use std::collections::VecDeque; use std::rc::Rc; use rustc_data_structures::binary_search_util; use rustc_data_structures::frozen::Frozen; use rustc_data_structures::fx::{FxHashMap, FxHashSet}; use rustc_data_structures::graph::scc::Sccs; use rustc_errors::Diagnostic; use rustc_hir::def_id::CRATE_DEF_ID; use rustc_hir::CRATE_HIR_ID; use rustc_index::vec::IndexVec; use rustc_infer::infer::outlives::test_type_match; use rustc_infer::infer::region_constraints::{GenericKind, VarInfos, VerifyBound, VerifyIfEq}; use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin}; use rustc_middle::mir::{ Body, ClosureOutlivesRequirement, ClosureOutlivesSubject, ClosureRegionRequirements, ConstraintCategory, Local, Location, ReturnConstraint, TerminatorKind, }; use rustc_middle::traits::ObligationCause; use rustc_middle::traits::ObligationCauseCode; use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt, TypeFoldable, TypeVisitable}; use rustc_span::Span; use crate::{ constraints::{ graph::NormalConstraintGraph, ConstraintSccIndex, OutlivesConstraint, OutlivesConstraintSet, }, diagnostics::{RegionErrorKind, RegionErrors, UniverseInfo}, member_constraints::{MemberConstraintSet, NllMemberConstraintIndex}, nll::{PoloniusOutput, ToRegionVid}, region_infer::reverse_sccs::ReverseSccGraph, region_infer::values::{ LivenessValues, PlaceholderIndices, RegionElement, RegionValueElements, RegionValues, ToElementIndex, }, type_check::{free_region_relations::UniversalRegionRelations, Locations}, universal_regions::UniversalRegions, }; mod dump_mir; mod graphviz; mod opaque_types; mod reverse_sccs; pub mod values; pub struct RegionInferenceContext<'tcx> { pub var_infos: VarInfos, /// Contains the definition for every region variable. Region /// variables are identified by their index (`RegionVid`). The /// definition contains information about where the region came /// from as well as its final inferred value. definitions: IndexVec>, /// The liveness constraints added to each region. For most /// regions, these start out empty and steadily grow, though for /// each universally quantified region R they start out containing /// the entire CFG and `end(R)`. liveness_constraints: LivenessValues, /// The outlives constraints computed by the type-check. constraints: Frozen>, /// The constraint-set, but in graph form, making it easy to traverse /// the constraints adjacent to a particular region. Used to construct /// the SCC (see `constraint_sccs`) and for error reporting. constraint_graph: Frozen, /// 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>, /// 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 /// to outlive a given SCC. Computed lazily. rev_scc_graph: Option>, /// The "R0 member of [R1..Rn]" constraints, indexed by SCC. member_constraints: Rc>, /// Records the member constraints that we applied to each scc. /// This is useful for error reporting. Once constraint /// propagation is done, this vector is sorted according to /// `member_region_scc`. member_constraints_applied: Vec, /// Map universe indexes to information on why we created it. universe_causes: FxHashMap>, /// Contains the minimum universe of any variable within the same /// SCC. We will ensure that no SCC contains values that are not /// visible from this index. scc_universes: IndexVec, /// Contains a "representative" from each SCC. This will be the /// minimal RegionVid belonging to that universe. It is used as a /// kind of hacky way to manage checking outlives relationships, /// since we can 'canonicalize' each region to the representative /// of its SCC and be sure that -- if they have the same repr -- /// they *must* be equal (though not having the same repr does not /// mean they are unequal). scc_representatives: IndexVec, /// The final inferred values of the region variables; we compute /// one value per SCC. To get the value for any given *region*, /// you first find which scc it is a part of. scc_values: RegionValues, /// Type constraints that we check after solving. type_tests: Vec>, /// Information about the universally quantified regions in scope /// on this function. universal_regions: Rc>, /// Information about how the universally quantified regions in /// scope on this function relate to one another. universal_region_relations: Frozen>, } /// Each time that `apply_member_constraint` is successful, it appends /// one of these structs to the `member_constraints_applied` field. /// This is used in error reporting to trace out what happened. /// /// The way that `apply_member_constraint` works is that it effectively /// adds a new lower bound to the SCC it is analyzing: so you wind up /// with `'R: 'O` where `'R` is the pick-region and `'O` is the /// minimal viable option. #[derive(Debug)] pub(crate) struct AppliedMemberConstraint { /// The SCC that was affected. (The "member region".) /// /// The vector if `AppliedMemberConstraint` elements is kept sorted /// by this field. pub(crate) member_region_scc: ConstraintSccIndex, /// The "best option" that `apply_member_constraint` found -- this was /// added as an "ad-hoc" lower-bound to `member_region_scc`. pub(crate) min_choice: ty::RegionVid, /// The "member constraint index" -- we can find out details about /// the constraint from /// `set.member_constraints[member_constraint_index]`. pub(crate) member_constraint_index: NllMemberConstraintIndex, } pub(crate) struct RegionDefinition<'tcx> { /// What kind of variable is this -- a free region? existential /// variable? etc. (See the `NllRegionVariableOrigin` for more /// info.) pub(crate) origin: NllRegionVariableOrigin, /// Which universe is this region variable defined in? This is /// most often `ty::UniverseIndex::ROOT`, but when we encounter /// forall-quantifiers like `for<'a> { 'a = 'b }`, we would create /// the variable for `'a` in a fresh universe that extends ROOT. pub(crate) universe: ty::UniverseIndex, /// If this is 'static or an early-bound region, then this is /// `Some(X)` where `X` is the name of the region. pub(crate) external_name: Option>, } /// N.B., the variants in `Cause` are intentionally ordered. Lower /// values are preferred when it comes to error messages. Do not /// reorder willy nilly. #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)] pub(crate) enum Cause { /// point inserted because Local was live at the given Location LiveVar(Local, Location), /// point inserted because Local was dropped at the given Location DropVar(Local, Location), } /// A "type test" corresponds to an outlives constraint between a type /// and a lifetime, like `T: 'x` or `::Bar: 'x`. They are /// translated from the `Verify` region constraints in the ordinary /// inference context. /// /// These sorts of constraints are handled differently than ordinary /// constraints, at least at present. During type checking, the /// `InferCtxt::process_registered_region_obligations` method will /// attempt to convert a type test like `T: 'x` into an ordinary /// outlives constraint when possible (for example, `&'a T: 'b` will /// be converted into `'a: 'b` and registered as a `Constraint`). /// /// In some cases, however, there are outlives relationships that are /// not converted into a region constraint, but rather into one of /// these "type tests". The distinction is that a type test does not /// influence the inference result, but instead just examines the /// values that we ultimately inferred for each region variable and /// checks that they meet certain extra criteria. If not, an error /// can be issued. /// /// One reason for this is that these type tests typically boil down /// to a check like `'a: 'x` where `'a` is a universally quantified /// region -- and therefore not one whose value is really meant to be /// *inferred*, precisely (this is not always the case: one can have a /// type test like `>::Bar: 'x`, where `'?0` is an /// inference variable). Another reason is that these type tests can /// involve *disjunction* -- that is, they can be satisfied in more /// than one way. /// /// For more information about this translation, see /// `InferCtxt::process_registered_region_obligations` and /// `InferCtxt::type_must_outlive` in `rustc_infer::infer::InferCtxt`. #[derive(Clone, Debug)] pub struct TypeTest<'tcx> { /// The type `T` that must outlive the region. pub generic_kind: GenericKind<'tcx>, /// The region `'x` that the type must outlive. pub lower_bound: RegionVid, /// The span to blame. pub span: Span, /// A test which, if met by the region `'x`, proves that this type /// constraint is satisfied. pub verify_bound: VerifyBound<'tcx>, } /// When we have an unmet lifetime constraint, we try to propagate it outward (e.g. to a closure /// environment). If we can't, it is an error. #[derive(Clone, Copy, Debug, Eq, PartialEq)] enum RegionRelationCheckResult { Ok, Propagated, Error, } #[derive(Clone, PartialEq, Eq, Debug)] enum Trace<'tcx> { StartRegion, FromOutlivesConstraint(OutlivesConstraint<'tcx>), NotVisited, } #[derive(Clone, PartialEq, Eq, Debug)] pub enum ExtraConstraintInfo { PlaceholderFromPredicate(Span), } impl<'tcx> RegionInferenceContext<'tcx> { /// Creates a new region inference context with a total of /// `num_region_variables` valid inference variables; the first N /// of those will be constant regions representing the free /// regions defined in `universal_regions`. /// /// The `outlives_constraints` and `type_tests` are an initial set /// of constraints produced by the MIR type check. pub(crate) fn new( var_infos: VarInfos, universal_regions: Rc>, placeholder_indices: Rc, universal_region_relations: Frozen>, outlives_constraints: OutlivesConstraintSet<'tcx>, member_constraints_in: MemberConstraintSet<'tcx, RegionVid>, universe_causes: FxHashMap>, type_tests: Vec>, liveness_constraints: LivenessValues, elements: &Rc, ) -> Self { // Create a RegionDefinition for each inference variable. let definitions: IndexVec<_, _> = var_infos .iter() .map(|info| RegionDefinition::new(info.universe, info.origin)) .collect(); let constraints = Frozen::freeze(outlives_constraints); let constraint_graph = Frozen::freeze(constraints.graph(definitions.len())); let fr_static = universal_regions.fr_static; let constraint_sccs = Rc::new(constraints.compute_sccs(&constraint_graph, fr_static)); let mut scc_values = RegionValues::new(elements, universal_regions.len(), &placeholder_indices); for region in liveness_constraints.rows() { let scc = constraint_sccs.scc(region); scc_values.merge_liveness(scc, region, &liveness_constraints); } let scc_universes = Self::compute_scc_universes(&constraint_sccs, &definitions); let scc_representatives = Self::compute_scc_representatives(&constraint_sccs, &definitions); let member_constraints = Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r))); let mut result = Self { var_infos, definitions, liveness_constraints, constraints, constraint_graph, constraint_sccs, rev_scc_graph: None, member_constraints, member_constraints_applied: Vec::new(), universe_causes, scc_universes, scc_representatives, scc_values, type_tests, universal_regions, universal_region_relations, }; result.init_free_and_bound_regions(); result } /// Each SCC is the combination of many region variables which /// have been equated. Therefore, we can associate a universe with /// each SCC which is minimum of all the universes of its /// constituent regions -- this is because whatever value the SCC /// takes on must be a value that each of the regions within the /// SCC could have as well. This implies that the SCC must have /// the minimum, or narrowest, universe. fn compute_scc_universes( constraint_sccs: &Sccs, definitions: &IndexVec>, ) -> IndexVec { let num_sccs = constraint_sccs.num_sccs(); let mut scc_universes = IndexVec::from_elem_n(ty::UniverseIndex::MAX, num_sccs); debug!("compute_scc_universes()"); // For each region R in universe U, ensure that the universe for the SCC // that contains R is "no bigger" than U. This effectively sets the universe // for each SCC to be the minimum of the regions within. for (region_vid, region_definition) in definitions.iter_enumerated() { let scc = constraint_sccs.scc(region_vid); let scc_universe = &mut scc_universes[scc]; let scc_min = std::cmp::min(region_definition.universe, *scc_universe); if scc_min != *scc_universe { *scc_universe = scc_min; debug!( "compute_scc_universes: lowered universe of {scc:?} to {scc_min:?} \ because it contains {region_vid:?} in {region_universe:?}", scc = scc, scc_min = scc_min, region_vid = region_vid, region_universe = region_definition.universe, ); } } // Walk each SCC `A` and `B` such that `A: B` // and ensure that universe(A) can see universe(B). // // This serves to enforce the 'empty/placeholder' hierarchy // (described in more detail on `RegionKind`): // // ``` // static -----+ // | | // empty(U0) placeholder(U1) // | / // empty(U1) // ``` // // In particular, imagine we have variables R0 in U0 and R1 // created in U1, and constraints like this; // // ``` // R1: !1 // R1 outlives the placeholder in U1 // R1: R0 // R1 outlives R0 // ``` // // Here, we wish for R1 to be `'static`, because it // cannot outlive `placeholder(U1)` and `empty(U0)` any other way. // // Thanks to this loop, what happens is that the `R1: R0` // constraint lowers the universe of `R1` to `U0`, which in turn // means that the `R1: !1` constraint will (later) cause // `R1` to become `'static`. for scc_a in constraint_sccs.all_sccs() { for &scc_b in constraint_sccs.successors(scc_a) { let scc_universe_a = scc_universes[scc_a]; let scc_universe_b = scc_universes[scc_b]; let scc_universe_min = std::cmp::min(scc_universe_a, scc_universe_b); if scc_universe_a != scc_universe_min { scc_universes[scc_a] = scc_universe_min; debug!( "compute_scc_universes: lowered universe of {scc_a:?} to {scc_universe_min:?} \ because {scc_a:?}: {scc_b:?} and {scc_b:?} is in universe {scc_universe_b:?}", scc_a = scc_a, scc_b = scc_b, scc_universe_min = scc_universe_min, scc_universe_b = scc_universe_b ); } } } debug!("compute_scc_universes: scc_universe = {:#?}", scc_universes); scc_universes } /// For each SCC, we compute a unique `RegionVid` (in fact, the /// minimal one that belongs to the SCC). See /// `scc_representatives` field of `RegionInferenceContext` for /// more details. fn compute_scc_representatives( constraints_scc: &Sccs, definitions: &IndexVec>, ) -> IndexVec { let num_sccs = constraints_scc.num_sccs(); let next_region_vid = definitions.next_index(); let mut scc_representatives = IndexVec::from_elem_n(next_region_vid, num_sccs); for region_vid in definitions.indices() { let scc = constraints_scc.scc(region_vid); let prev_min = scc_representatives[scc]; scc_representatives[scc] = region_vid.min(prev_min); } scc_representatives } /// Initializes the region variables for each universally /// quantified region (lifetime parameter). The first N variables /// always correspond to the regions appearing in the function /// signature (both named and anonymous) and where-clauses. This /// function iterates over those regions and initializes them with /// minimum values. /// /// For example: /// ``` /// fn foo<'a, 'b>( /* ... */ ) where 'a: 'b { /* ... */ } /// ``` /// would initialize two variables like so: /// ```ignore (illustrative) /// R0 = { CFG, R0 } // 'a /// R1 = { CFG, R0, R1 } // 'b /// ``` /// Here, R0 represents `'a`, and it contains (a) the entire CFG /// and (b) any universally quantified regions that it outlives, /// which in this case is just itself. R1 (`'b`) in contrast also /// outlives `'a` and hence contains R0 and R1. fn init_free_and_bound_regions(&mut self) { // Update the names (if any) for (external_name, variable) in self.universal_regions.named_universal_regions() { debug!( "init_universal_regions: region {:?} has external name {:?}", variable, external_name ); self.definitions[variable].external_name = Some(external_name); } for variable in self.definitions.indices() { let scc = self.constraint_sccs.scc(variable); match self.definitions[variable].origin { NllRegionVariableOrigin::FreeRegion => { // For each free, universally quantified region X: // Add all nodes in the CFG to liveness constraints self.liveness_constraints.add_all_points(variable); self.scc_values.add_all_points(scc); // Add `end(X)` into the set for X. self.scc_values.add_element(scc, variable); } 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_universes[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); } } NllRegionVariableOrigin::Existential { .. } => { // For existential, regions, nothing to do. } } } } /// Returns an iterator over all the region indices. pub fn regions(&self) -> impl Iterator + 'tcx { self.definitions.indices() } /// Given a universal region in scope on the MIR, returns the /// corresponding index. /// /// (Panics if `r` is not a registered universal region.) pub fn to_region_vid(&self, r: ty::Region<'tcx>) -> RegionVid { self.universal_regions.to_region_vid(r) } /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`. pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) { self.universal_regions.annotate(tcx, err) } /// Returns `true` if the region `r` contains the point `p`. /// /// Panics if called before `solve()` executes, pub(crate) fn region_contains(&self, r: impl ToRegionVid, p: impl ToElementIndex) -> bool { let scc = self.constraint_sccs.scc(r.to_region_vid()); self.scc_values.contains(scc, p) } /// Returns access to the value of `r` for debugging purposes. pub(crate) fn region_value_str(&self, r: RegionVid) -> String { let scc = self.constraint_sccs.scc(r.to_region_vid()); self.scc_values.region_value_str(scc) } pub(crate) fn placeholders_contained_in<'a>( &'a self, r: RegionVid, ) -> impl Iterator + 'a { let scc = self.constraint_sccs.scc(r.to_region_vid()); self.scc_values.placeholders_contained_in(scc) } /// Returns access to the value of `r` for debugging purposes. pub(crate) fn region_universe(&self, r: RegionVid) -> ty::UniverseIndex { let scc = self.constraint_sccs.scc(r.to_region_vid()); self.scc_universes[scc] } /// Once region solving has completed, this function will return /// the member constraints that were applied to the value of a given /// region `r`. See `AppliedMemberConstraint`. pub(crate) fn applied_member_constraints( &self, r: impl ToRegionVid, ) -> &[AppliedMemberConstraint] { let scc = self.constraint_sccs.scc(r.to_region_vid()); binary_search_util::binary_search_slice( &self.member_constraints_applied, |applied| applied.member_region_scc, &scc, ) } /// Performs region inference and report errors if we see any /// unsatisfiable constraints. If this is a closure, returns the /// region requirements to propagate to our creator, if any. #[instrument(skip(self, infcx, body, polonius_output), level = "debug")] pub(super) fn solve( &mut self, infcx: &InferCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>, body: &Body<'tcx>, polonius_output: Option>, ) -> (Option>, RegionErrors<'tcx>) { let mir_def_id = body.source.def_id(); self.propagate_constraints(body); let mut errors_buffer = RegionErrors::new(infcx.tcx); // If this is a closure, we can propagate unsatisfied // `outlives_requirements` to our creator, so create a vector // to store those. Otherwise, we'll pass in `None` to the // functions below, which will trigger them to report errors // eagerly. let mut outlives_requirements = infcx.tcx.is_typeck_child(mir_def_id).then(Vec::new); self.check_type_tests( infcx, param_env, body, outlives_requirements.as_mut(), &mut errors_buffer, ); // In Polonius mode, the errors about missing universal region relations are in the output // and need to be emitted or propagated. Otherwise, we need to check whether the // constraints were too strong, and if so, emit or propagate those errors. if infcx.tcx.sess.opts.unstable_opts.polonius { self.check_polonius_subset_errors( outlives_requirements.as_mut(), &mut errors_buffer, polonius_output.expect("Polonius output is unavailable despite `-Z polonius`"), ); } else { self.check_universal_regions(outlives_requirements.as_mut(), &mut errors_buffer); } if errors_buffer.is_empty() { self.check_member_constraints(infcx, &mut errors_buffer); } let outlives_requirements = outlives_requirements.unwrap_or_default(); if outlives_requirements.is_empty() { (None, errors_buffer) } else { let num_external_vids = self.universal_regions.num_global_and_external_regions(); ( Some(ClosureRegionRequirements { num_external_vids, outlives_requirements }), errors_buffer, ) } } /// Propagate the region constraints: this will grow the values /// for each region variable until all the constraints are /// satisfied. Note that some values may grow **too** large to be /// feasible, but we check this later. #[instrument(skip(self, _body), level = "debug")] fn propagate_constraints(&mut self, _body: &Body<'tcx>) { debug!("constraints={:#?}", { let mut constraints: Vec<_> = self.constraints.outlives().iter().collect(); constraints.sort_by_key(|c| (c.sup, c.sub)); constraints .into_iter() .map(|c| (c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub))) .collect::>() }); // To propagate constraints, we walk the DAG induced by the // SCC. For each SCC, we visit its successors and compute // their values, then we union all those values to get our // own. let constraint_sccs = self.constraint_sccs.clone(); for scc in constraint_sccs.all_sccs() { self.compute_value_for_scc(scc); } // Sort the applied member constraints so we can binary search // through them later. self.member_constraints_applied.sort_by_key(|applied| applied.member_region_scc); } /// Computes the value of the SCC `scc_a`, which has not yet been /// computed, by unioning the values of its successors. /// Assumes that all successors have been computed already /// (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) { 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); } } // Now take member constraints into account. let member_constraints = self.member_constraints.clone(); for m_c_i in member_constraints.indices(scc_a) { self.apply_member_constraint(scc_a, m_c_i, member_constraints.choice_regions(m_c_i)); } debug!(value = ?self.scc_values.region_value_str(scc_a)); } /// Invoked for each `R0 member of [R1..Rn]` constraint. /// /// `scc` is the SCC containing R0, and `choice_regions` are the /// `R1..Rn` regions -- they are always known to be universal /// regions (and if that's not true, we just don't attempt to /// enforce the constraint). /// /// The current value of `scc` at the time the method is invoked /// is considered a *lower bound*. If possible, we will modify /// the constraint to set it equal to one of the option regions. /// If we make any changes, returns true, else false. #[instrument(skip(self, member_constraint_index), level = "debug")] fn apply_member_constraint( &mut self, scc: ConstraintSccIndex, member_constraint_index: NllMemberConstraintIndex, choice_regions: &[ty::RegionVid], ) -> bool { // Create a mutable vector of the options. We'll try to winnow // them down. let mut choice_regions: Vec = choice_regions.to_vec(); // Convert to the SCC representative: sometimes we have inference // variables in the member constraint that wind up equated with // universal regions. The scc representative is the minimal numbered // one from the corresponding scc so it will be the universal region // if one exists. for c_r in &mut choice_regions { let scc = self.constraint_sccs.scc(*c_r); *c_r = self.scc_representatives[scc]; } // The 'member region' in a member constraint is part of the // hidden type, which must be in the root universe. Therefore, // it cannot have any placeholders in its value. assert!(self.scc_universes[scc] == ty::UniverseIndex::ROOT); debug_assert!( self.scc_values.placeholders_contained_in(scc).next().is_none(), "scc {:?} in a member constraint has placeholder value: {:?}", scc, self.scc_values.region_value_str(scc), ); // The existing value for `scc` is a lower-bound. This will // consist of some set `{P} + {LB}` of points `{P}` and // lower-bound free regions `{LB}`. As each choice region `O` // is a free region, it will outlive the points. But we can // only consider the option `O` if `O: LB`. choice_regions.retain(|&o_r| { self.scc_values .universal_regions_outlived_by(scc) .all(|lb| self.universal_region_relations.outlives(o_r, lb)) }); debug!(?choice_regions, "after lb"); // Now find all the *upper bounds* -- that is, each UB is a // free region that must outlive the member region `R0` (`UB: // R0`). Therefore, we need only keep an option `O` if `UB: O` // for all UB. let rev_scc_graph = self.reverse_scc_graph(); let universal_region_relations = &self.universal_region_relations; for ub in rev_scc_graph.upper_bounds(scc) { debug!(?ub); choice_regions.retain(|&o_r| universal_region_relations.outlives(ub, o_r)); } debug!(?choice_regions, "after ub"); // If we ruled everything out, we're done. if choice_regions.is_empty() { return false; } // Otherwise, we need to find the minimum remaining choice, if // any, and take that. debug!("choice_regions remaining are {:#?}", choice_regions); let Some(&min_choice) = choice_regions.iter().find(|&r1| { choice_regions.iter().all(|&r2| { self.universal_region_relations.outlives(r2, *r1) }) }) else { debug!("no choice region outlived by all others"); return false; }; let min_choice_scc = self.constraint_sccs.scc(min_choice); debug!(?min_choice, ?min_choice_scc); if self.scc_values.add_region(scc, min_choice_scc) { self.member_constraints_applied.push(AppliedMemberConstraint { member_region_scc: scc, min_choice, member_constraint_index, }); true } else { false } } /// Returns `true` if all the elements in the value of `scc_b` are nameable /// 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.scc_universes[scc_a]; // Quick check: 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(self.scc_universes[scc_b]) { 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); } /// Once regions have been propagated, this method is used to see /// whether the "type tests" produced by typeck were satisfied; /// type tests encode type-outlives relationships like `T: /// 'a`. See `TypeTest` for more details. fn check_type_tests( &self, infcx: &InferCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>, body: &Body<'tcx>, mut propagated_outlives_requirements: Option<&mut Vec>>, errors_buffer: &mut RegionErrors<'tcx>, ) { let tcx = infcx.tcx; // Sometimes we register equivalent type-tests that would // result in basically the exact same error being reported to // the user. Avoid that. let mut deduplicate_errors = FxHashSet::default(); for type_test in &self.type_tests { debug!("check_type_test: {:?}", type_test); let generic_ty = type_test.generic_kind.to_ty(tcx); if self.eval_verify_bound( infcx, param_env, generic_ty, type_test.lower_bound, &type_test.verify_bound, ) { continue; } if let Some(propagated_outlives_requirements) = &mut propagated_outlives_requirements { if self.try_promote_type_test( infcx, param_env, body, type_test, propagated_outlives_requirements, ) { continue; } } // Type-test failed. Report the error. let erased_generic_kind = infcx.tcx.erase_regions(type_test.generic_kind); // Skip duplicate-ish errors. if deduplicate_errors.insert(( erased_generic_kind, type_test.lower_bound, type_test.span, )) { debug!( "check_type_test: reporting error for erased_generic_kind={:?}, \ lower_bound_region={:?}, \ type_test.span={:?}", erased_generic_kind, type_test.lower_bound, type_test.span, ); errors_buffer.push(RegionErrorKind::TypeTestError { type_test: type_test.clone() }); } } } /// Invoked when we have some type-test (e.g., `T: 'X`) that we cannot /// prove to be satisfied. If this is a closure, we will attempt to /// "promote" this type-test into our `ClosureRegionRequirements` and /// hence pass it up the creator. To do this, we have to phrase the /// type-test in terms of external free regions, as local free /// regions are not nameable by the closure's creator. /// /// Promotion works as follows: we first check that the type `T` /// contains only regions that the creator knows about. If this is /// true, then -- as a consequence -- we know that all regions in /// the type `T` are free regions that outlive the closure body. If /// false, then promotion fails. /// /// Once we've promoted T, we have to "promote" `'X` to some region /// that is "external" to the closure. Generally speaking, a region /// may be the union of some points in the closure body as well as /// various free lifetimes. We can ignore the points in the closure /// body: if the type T can be expressed in terms of external regions, /// we know it outlives the points in the closure body. That /// just leaves the free regions. /// /// The idea then is to lower the `T: 'X` constraint into multiple /// bounds -- e.g., if `'X` is the union of two free lifetimes, /// `'1` and `'2`, then we would create `T: '1` and `T: '2`. #[instrument(level = "debug", skip(self, infcx, propagated_outlives_requirements))] fn try_promote_type_test( &self, infcx: &InferCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>, body: &Body<'tcx>, type_test: &TypeTest<'tcx>, propagated_outlives_requirements: &mut Vec>, ) -> bool { let tcx = infcx.tcx; let TypeTest { generic_kind, lower_bound, span: _, verify_bound: _ } = type_test; let generic_ty = generic_kind.to_ty(tcx); let Some(subject) = self.try_promote_type_test_subject(infcx, generic_ty) else { return false; }; debug!("subject = {:?}", subject); let r_scc = self.constraint_sccs.scc(*lower_bound); debug!( "lower_bound = {:?} r_scc={:?} universe={:?}", lower_bound, r_scc, self.scc_universes[r_scc] ); // If the type test requires that `T: 'a` where `'a` is a // placeholder from another universe, that effectively requires // `T: 'static`, so we have to propagate that requirement. // // It doesn't matter *what* universe because the promoted `T` will // always be in the root universe. if let Some(p) = self.scc_values.placeholders_contained_in(r_scc).next() { debug!("encountered placeholder in higher universe: {:?}, requiring 'static", p); let static_r = self.universal_regions.fr_static; propagated_outlives_requirements.push(ClosureOutlivesRequirement { subject, outlived_free_region: static_r, blame_span: type_test.span, category: ConstraintCategory::Boring, }); // we can return here -- the code below might push add'l constraints // but they would all be weaker than this one. return true; } // For each region outlived by lower_bound find a non-local, // universal region (it may be the same region) and add it to // `ClosureOutlivesRequirement`. for ur in self.scc_values.universal_regions_outlived_by(r_scc) { debug!("universal_region_outlived_by ur={:?}", ur); // Check whether we can already prove that the "subject" outlives `ur`. // If so, we don't have to propagate this requirement to our caller. // // To continue the example from the function, if we are trying to promote // a requirement that `T: 'X`, and we know that `'X = '1 + '2` (i.e., the union // `'1` and `'2`), then in this loop `ur` will be `'1` (and `'2`). So here // we check whether `T: '1` is something we *can* prove. If so, no need // to propagate that requirement. // // This is needed because -- particularly in the case // where `ur` is a local bound -- we are sometimes in a // position to prove things that our caller cannot. See // #53570 for an example. if self.eval_verify_bound(infcx, param_env, generic_ty, ur, &type_test.verify_bound) { continue; } let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur); debug!("try_promote_type_test: non_local_ub={:?}", non_local_ub); // This is slightly too conservative. To show T: '1, given `'2: '1` // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to // avoid potential non-determinism we approximate this by requiring // T: '1 and T: '2. for upper_bound in non_local_ub { debug_assert!(self.universal_regions.is_universal_region(upper_bound)); debug_assert!(!self.universal_regions.is_local_free_region(upper_bound)); let requirement = ClosureOutlivesRequirement { subject, outlived_free_region: upper_bound, blame_span: type_test.span, category: ConstraintCategory::Boring, }; debug!("try_promote_type_test: pushing {:#?}", requirement); propagated_outlives_requirements.push(requirement); } } true } /// When we promote a type test `T: 'r`, we have to convert the /// type `T` into something we can store in a query result (so /// something allocated for `'tcx`). This is problematic if `ty` /// contains regions. During the course of NLL region checking, we /// will have replaced all of those regions with fresh inference /// variables. To create a test subject, we want to replace those /// inference variables with some region from the closure /// signature -- this is not always possible, so this is a /// fallible process. Presuming we do find a suitable region, we /// will use it's *external name*, which will be a `RegionKind` /// variant that can be used in query responses such as /// `ReEarlyBound`. #[instrument(level = "debug", skip(self, infcx))] fn try_promote_type_test_subject( &self, infcx: &InferCtxt<'tcx>, ty: Ty<'tcx>, ) -> Option> { let tcx = infcx.tcx; let ty = tcx.fold_regions(ty, |r, _depth| { let region_vid = self.to_region_vid(r); // The challenge if this. We have some region variable `r` // whose value is a set of CFG points and universal // regions. We want to find if that set is *equivalent* to // any of the named regions found in the closure. // // To do so, we compute the // `non_local_universal_upper_bound`. This will be a // non-local, universal region that is greater than `r`. // However, it might not be *contained* within `r`, so // then we further check whether this bound is contained // in `r`. If so, we can say that `r` is equivalent to the // bound. // // Let's work through a few examples. For these, imagine // that we have 3 non-local regions (I'll denote them as // `'static`, `'a`, and `'b`, though of course in the code // they would be represented with indices) where: // // - `'static: 'a` // - `'static: 'b` // // First, let's assume that `r` is some existential // variable with an inferred value `{'a, 'static}` (plus // some CFG nodes). In this case, the non-local upper // bound is `'static`, since that outlives `'a`. `'static` // is also a member of `r` and hence we consider `r` // equivalent to `'static` (and replace it with // `'static`). // // Now let's consider the inferred value `{'a, 'b}`. This // means `r` is effectively `'a | 'b`. I'm not sure if // this can come about, actually, but assuming it did, we // would get a non-local upper bound of `'static`. Since // `'static` is not contained in `r`, we would fail to // find an equivalent. let upper_bound = self.non_local_universal_upper_bound(region_vid); if self.region_contains(region_vid, upper_bound) { self.definitions[upper_bound].external_name.unwrap_or(r) } else { // In the case of a failure, use a `ReVar` result. This will // cause the `needs_infer` later on to return `None`. r } }); debug!("try_promote_type_test_subject: folded ty = {:?}", ty); // `needs_infer` will only be true if we failed to promote some region. if ty.needs_infer() { return None; } Some(ClosureOutlivesSubject::Ty(ty)) } /// Given some universal or existential region `r`, finds a /// non-local, universal region `r+` that outlives `r` at entry to (and /// exit from) the closure. In the worst case, this will be /// `'static`. /// /// This is used for two purposes. First, if we are propagated /// some requirement `T: r`, we can use this method to enlarge `r` /// to something we can encode for our creator (which only knows /// about non-local, universal regions). It is also used when /// encoding `T` as part of `try_promote_type_test_subject` (see /// that fn for details). /// /// This is based on the result `'y` of `universal_upper_bound`, /// except that it converts further takes the non-local upper /// bound of `'y`, so that the final result is non-local. fn non_local_universal_upper_bound(&self, r: RegionVid) -> RegionVid { debug!("non_local_universal_upper_bound(r={:?}={})", r, self.region_value_str(r)); let lub = self.universal_upper_bound(r); // Grow further to get smallest universal region known to // creator. let non_local_lub = self.universal_region_relations.non_local_upper_bound(lub); debug!("non_local_universal_upper_bound: non_local_lub={:?}", non_local_lub); non_local_lub } /// Returns a universally quantified region that outlives the /// value of `r` (`r` may be existentially or universally /// quantified). /// /// Since `r` is (potentially) an existential region, it has some /// value which may include (a) any number of points in the CFG /// and (b) any number of `end('x)` elements of universally /// quantified regions. To convert this into a single universal /// region we do as follows: /// /// - Ignore the CFG points in `'r`. All universally quantified regions /// include the CFG anyhow. /// - For each `end('x)` element in `'r`, compute the mutual LUB, yielding /// a result `'y`. #[instrument(skip(self), level = "debug", ret)] pub(crate) fn universal_upper_bound(&self, r: RegionVid) -> RegionVid { debug!(r = %self.region_value_str(r)); // Find the smallest universal region that contains all other // universal regions within `region`. let mut lub = self.universal_regions.fr_fn_body; let r_scc = self.constraint_sccs.scc(r); for ur in self.scc_values.universal_regions_outlived_by(r_scc) { lub = self.universal_region_relations.postdom_upper_bound(lub, ur); } lub } /// Like `universal_upper_bound`, but returns an approximation more suitable /// for diagnostics. If `r` contains multiple disjoint universal regions /// (e.g. 'a and 'b in `fn foo<'a, 'b> { ... }`, we pick the lower-numbered region. /// This corresponds to picking named regions over unnamed regions /// (e.g. picking early-bound regions over a closure late-bound region). /// /// This means that the returned value may not be a true upper bound, since /// only 'static is known to outlive disjoint universal regions. /// Therefore, this method should only be used in diagnostic code, /// where displaying *some* named universal region is better than /// falling back to 'static. #[instrument(level = "debug", skip(self))] pub(crate) fn approx_universal_upper_bound(&self, r: RegionVid) -> RegionVid { debug!("{}", self.region_value_str(r)); // Find the smallest universal region that contains all other // universal regions within `region`. let mut lub = self.universal_regions.fr_fn_body; let r_scc = self.constraint_sccs.scc(r); let static_r = self.universal_regions.fr_static; for ur in self.scc_values.universal_regions_outlived_by(r_scc) { let new_lub = self.universal_region_relations.postdom_upper_bound(lub, ur); debug!(?ur, ?lub, ?new_lub); // The upper bound of two non-static regions is static: this // means we know nothing about the relationship between these // two regions. Pick a 'better' one to use when constructing // a diagnostic if ur != static_r && lub != static_r && new_lub == static_r { // Prefer the region with an `external_name` - this // indicates that the region is early-bound, so working with // it can produce a nicer error. if self.region_definition(ur).external_name.is_some() { lub = ur; } else if self.region_definition(lub).external_name.is_some() { // Leave lub unchanged } else { // If we get here, we don't have any reason to prefer // one region over the other. Just pick the // one with the lower index for now. lub = std::cmp::min(ur, lub); } } else { lub = new_lub; } } debug!(?r, ?lub); lub } /// Tests if `test` is true when applied to `lower_bound` at /// `point`. fn eval_verify_bound( &self, infcx: &InferCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>, generic_ty: Ty<'tcx>, lower_bound: RegionVid, verify_bound: &VerifyBound<'tcx>, ) -> bool { debug!("eval_verify_bound(lower_bound={:?}, verify_bound={:?})", lower_bound, verify_bound); match verify_bound { VerifyBound::IfEq(verify_if_eq_b) => { self.eval_if_eq(infcx, param_env, generic_ty, lower_bound, *verify_if_eq_b) } VerifyBound::IsEmpty => { let lower_bound_scc = self.constraint_sccs.scc(lower_bound); self.scc_values.elements_contained_in(lower_bound_scc).next().is_none() } VerifyBound::OutlivedBy(r) => { let r_vid = self.to_region_vid(*r); self.eval_outlives(r_vid, lower_bound) } VerifyBound::AnyBound(verify_bounds) => verify_bounds.iter().any(|verify_bound| { self.eval_verify_bound(infcx, param_env, generic_ty, lower_bound, verify_bound) }), VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| { self.eval_verify_bound(infcx, param_env, generic_ty, lower_bound, verify_bound) }), } } fn eval_if_eq( &self, infcx: &InferCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>, generic_ty: Ty<'tcx>, lower_bound: RegionVid, verify_if_eq_b: ty::Binder<'tcx, VerifyIfEq<'tcx>>, ) -> bool { let generic_ty = self.normalize_to_scc_representatives(infcx.tcx, generic_ty); let verify_if_eq_b = self.normalize_to_scc_representatives(infcx.tcx, verify_if_eq_b); match test_type_match::extract_verify_if_eq( infcx.tcx, param_env, &verify_if_eq_b, generic_ty, ) { Some(r) => { let r_vid = self.to_region_vid(r); self.eval_outlives(r_vid, lower_bound) } None => false, } } /// This is a conservative normalization procedure. It takes every /// free region in `value` and replaces it with the /// "representative" of its SCC (see `scc_representatives` field). /// We are guaranteed that if two values normalize to the same /// thing, then they are equal; this is a conservative check in /// that they could still be equal even if they normalize to /// different results. (For example, there might be two regions /// with the same value that are not in the same SCC). /// /// N.B., this is not an ideal approach and I would like to revisit /// it. However, it works pretty well in practice. In particular, /// this is needed to deal with projection outlives bounds like /// /// ```text /// >::Item: '1 /// ``` /// /// In particular, this routine winds up being important when /// there are bounds like `where >::Item: 'b` in the /// environment. In this case, if we can show that `'0 == 'a`, /// and that `'b: '1`, then we know that the clause is /// satisfied. In such cases, particularly due to limitations of /// the trait solver =), we usually wind up with a where-clause like /// `T: Foo<'a>` in scope, which thus forces `'0 == 'a` to be added as /// a constraint, and thus ensures that they are in the same SCC. /// /// So why can't we do a more correct routine? Well, we could /// *almost* use the `relate_tys` code, but the way it is /// currently setup it creates inference variables to deal with /// higher-ranked things and so forth, and right now the inference /// context is not permitted to make more inference variables. So /// we use this kind of hacky solution. fn normalize_to_scc_representatives(&self, tcx: TyCtxt<'tcx>, value: T) -> T where T: TypeFoldable<'tcx>, { tcx.fold_regions(value, |r, _db| { let vid = self.to_region_vid(r); let scc = self.constraint_sccs.scc(vid); let repr = self.scc_representatives[scc]; tcx.mk_region(ty::ReVar(repr)) }) } // Evaluate whether `sup_region == sub_region`. fn eval_equal(&self, r1: RegionVid, r2: RegionVid) -> bool { self.eval_outlives(r1, r2) && self.eval_outlives(r2, r1) } // Evaluate whether `sup_region: sub_region`. #[instrument(skip(self), level = "debug", ret)] fn eval_outlives(&self, sup_region: RegionVid, sub_region: RegionVid) -> bool { debug!( "sup_region's value = {:?} universal={:?}", self.region_value_str(sup_region), self.universal_regions.is_universal_region(sup_region), ); debug!( "sub_region's value = {:?} universal={:?}", self.region_value_str(sub_region), self.universal_regions.is_universal_region(sub_region), ); let sub_region_scc = self.constraint_sccs.scc(sub_region); let sup_region_scc = self.constraint_sccs.scc(sup_region); // If we are checking that `'sup: 'sub`, and `'sub` contains // some placeholder that `'sup` cannot name, then this is only // true if `'sup` outlives static. if !self.universe_compatible(sub_region_scc, sup_region_scc) { debug!( "sub universe `{sub_region_scc:?}` is not nameable \ by super `{sup_region_scc:?}`, promoting to static", ); return self.eval_outlives(sup_region, self.universal_regions.fr_static); } // Both the `sub_region` and `sup_region` consist of the union // of some number of universal regions (along with the union // of various points in the CFG; ignore those points for // now). Therefore, the sup-region outlives the sub-region if, // for each universal region R1 in the sub-region, there // exists some region R2 in the sup-region that outlives R1. let universal_outlives = self.scc_values.universal_regions_outlived_by(sub_region_scc).all(|r1| { self.scc_values .universal_regions_outlived_by(sup_region_scc) .any(|r2| self.universal_region_relations.outlives(r2, r1)) }); if !universal_outlives { debug!("sub region contains a universal region not present in super"); return false; } // Now we have to compare all the points in the sub region and make // sure they exist in the sup region. if self.universal_regions.is_universal_region(sup_region) { // Micro-opt: universal regions contain all points. debug!("super is universal and hence contains all points"); return true; } debug!("comparison between points in sup/sub"); self.scc_values.contains_points(sup_region_scc, sub_region_scc) } /// Once regions have been propagated, this method is used to see /// whether any of the constraints were too strong. In particular, /// we want to check for a case where a universally quantified /// region exceeded its bounds. Consider: /// ```compile_fail /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x } /// ``` /// In this case, returning `x` requires `&'a u32 <: &'b u32` /// and hence we establish (transitively) a constraint that /// `'a: 'b`. The `propagate_constraints` code above will /// therefore add `end('a)` into the region for `'b` -- but we /// have no evidence that `'b` outlives `'a`, so we want to report /// an error. /// /// If `propagated_outlives_requirements` is `Some`, then we will /// push unsatisfied obligations into there. Otherwise, we'll /// report them as errors. fn check_universal_regions( &self, mut propagated_outlives_requirements: Option<&mut Vec>>, errors_buffer: &mut RegionErrors<'tcx>, ) { for (fr, fr_definition) in self.definitions.iter_enumerated() { match fr_definition.origin { NllRegionVariableOrigin::FreeRegion => { // Go through each of the universal regions `fr` and check that // they did not grow too large, accumulating any requirements // for our caller into the `outlives_requirements` vector. self.check_universal_region( fr, &mut propagated_outlives_requirements, errors_buffer, ); } NllRegionVariableOrigin::Placeholder(placeholder) => { self.check_bound_universal_region(fr, placeholder, errors_buffer); } NllRegionVariableOrigin::Existential { .. } => { // nothing to check here } } } } /// Checks if Polonius has found any unexpected free region relations. /// /// In Polonius terms, a "subset error" (or "illegal subset relation error") is the equivalent /// of NLL's "checking if any region constraints were too strong": a placeholder origin `'a` /// was unexpectedly found to be a subset of another placeholder origin `'b`, and means in NLL /// terms that the "longer free region" `'a` outlived the "shorter free region" `'b`. /// /// More details can be found in this blog post by Niko: /// /// /// In the canonical example /// ```compile_fail /// fn foo<'a, 'b>(x: &'a u32) -> &'b u32 { x } /// ``` /// returning `x` requires `&'a u32 <: &'b u32` and hence we establish (transitively) a /// constraint that `'a: 'b`. It is an error that we have no evidence that this /// constraint holds. /// /// If `propagated_outlives_requirements` is `Some`, then we will /// push unsatisfied obligations into there. Otherwise, we'll /// report them as errors. fn check_polonius_subset_errors( &self, mut propagated_outlives_requirements: Option<&mut Vec>>, errors_buffer: &mut RegionErrors<'tcx>, polonius_output: Rc, ) { debug!( "check_polonius_subset_errors: {} subset_errors", polonius_output.subset_errors.len() ); // Similarly to `check_universal_regions`: a free region relation, which was not explicitly // declared ("known") was found by Polonius, so emit an error, or propagate the // requirements for our caller into the `propagated_outlives_requirements` vector. // // Polonius doesn't model regions ("origins") as CFG-subsets or durations, but the // `longer_fr` and `shorter_fr` terminology will still be used here, for consistency with // the rest of the NLL infrastructure. The "subset origin" is the "longer free region", // and the "superset origin" is the outlived "shorter free region". // // Note: Polonius will produce a subset error at every point where the unexpected // `longer_fr`'s "placeholder loan" is contained in the `shorter_fr`. This can be helpful // for diagnostics in the future, e.g. to point more precisely at the key locations // requiring this constraint to hold. However, the error and diagnostics code downstream // expects that these errors are not duplicated (and that they are in a certain order). // Otherwise, diagnostics messages such as the ones giving names like `'1` to elided or // anonymous lifetimes for example, could give these names differently, while others like // the outlives suggestions or the debug output from `#[rustc_regions]` would be // duplicated. The polonius subset errors are deduplicated here, while keeping the // CFG-location ordering. let mut subset_errors: Vec<_> = polonius_output .subset_errors .iter() .flat_map(|(_location, subset_errors)| subset_errors.iter()) .collect(); subset_errors.sort(); subset_errors.dedup(); for (longer_fr, shorter_fr) in subset_errors.into_iter() { debug!( "check_polonius_subset_errors: subset_error longer_fr={:?},\ shorter_fr={:?}", longer_fr, shorter_fr ); let propagated = self.try_propagate_universal_region_error( *longer_fr, *shorter_fr, &mut propagated_outlives_requirements, ); if propagated == RegionRelationCheckResult::Error { errors_buffer.push(RegionErrorKind::RegionError { longer_fr: *longer_fr, shorter_fr: *shorter_fr, fr_origin: NllRegionVariableOrigin::FreeRegion, is_reported: true, }); } } // Handle the placeholder errors as usual, until the chalk-rustc-polonius triumvirate has // a more complete picture on how to separate this responsibility. for (fr, fr_definition) in self.definitions.iter_enumerated() { match fr_definition.origin { NllRegionVariableOrigin::FreeRegion => { // handled by polonius above } NllRegionVariableOrigin::Placeholder(placeholder) => { self.check_bound_universal_region(fr, placeholder, errors_buffer); } NllRegionVariableOrigin::Existential { .. } => { // nothing to check here } } } } /// Checks the final value for the free region `fr` to see if it /// grew too large. In particular, examine what `end(X)` points /// wound up in `fr`'s final value; for each `end(X)` where `X != /// fr`, we want to check that `fr: X`. If not, that's either an /// error, or something we have to propagate to our creator. /// /// Things that are to be propagated are accumulated into the /// `outlives_requirements` vector. #[instrument(skip(self, propagated_outlives_requirements, errors_buffer), level = "debug")] fn check_universal_region( &self, longer_fr: RegionVid, propagated_outlives_requirements: &mut Option<&mut Vec>>, errors_buffer: &mut RegionErrors<'tcx>, ) { let longer_fr_scc = self.constraint_sccs.scc(longer_fr); // Because this free region must be in the ROOT universe, we // know it cannot contain any bound universes. assert!(self.scc_universes[longer_fr_scc] == ty::UniverseIndex::ROOT); debug_assert!(self.scc_values.placeholders_contained_in(longer_fr_scc).next().is_none()); // Only check all of the relations for the main representative of each // SCC, otherwise just check that we outlive said representative. This // reduces the number of redundant relations propagated out of // closures. // Note that the representative will be a universal region if there is // one in this SCC, so we will always check the representative here. let representative = self.scc_representatives[longer_fr_scc]; if representative != longer_fr { if let RegionRelationCheckResult::Error = self.check_universal_region_relation( longer_fr, representative, propagated_outlives_requirements, ) { errors_buffer.push(RegionErrorKind::RegionError { longer_fr, shorter_fr: representative, fr_origin: NllRegionVariableOrigin::FreeRegion, is_reported: true, }); } return; } // Find every region `o` such that `fr: o` // (because `fr` includes `end(o)`). let mut error_reported = false; for shorter_fr in self.scc_values.universal_regions_outlived_by(longer_fr_scc) { if let RegionRelationCheckResult::Error = self.check_universal_region_relation( longer_fr, shorter_fr, propagated_outlives_requirements, ) { // We only report the first region error. Subsequent errors are hidden so as // not to overwhelm the user, but we do record them so as to potentially print // better diagnostics elsewhere... errors_buffer.push(RegionErrorKind::RegionError { longer_fr, shorter_fr, fr_origin: NllRegionVariableOrigin::FreeRegion, is_reported: !error_reported, }); error_reported = true; } } } /// Checks that we can prove that `longer_fr: shorter_fr`. If we can't we attempt to propagate /// the constraint outward (e.g. to a closure environment), but if that fails, there is an /// error. fn check_universal_region_relation( &self, longer_fr: RegionVid, shorter_fr: RegionVid, propagated_outlives_requirements: &mut Option<&mut Vec>>, ) -> RegionRelationCheckResult { // If it is known that `fr: o`, carry on. if self.universal_region_relations.outlives(longer_fr, shorter_fr) { RegionRelationCheckResult::Ok } else { // If we are not in a context where we can't propagate errors, or we // could not shrink `fr` to something smaller, then just report an // error. // // Note: in this case, we use the unapproximated regions to report the // error. This gives better error messages in some cases. self.try_propagate_universal_region_error( longer_fr, shorter_fr, propagated_outlives_requirements, ) } } /// Attempt to propagate a region error (e.g. `'a: 'b`) that is not met to a closure's /// creator. If we cannot, then the caller should report an error to the user. fn try_propagate_universal_region_error( &self, longer_fr: RegionVid, shorter_fr: RegionVid, propagated_outlives_requirements: &mut Option<&mut Vec>>, ) -> RegionRelationCheckResult { if let Some(propagated_outlives_requirements) = propagated_outlives_requirements { // Shrink `longer_fr` until we find a non-local region (if we do). // We'll call it `fr-` -- it's ever so slightly smaller than // `longer_fr`. if let Some(fr_minus) = self.universal_region_relations.non_local_lower_bound(longer_fr) { 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, ); // Grow `shorter_fr` until we find some non-local regions. (We // always will.) We'll call them `shorter_fr+` -- they're ever // so slightly larger than `shorter_fr`. let shorter_fr_plus = self.universal_region_relations.non_local_upper_bounds(shorter_fr); debug!( "try_propagate_universal_region_error: shorter_fr_plus={:?}", shorter_fr_plus ); for fr in shorter_fr_plus { // Push the constraint `fr-: shorter_fr+` 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, }); } return RegionRelationCheckResult::Propagated; } } RegionRelationCheckResult::Error } fn check_bound_universal_region( &self, longer_fr: RegionVid, placeholder: ty::PlaceholderRegion, errors_buffer: &mut RegionErrors<'tcx>, ) { debug!("check_bound_universal_region(fr={:?}, placeholder={:?})", longer_fr, placeholder,); let longer_fr_scc = self.constraint_sccs.scc(longer_fr); debug!("check_bound_universal_region: longer_fr_scc={:?}", longer_fr_scc,); for error_element in self.scc_values.elements_contained_in(longer_fr_scc) { match error_element { RegionElement::Location(_) | RegionElement::RootUniversalRegion(_) => {} // If we have some bound universal region `'a`, then the only // elements it can contain is itself -- we don't know anything // else about it! RegionElement::PlaceholderRegion(placeholder1) => { if placeholder == placeholder1 { continue; } } } errors_buffer.push(RegionErrorKind::BoundUniversalRegionError { longer_fr, error_element, placeholder, }); // Stop after the first error, it gets too noisy otherwise, and does not provide more information. break; } debug!("check_bound_universal_region: all bounds satisfied"); } #[instrument(level = "debug", skip(self, infcx, errors_buffer))] fn check_member_constraints( &self, infcx: &InferCtxt<'tcx>, errors_buffer: &mut RegionErrors<'tcx>, ) { let member_constraints = self.member_constraints.clone(); for m_c_i in member_constraints.all_indices() { debug!(?m_c_i); let m_c = &member_constraints[m_c_i]; let member_region_vid = m_c.member_region_vid; debug!( ?member_region_vid, value = ?self.region_value_str(member_region_vid), ); let choice_regions = member_constraints.choice_regions(m_c_i); debug!(?choice_regions); // Did the member region wind up equal to any of the option regions? if let Some(o) = choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid)) { debug!("evaluated as equal to {:?}", o); continue; } // If not, report an error. let member_region = infcx.tcx.mk_region(ty::ReVar(member_region_vid)); errors_buffer.push(RegionErrorKind::UnexpectedHiddenRegion { span: m_c.definition_span, hidden_ty: m_c.hidden_ty, key: m_c.key, member_region, }); } } /// 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( &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 { debug!("cannot_name_value_of(r1={:?}, r2={:?})", r1, r2); match self.definitions[r2].origin { NllRegionVariableOrigin::Placeholder(placeholder) => { let universe1 = self.definitions[r1].universe; debug!( "cannot_name_value_of: universe1={:?} placeholder={:?}", universe1, placeholder ); universe1.cannot_name(placeholder.universe) } NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { .. } => { false } } } /// 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) } /// Walks the graph of constraints (where `'a: 'b` is considered /// an edge `'a -> 'b`) to find all paths from `from_region` to /// `to_region`. The paths are accumulated into the vector /// `results`. The paths are stored as a series of /// `ConstraintIndex` values -- in other words, a list of *edges*. /// /// Returns: a series of constraints as well as the region `R` /// that passed the target test. pub(crate) fn find_constraint_paths_between_regions( &self, from_region: RegionVid, target_test: impl Fn(RegionVid) -> bool, ) -> Option<(Vec>, RegionVid)> { let mut context = IndexVec::from_elem(Trace::NotVisited, &self.definitions); context[from_region] = Trace::StartRegion; // Use a deque so that we do a breadth-first search. We will // stop at the first match, which ought to be the shortest // path (fewest constraints). let mut deque = VecDeque::new(); deque.push_back(from_region); while let Some(r) = deque.pop_front() { debug!( "find_constraint_paths_between_regions: from_region={:?} r={:?} value={}", from_region, r, self.region_value_str(r), ); // Check if we reached the region we were looking for. If so, // we can reconstruct the path that led to it and return it. if target_test(r) { let mut result = vec![]; let mut p = r; loop { match context[p].clone() { Trace::NotVisited => { bug!("found unvisited region {:?} on path to {:?}", p, r) } Trace::FromOutlivesConstraint(c) => { p = c.sup; result.push(c); } Trace::StartRegion => { result.reverse(); return Some((result, r)); } } } } // Otherwise, walk over the outgoing constraints and // enqueue any regions we find, keeping track of how we // reached them. // A constraint like `'r: 'x` can come from our constraint // graph. let fr_static = self.universal_regions.fr_static; let outgoing_edges_from_graph = self.constraint_graph.outgoing_edges(r, &self.constraints, fr_static); // Always inline this closure because it can be hot. let mut handle_constraint = #[inline(always)] |constraint: OutlivesConstraint<'tcx>| { debug_assert_eq!(constraint.sup, r); let sub_region = constraint.sub; if let Trace::NotVisited = context[sub_region] { context[sub_region] = Trace::FromOutlivesConstraint(constraint); deque.push_back(sub_region); } }; // This loop can be hot. for constraint in outgoing_edges_from_graph { handle_constraint(constraint); } // Member constraints can also give rise to `'r: 'x` edges that // were not part of the graph initially, so watch out for those. // (But they are extremely rare; this loop is very cold.) for constraint in self.applied_member_constraints(r) { let p_c = &self.member_constraints[constraint.member_constraint_index]; let constraint = OutlivesConstraint { sup: r, sub: constraint.min_choice, locations: Locations::All(p_c.definition_span), span: p_c.definition_span, category: ConstraintCategory::OpaqueType, variance_info: ty::VarianceDiagInfo::default(), from_closure: false, }; handle_constraint(constraint); } } None } /// Finds some region R such that `fr1: R` and `R` is live at `elem`. #[instrument(skip(self), level = "trace", ret)] pub(crate) fn find_sub_region_live_at(&self, fr1: RegionVid, elem: Location) -> RegionVid { trace!(scc = ?self.constraint_sccs.scc(fr1)); trace!(universe = ?self.scc_universes[self.constraint_sccs.scc(fr1)]); self.find_constraint_paths_between_regions(fr1, |r| { // First look for some `r` such that `fr1: r` and `r` is live at `elem` trace!(?r, liveness_constraints=?self.liveness_constraints.region_value_str(r)); self.liveness_constraints.contains(r, elem) }) .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_paths_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_paths_between_regions(fr1, |r| { self.cannot_name_placeholder(r, fr1) }) }) .map(|(_path, r)| r) .unwrap() } /// Get the region outlived by `longer_fr` and live at `element`. pub(crate) fn region_from_element( &self, longer_fr: RegionVid, element: &RegionElement, ) -> RegionVid { match *element { RegionElement::Location(l) => self.find_sub_region_live_at(longer_fr, l), RegionElement::RootUniversalRegion(r) => r, RegionElement::PlaceholderRegion(error_placeholder) => self .definitions .iter_enumerated() .find_map(|(r, definition)| match definition.origin { NllRegionVariableOrigin::Placeholder(p) if p == error_placeholder => Some(r), _ => None, }) .unwrap(), } } /// Get the region definition of `r`. pub(crate) fn region_definition(&self, r: RegionVid) -> &RegionDefinition<'tcx> { &self.definitions[r] } /// Check if the SCC of `r` contains `upper`. pub(crate) fn upper_bound_in_region_scc(&self, r: RegionVid, upper: RegionVid) -> bool { let r_scc = self.constraint_sccs.scc(r); self.scc_values.contains(r_scc, upper) } pub(crate) fn universal_regions(&self) -> &UniversalRegions<'tcx> { self.universal_regions.as_ref() } /// Tries to find the best constraint to blame for the fact that /// `R: from_region`, where `R` is some region that meets /// `target_test`. This works by following the constraint graph, /// 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))] pub(crate) fn best_blame_constraint( &self, from_region: RegionVid, from_region_origin: NllRegionVariableOrigin, target_test: impl Fn(RegionVid) -> bool, ) -> (BlameConstraint<'tcx>, Vec) { // Find all paths let (path, target_region) = self.find_constraint_paths_between_regions(from_region, target_test).unwrap(); debug!( "path={:#?}", path.iter() .map(|c| format!( "{:?} ({:?}: {:?})", c, self.constraint_sccs.scc(c.sup), self.constraint_sccs.scc(c.sub), )) .collect::>() ); let mut extra_info = vec![]; for constraint in path.iter() { let outlived = constraint.sub; let Some(origin) = self.var_infos.get(outlived) else { continue; }; let RegionVariableOrigin::Nll(NllRegionVariableOrigin::Placeholder(p)) = origin.origin else { continue; }; debug!(?constraint, ?p); let ConstraintCategory::Predicate(span) = constraint.category else { continue; }; extra_info.push(ExtraConstraintInfo::PlaceholderFromPredicate(span)); // We only want to point to one break; } // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint. // Instead, we use it to produce an improved `ObligationCauseCode`. // FIXME - determine what we should do if we encounter multiple `ConstraintCategory::Predicate` // constraints. Currently, we just pick the first one. let cause_code = path .iter() .find_map(|constraint| { if let ConstraintCategory::Predicate(predicate_span) = constraint.category { // We currently do not store the `DefId` in the `ConstraintCategory` // for performances reasons. The error reporting code used by NLL only // uses the span, so this doesn't cause any problems at the moment. Some(ObligationCauseCode::BindingObligation( CRATE_DEF_ID.to_def_id(), predicate_span, )) } else { None } }) .unwrap_or_else(|| ObligationCauseCode::MiscObligation); // Classify each of the constraints along the path. let mut categorized_path: Vec> = path .iter() .map(|constraint| BlameConstraint { category: constraint.category, from_closure: constraint.from_closure, cause: ObligationCause::new(constraint.span, CRATE_HIR_ID, cause_code.clone()), variance_info: constraint.variance_info, outlives_constraint: *constraint, }) .collect(); debug!("categorized_path={:#?}", categorized_path); // To find the best span to cite, we first try to look for the // final constraint that is interesting and where the `sup` is // not unified with the ultimate target region. The reason // for this is that we have a chain of constraints that lead // from the source to the target region, something like: // // '0: '1 ('0 is the source) // '1: '2 // '2: '3 // '3: '4 // '4: '5 // '5: '6 ('6 is the target) // // Some of those regions are unified with `'6` (in the same // SCC). We want to screen those out. After that point, the // "closest" constraint we have to the end is going to be the // most likely to be the point where the value escapes -- but // we still want to screen for an "interesting" point to // highlight (e.g., a call site or something). let target_scc = self.constraint_sccs.scc(target_region); let mut range = 0..path.len(); // As noted above, when reporting an error, there is typically a chain of constraints // leading from some "source" region which must outlive some "target" region. // In most cases, we prefer to "blame" the constraints closer to the target -- // but there is one exception. When constraints arise from higher-ranked subtyping, // we generally prefer to blame the source value, // as the "target" in this case tends to be some type annotation that the user gave. // Therefore, if we find that the region origin is some instantiation // of a higher-ranked region, we start our search from the "source" point // rather than the "target", and we also tweak a few other things. // // An example might be this bit of Rust code: // // ```rust // let x: fn(&'static ()) = |_| {}; // let y: for<'a> fn(&'a ()) = x; // ``` // // In MIR, this will be converted into a combination of assignments and type ascriptions. // In particular, the 'static is imposed through a type ascription: // // ```rust // x = ...; // AscribeUserType(x, fn(&'static ()) // y = x; // ``` // // We wind up ultimately with constraints like // // ```rust // !a: 'temp1 // from the `y = x` statement // 'temp1: 'temp2 // 'temp2: 'static // from the AscribeUserType // ``` // // and here we prefer to blame the source (the y = x statement). let blame_source = match from_region_origin { NllRegionVariableOrigin::FreeRegion | NllRegionVariableOrigin::Existential { from_forall: false } => true, NllRegionVariableOrigin::Placeholder(_) | NllRegionVariableOrigin::Existential { from_forall: true } => false, }; let find_region = |i: &usize| { let constraint = &path[*i]; let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup); if blame_source { match categorized_path[*i].category { ConstraintCategory::OpaqueType | ConstraintCategory::Boring | ConstraintCategory::BoringNoLocation | ConstraintCategory::Internal | ConstraintCategory::Predicate(_) => false, ConstraintCategory::TypeAnnotation | ConstraintCategory::Return(_) | ConstraintCategory::Yield => true, _ => constraint_sup_scc != target_scc, } } else { !matches!( categorized_path[*i].category, ConstraintCategory::OpaqueType | ConstraintCategory::Boring | ConstraintCategory::BoringNoLocation | ConstraintCategory::Internal | ConstraintCategory::Predicate(_) ) } }; let best_choice = if blame_source { range.rev().find(find_region) } else { range.find(find_region) }; debug!(?best_choice, ?blame_source, ?extra_info); if let Some(i) = best_choice { if let Some(next) = categorized_path.get(i + 1) { if matches!(categorized_path[i].category, ConstraintCategory::Return(_)) && next.category == ConstraintCategory::OpaqueType { // The return expression is being influenced by the return type being // impl Trait, point at the return type and not the return expr. return (next.clone(), extra_info); } } if categorized_path[i].category == ConstraintCategory::Return(ReturnConstraint::Normal) { let field = categorized_path.iter().find_map(|p| { if let ConstraintCategory::ClosureUpvar(f) = p.category { Some(f) } else { None } }); if let Some(field) = field { categorized_path[i].category = ConstraintCategory::Return(ReturnConstraint::ClosureUpvar(field)); } } return (categorized_path[i].clone(), extra_info); } // If that search fails, that is.. unusual. Maybe everything // is in the same SCC or something. In that case, find what // appears to be the most interesting point to report to the // user via an even more ad-hoc guess. categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category)); debug!("sorted_path={:#?}", categorized_path); (categorized_path.remove(0), extra_info) } pub(crate) fn universe_info(&self, universe: ty::UniverseIndex) -> UniverseInfo<'tcx> { self.universe_causes[&universe].clone() } /// Tries to find the terminator of the loop in which the region 'r' resides. /// Returns the location of the terminator if found. pub(crate) fn find_loop_terminator_location( &self, r: RegionVid, body: &Body<'_>, ) -> Option { let scc = self.constraint_sccs.scc(r.to_region_vid()); let locations = self.scc_values.locations_outlived_by(scc); for location in locations { let bb = &body[location.block]; if let Some(terminator) = &bb.terminator { // terminator of a loop should be TerminatorKind::FalseUnwind if let TerminatorKind::FalseUnwind { .. } = terminator.kind { return Some(location); } } } None } } impl<'tcx> RegionDefinition<'tcx> { fn new(universe: ty::UniverseIndex, rv_origin: RegionVariableOrigin) -> Self { // Create a new region definition. Note that, for free // regions, the `external_name` field gets updated later in // `init_universal_regions`. let origin = match rv_origin { RegionVariableOrigin::Nll(origin) => origin, _ => NllRegionVariableOrigin::Existential { from_forall: false }, }; Self { origin, universe, external_name: None } } } #[derive(Clone, Debug)] pub struct BlameConstraint<'tcx> { pub category: ConstraintCategory<'tcx>, pub from_closure: bool, pub cause: ObligationCause<'tcx>, pub variance_info: ty::VarianceDiagInfo<'tcx>, pub outlives_constraint: OutlivesConstraint<'tcx>, }