ impl RegionTracker {
/// Returns `true` if during the annotated SCC reaches a placeholder
/// with a universe larger than the smallest reachable one, `false` otherwise.
pub fn has_incompatible_universes(&self) -> bool {
self.universe().cannot_name(self.max_placeholder_universe_reached)
pub(crate) fn has_incompatible_universes(&self) -> bool {
self.min_universe().cannot_name(self.max_placeholder_universe_reached)
}
}
@ -163,7 +165,7 @@ pub struct RegionInferenceContext<'tcx> {
/// The SCC computed from `constraints` and the constraint
/// graph. We have an edge from SCC A to SCC B if `A: B`. Used to
/// compute the values of each region.
constraint_sccs: Rc<ConstraintSccs>,
constraint_sccs: ConstraintSccs,
/// Reverse of the SCC constraint graph -- i.e., an edge `A -> B` exists if
/// `B: A`. This is used to compute the universal regions that are required
@ -401,7 +403,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
universal_regions: Rc<UniversalRegions<'tcx>>,
placeholder_indices: Rc<PlaceholderIndices>,
universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
outlives_constraints: OutlivesConstraintSet<'tcx>,
mut outlives_constraints: OutlivesConstraintSet<'tcx>,
member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
universe_causes: FxIndexMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
type_tests: Vec<TypeTest<'tcx>>,
@ -419,17 +421,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
.map(|info| RegionDefinition::new(info.universe, info.origin))
.collect();
let fr_static = universal_regions.fr_static;
let constraint_sccs =
outlives_constraints.add_outlives_static(&universal_regions, &definitions);
let constraints = Frozen::freeze(outlives_constraints);
let constraint_graph = Frozen::freeze(constraints.graph(definitions.len()));
let constraint_sccs = {
let constraint_graph = constraints.graph(definitions.len());
let region_graph = &constraint_graph.region_graph(&constraints, fr_static);
let sccs = ConstraintSccs::new_with_annotation(&region_graph, |r| {
RegionTracker::new(r, &definitions[r])
});
Rc::new(sccs)
};
if cfg!(debug_assertions) {
sccs_info(infcx, &constraint_sccs);
@ -548,21 +543,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
}
NllRegionVariableOrigin::Placeholder(placeholder) => {
// Each placeholder region is only visible from
// its universe `ui` and its extensions. So we
// can't just add it into `scc` unless the
// universe of the scc can name this region.
let scc_universe = self.scc_universe(scc);
if scc_universe.can_name(placeholder.universe) {
self.scc_values.add_element(scc, placeholder);
} else {
debug!(
"init_free_and_bound_regions: placeholder {:?} is \
not compatible with universe {:?} of its SCC {:?}",
placeholder, scc_universe, scc,
);
self.add_incompatible_universe(scc);
}
self.scc_values.add_element(scc, placeholder);
}
NllRegionVariableOrigin::Existential { .. } => {
@ -744,23 +725,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// (which is assured by iterating over SCCs in dependency order).
#[instrument(skip(self), level = "debug")]
fn compute_value_for_scc(&mut self, scc_a: ConstraintSccIndex) {
let constraint_sccs = self.constraint_sccs.clone();
// Walk each SCC `B` such that `A: B`...
for &scc_b in constraint_sccs.successors(scc_a) {
for &scc_b in self.constraint_sccs.successors(scc_a) {
debug!(?scc_b);
// ...and add elements from `B` into `A`. One complication
// arises because of universes: If `B` contains something
// that `A` cannot name, then `A` can only contain `B` if
// it outlives static.
if self.universe_compatible(scc_b, scc_a) {
// `A` can name everything that is in `B`, so just
// merge the bits.
self.scc_values.add_region(scc_a, scc_b);
} else {
self.add_incompatible_universe(scc_a);
}
self.scc_values.add_region(scc_a, scc_b);
}
// Now take member constraints into account.
@ -814,7 +782,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
// If the member region lives in a higher universe, we currently choose
// the most conservative option by leaving it unchanged.
if !self.constraint_sccs().annotation(scc).universe().is_root() {
if !self.constraint_sccs().annotation(scc).min_universe().is_root() {
return;
}
@ -886,35 +854,20 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// in `scc_a`. Used during constraint propagation, and only once
/// the value of `scc_b` has been computed.
fn universe_compatible(&self, scc_b: ConstraintSccIndex, scc_a: ConstraintSccIndex) -> bool {
let universe_a = self.constraint_sccs().annotation(scc_a).universe();
let universe_b = self.constraint_sccs().annotation(scc_b).universe();
let a_annotation = self.constraint_sccs().annotation(scc_a);
let b_annotation = self.constraint_sccs().annotation(scc_b);
let a_universe = a_annotation.min_universe();
// Quick check: if scc_b's declared universe is a subset of
// If scc_b's declared universe is a subset of
// scc_a's declared universe (typically, both are ROOT), then
// it cannot contain any problematic universe elements.
if universe_a.can_name(universe_b) {
if a_universe.can_name(b_annotation.min_universe()) {
return true;
}
// Otherwise, we have to iterate over the universe elements in
// B's value, and check whether all of them are nameable
// from universe_a
self.scc_values.placeholders_contained_in(scc_b).all(|p| universe_a.can_name(p.universe))
}
/// Extend `scc` so that it can outlive some placeholder region
/// from a universe it can't name; at present, the only way for
/// this to be true is if `scc` outlives `'static`. This is
/// actually stricter than necessary: ideally, we'd support bounds
/// like `for<'a: 'b>` that might then allow us to approximate
/// `'a` with `'b` and not `'static`. But it will have to do for
/// now.
fn add_incompatible_universe(&mut self, scc: ConstraintSccIndex) {
debug!("add_incompatible_universe(scc={:?})", scc);
let fr_static = self.universal_regions.fr_static;
self.scc_values.add_all_points(scc);
self.scc_values.add_element(scc, fr_static);
// Otherwise, there can be no placeholder in `b` with a too high
// universe index to name from `a`.
a_universe.can_name(b_annotation.max_placeholder_universe_reached)
}
/// Once regions have been propagated, this method is used to see
@ -1022,7 +975,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
"lower_bound = {:?} r_scc={:?} universe={:?}",
lower_bound,
r_scc,
self.constraint_sccs.annotation(r_scc).universe()
self.constraint_sccs.annotation(r_scc).min_universe()
);
// If the type test requires that `T: 'a` where `'a` is a
@ -1539,7 +1492,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
/// The minimum universe of any variable reachable from this
/// SCC, inside or outside of it.
fn scc_universe(&self, scc: ConstraintSccIndex) -> UniverseIndex {
self.constraint_sccs().annotation(scc).universe()
self.constraint_sccs().annotation(scc).min_universe()
}
/// Checks the final value for the free region `fr` to see if it
/// grew too large. In particular, examine what `end(X)` points
@ -1896,6 +1849,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
// This loop can be hot.
for constraint in outgoing_edges_from_graph {
if matches!(constraint.category, ConstraintCategory::IllegalUniverse) {
debug!("Ignoring illegal universe constraint: {constraint:?}");
continue;
}
handle_constraint(constraint);
}

3
compiler/rustc_middle/src/mir/query.rs
View File

@ -274,6 +274,9 @@ pub enum ConstraintCategory<'tcx> {
/// A constraint that doesn't correspond to anything the user sees.
Internal,
/// An internal constraint derived from an illegal universe relation.
IllegalUniverse,
}
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash)]