Google Git
Sign in
android / toolchain / rustc / 89a0a0cd9cbd0a0138a09bd877bbc73859a8c330 / . / compiler / rustc_borrowck / src / region_infer / mod.rs
blob: 8b63294fbab0ee3af1abd29c58269aa2546352e5 [file] [log] [blame]
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::{DefId, CRATE_DEF_ID};
use rustc_hir::CRATE_HIR_ID;
use rustc_index::vec::IndexVec;
use rustc_infer::infer::canonical::QueryOutlivesConstraint;
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, subst::SubstsRef, 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<RegionVid, RegionDefinition<'tcx>>,
/// 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<RegionVid>,
/// The outlives constraints computed by the type-check.
constraints: Frozen<OutlivesConstraintSet<'tcx>>,
/// 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<NormalConstraintGraph>,
/// 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<Sccs<RegionVid, ConstraintSccIndex>>,
/// 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<Rc<ReverseSccGraph>>,
/// The "R0 member of [R1..Rn]" constraints, indexed by SCC.
member_constraints: Rc<MemberConstraintSet<'tcx, ConstraintSccIndex>>,
/// 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<AppliedMemberConstraint>,
/// Map closure bounds to a `Span` that should be used for error reporting.
closure_bounds_mapping:
FxHashMap<Location, FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>>,
/// Map universe indexes to information on why we created it.
universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
/// 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<ConstraintSccIndex, ty::UniverseIndex>,
/// 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<ConstraintSccIndex, ty::RegionVid>,
/// 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<ConstraintSccIndex>,
/// Type constraints that we check after solving.
type_tests: Vec<TypeTest<'tcx>>,
/// Information about the universally quantified regions in scope
/// on this function.
universal_regions: Rc<UniversalRegions<'tcx>>,
/// Information about how the universally quantified regions in
/// scope on this function relate to one another.
universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
}
/// 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.
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<ty::Region<'tcx>>,
}
/// 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 `<T as Foo>::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 `<Foo as Trait<'?0>>::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,
/// Where did this constraint arise and why?
pub locations: Locations,
/// 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<UniversalRegions<'tcx>>,
placeholder_indices: Rc<PlaceholderIndices>,
universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
outlives_constraints: OutlivesConstraintSet<'tcx>,
member_constraints_in: MemberConstraintSet<'tcx, RegionVid>,
closure_bounds_mapping: FxHashMap<
Location,
FxHashMap<(RegionVid, RegionVid), (ConstraintCategory<'tcx>, Span)>,
>,
universe_causes: FxHashMap<ty::UniverseIndex, UniverseInfo<'tcx>>,
type_tests: Vec<TypeTest<'tcx>>,
liveness_constraints: LivenessValues<RegionVid>,
elements: &Rc<RegionValueElements>,
) -> 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(),
closure_bounds_mapping,
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<RegionVid, ConstraintSccIndex>,
definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
) -> IndexVec<ConstraintSccIndex, ty::UniverseIndex> {
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<RegionVid, ConstraintSccIndex>,
definitions: &IndexVec<RegionVid, RegionDefinition<'tcx>>,
) -> IndexVec<ConstraintSccIndex, ty::RegionVid> {
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<Item = RegionVid> + '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)
}
/// 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<Rc<PoloniusOutput>>,
) -> (Option<ClosureRegionRequirements<'tcx>>, RegionErrors<'tcx>) {
let mir_def_id = body.source.def_id();
self.propagate_constraints(body);
let mut errors_buffer = RegionErrors::new();
// 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::<Vec<_>>()
});
// 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<ty::RegionVid> = 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 min = |r1: ty::RegionVid, r2: ty::RegionVid| -> Option<ty::RegionVid> {
let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
match (r1_outlives_r2, r2_outlives_r1) {
(true, true) => Some(r1.min(r2)),
(true, false) => Some(r2),
(false, true) => Some(r1),
(false, false) => None,
}
};
let mut min_choice = choice_regions[0];
for &other_option in &choice_regions[1..] {
debug!(?min_choice, ?other_option,);
match min(min_choice, other_option) {
Some(m) => min_choice = m,
None => {
debug!(?min_choice, ?other_option, "incomparable; no min choice",);
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<ClosureOutlivesRequirement<'tcx>>>,
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,
body,
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.locations,
)) {
debug!(
"check_type_test: reporting error for erased_generic_kind={:?}, \
lower_bound_region={:?}, \
type_test.locations={:?}",
erased_generic_kind, type_test.lower_bound, type_test.locations,
);
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<ClosureOutlivesRequirement<'tcx>>,
) -> bool {
let tcx = infcx.tcx;
let TypeTest { generic_kind, lower_bound, locations, 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: locations.span(body),
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,
body,
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: locations.span(body),
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<ClosureOutlivesSubject<'tcx>> {
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>,
body: &Body<'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,
body,
generic_ty,
lower_bound,
verify_bound,
)
}),
VerifyBound::AllBounds(verify_bounds) => verify_bounds.iter().all(|verify_bound| {
self.eval_verify_bound(
infcx,
param_env,
body,
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
/// <T as Foo<'0>>::Item: '1
/// ```
///
/// In particular, this routine winds up being important when
/// there are bounds like `where <T as Foo<'a>>::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<T>(&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<ClosureOutlivesRequirement<'tcx>>>,
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:
/// <https://smallcultfollowing.com/babysteps/blog/2019/01/17/polonius-and-region-errors/>
///
/// 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<ClosureOutlivesRequirement<'tcx>>>,
errors_buffer: &mut RegionErrors<'tcx>,
polonius_output: Rc<PoloniusOutput>,
) {
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<ClosureOutlivesRequirement<'tcx>>>,
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<ClosureOutlivesRequirement<'tcx>>>,
) -> 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<ClosureOutlivesRequirement<'tcx>>>,
) -> 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,);
// 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!
let Some(error_element) = ({
self.scc_values.elements_contained_in(longer_fr_scc).find(|element| match element {
RegionElement::Location(_) => true,
RegionElement::RootUniversalRegion(_) => true,
RegionElement::PlaceholderRegion(placeholder1) => placeholder != *placeholder1,
})
}) else {
return;
};
debug!("check_bound_universal_region: error_element = {:?}", error_element);
// Find the region that introduced this `error_element`.
errors_buffer.push(RegionErrorKind::BoundUniversalRegionError {
longer_fr,
error_element,
placeholder,
});
}
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!("check_member_constraint(m_c_i={:?})", m_c_i);
let m_c = &member_constraints[m_c_i];
let member_region_vid = m_c.member_region_vid;
debug!(
"check_member_constraint: member_region_vid={:?} with value {}",
member_region_vid,
self.region_value_str(member_region_vid),
);
let choice_regions = member_constraints.choice_regions(m_c_i);
debug!("check_member_constraint: choice_regions={:?}", 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!("check_member_constraint: 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
}
}
}
pub(crate) fn retrieve_closure_constraint_info(
&self,
constraint: OutlivesConstraint<'tcx>,
) -> Option<(ConstraintCategory<'tcx>, Span)> {
match constraint.locations {
Locations::All(_) => None,
Locations::Single(loc) => {
self.closure_bounds_mapping[&loc].get(&(constraint.sup, constraint.sub)).copied()
}
}
}
/// 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<OutlivesConstraint<'tcx>>, 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(),
};
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<ExtraConstraintInfo>) {
// 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::<Vec<_>>()
);
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<BlameConstraint<'tcx>> = path
.iter()
.map(|constraint| {
let (category, span, from_closure, cause_code) =
if constraint.category == ConstraintCategory::ClosureBounds {
if let Some((category, span)) =
self.retrieve_closure_constraint_info(*constraint)
{
(category, span, true, ObligationCauseCode::MiscObligation)
} else {
(
constraint.category,
constraint.span,
false,
ObligationCauseCode::MiscObligation,
)
}
} else {
(constraint.category, constraint.span, false, cause_code.clone())
};
BlameConstraint {
category,
from_closure,
cause: ObligationCause::new(span, CRATE_HIR_ID, cause_code),
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<Location> {
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 }
}
}
pub trait ClosureRegionRequirementsExt<'tcx> {
fn apply_requirements(
&self,
tcx: TyCtxt<'tcx>,
closure_def_id: DefId,
closure_substs: SubstsRef<'tcx>,
) -> Vec<QueryOutlivesConstraint<'tcx>>;
}
impl<'tcx> ClosureRegionRequirementsExt<'tcx> for ClosureRegionRequirements<'tcx> {
/// Given an instance T of the closure type, this method
/// instantiates the "extra" requirements that we computed for the
/// closure into the inference context. This has the effect of
/// adding new outlives obligations to existing variables.
///
/// As described on `ClosureRegionRequirements`, the extra
/// requirements are expressed in terms of regionvids that index
/// into the free regions that appear on the closure type. So, to
/// do this, we first copy those regions out from the type T into
/// a vector. Then we can just index into that vector to extract
/// out the corresponding region from T and apply the
/// requirements.
fn apply_requirements(
&self,
tcx: TyCtxt<'tcx>,
closure_def_id: DefId,
closure_substs: SubstsRef<'tcx>,
) -> Vec<QueryOutlivesConstraint<'tcx>> {
debug!(
"apply_requirements(closure_def_id={:?}, closure_substs={:?})",
closure_def_id, closure_substs
);
// Extract the values of the free regions in `closure_substs`
// into a vector. These are the regions that we will be
// relating to one another.
let closure_mapping = &UniversalRegions::closure_mapping(
tcx,
closure_substs,
self.num_external_vids,
tcx.typeck_root_def_id(closure_def_id),
);
debug!("apply_requirements: closure_mapping={:?}", closure_mapping);
// Create the predicates.
self.outlives_requirements
.iter()
.map(|outlives_requirement| {
let outlived_region = closure_mapping[outlives_requirement.outlived_free_region];
match outlives_requirement.subject {
ClosureOutlivesSubject::Region(region) => {
let region = closure_mapping[region];
debug!(
"apply_requirements: region={:?} \
outlived_region={:?} \
outlives_requirement={:?}",
region, outlived_region, outlives_requirement,
);
(
ty::Binder::dummy(ty::OutlivesPredicate(
region.into(),
outlived_region,
)),
ConstraintCategory::BoringNoLocation,
)
}
ClosureOutlivesSubject::Ty(ty) => {
debug!(
"apply_requirements: ty={:?} \
outlived_region={:?} \
outlives_requirement={:?}",
ty, outlived_region, outlives_requirement,
);
(
ty::Binder::dummy(ty::OutlivesPredicate(ty.into(), outlived_region)),
ConstraintCategory::BoringNoLocation,
)
}
}
})
.collect()
}
}
#[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>,
}