340 lines
13 KiB
Rust
340 lines
13 KiB
Rust
// FIXME(@lcnr): Move this module out of `rustc_hir_analysis`.
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//
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// We don't do any drop checking during hir typeck.
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use crate::hir::def_id::{DefId, LocalDefId};
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use rustc_errors::{struct_span_err, ErrorGuaranteed};
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use rustc_middle::ty::error::TypeError;
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use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
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use rustc_middle::ty::subst::SubstsRef;
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use rustc_middle::ty::util::IgnoreRegions;
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use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
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/// This function confirms that the `Drop` implementation identified by
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/// `drop_impl_did` is not any more specialized than the type it is
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/// attached to (Issue #8142).
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///
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/// This means:
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///
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/// 1. The self type must be nominal (this is already checked during
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/// coherence),
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///
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/// 2. The generic region/type parameters of the impl's self type must
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/// all be parameters of the Drop impl itself (i.e., no
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/// specialization like `impl Drop for Foo<i32>`), and,
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///
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/// 3. Any bounds on the generic parameters must be reflected in the
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/// struct/enum definition for the nominal type itself (i.e.
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/// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
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///
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pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorGuaranteed> {
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let dtor_self_type = tcx.bound_type_of(drop_impl_did).subst_identity();
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let dtor_predicates = tcx.predicates_of(drop_impl_did);
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match dtor_self_type.kind() {
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ty::Adt(adt_def, self_to_impl_substs) => {
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ensure_drop_params_and_item_params_correspond(
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tcx,
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drop_impl_did.expect_local(),
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adt_def.did(),
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self_to_impl_substs,
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)?;
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ensure_drop_predicates_are_implied_by_item_defn(
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tcx,
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dtor_predicates,
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adt_def.did().expect_local(),
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self_to_impl_substs,
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)
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}
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_ => {
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// Destructors only work on nominal types. This was
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// already checked by coherence, but compilation may
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// not have been terminated.
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let span = tcx.def_span(drop_impl_did);
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let reported = tcx.sess.delay_span_bug(
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span,
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&format!("should have been rejected by coherence check: {dtor_self_type}"),
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);
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Err(reported)
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}
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}
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}
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fn ensure_drop_params_and_item_params_correspond<'tcx>(
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tcx: TyCtxt<'tcx>,
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drop_impl_did: LocalDefId,
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self_type_did: DefId,
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drop_impl_substs: SubstsRef<'tcx>,
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) -> Result<(), ErrorGuaranteed> {
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let Err(arg) = tcx.uses_unique_generic_params(drop_impl_substs, IgnoreRegions::No) else {
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return Ok(())
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};
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let drop_impl_span = tcx.def_span(drop_impl_did);
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let item_span = tcx.def_span(self_type_did);
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let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
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let mut err =
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struct_span_err!(tcx.sess, drop_impl_span, E0366, "`Drop` impls cannot be specialized");
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match arg {
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ty::util::NotUniqueParam::DuplicateParam(arg) => {
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err.note(&format!("`{arg}` is mentioned multiple times"))
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}
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ty::util::NotUniqueParam::NotParam(arg) => {
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err.note(&format!("`{arg}` is not a generic parameter"))
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}
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};
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err.span_note(
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item_span,
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&format!(
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"use the same sequence of generic lifetime, type and const parameters \
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as the {self_descr} definition",
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),
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);
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Err(err.emit())
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}
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/// Confirms that every predicate imposed by dtor_predicates is
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/// implied by assuming the predicates attached to self_type_did.
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fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
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tcx: TyCtxt<'tcx>,
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dtor_predicates: ty::GenericPredicates<'tcx>,
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self_type_did: LocalDefId,
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self_to_impl_substs: SubstsRef<'tcx>,
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) -> Result<(), ErrorGuaranteed> {
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let mut result = Ok(());
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// Here is an example, analogous to that from
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// `compare_impl_method`.
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//
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// Consider a struct type:
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//
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// struct Type<'c, 'b:'c, 'a> {
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// x: &'a Contents // (contents are irrelevant;
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// y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
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// }
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//
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// and a Drop impl:
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//
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// impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
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// fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
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// }
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//
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// We start out with self_to_impl_substs, that maps the generic
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// parameters of Type to that of the Drop impl.
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//
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// self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
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//
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// Applying this to the predicates (i.e., assumptions) provided by the item
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// definition yields the instantiated assumptions:
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//
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// ['y : 'z]
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//
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// We then check all of the predicates of the Drop impl:
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//
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// ['y:'z, 'x:'y]
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//
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// and ensure each is in the list of instantiated
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// assumptions. Here, `'y:'z` is present, but `'x:'y` is
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// absent. So we report an error that the Drop impl injected a
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// predicate that is not present on the struct definition.
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// We can assume the predicates attached to struct/enum definition
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// hold.
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let generic_assumptions = tcx.predicates_of(self_type_did);
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let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
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let assumptions_in_impl_context = assumptions_in_impl_context.predicates;
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debug!(?assumptions_in_impl_context, ?dtor_predicates.predicates);
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let self_param_env = tcx.param_env(self_type_did);
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// An earlier version of this code attempted to do this checking
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// via the traits::fulfill machinery. However, it ran into trouble
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// since the fulfill machinery merely turns outlives-predicates
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// 'a:'b and T:'b into region inference constraints. It is simpler
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// just to look for all the predicates directly.
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assert_eq!(dtor_predicates.parent, None);
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for &(predicate, predicate_sp) in dtor_predicates.predicates {
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// (We do not need to worry about deep analysis of type
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// expressions etc because the Drop impls are already forced
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// to take on a structure that is roughly an alpha-renaming of
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// the generic parameters of the item definition.)
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// This path now just checks *all* predicates via an instantiation of
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// the `SimpleEqRelation`, which simply forwards to the `relate` machinery
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// after taking care of anonymizing late bound regions.
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//
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// However, it may be more efficient in the future to batch
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// the analysis together via the fulfill (see comment above regarding
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// the usage of the fulfill machinery), rather than the
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// repeated `.iter().any(..)` calls.
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// This closure is a more robust way to check `Predicate` equality
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// than simple `==` checks (which were the previous implementation).
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// It relies on `ty::relate` for `TraitPredicate`, `ProjectionPredicate`,
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// `ConstEvaluatable` and `TypeOutlives` (which implement the Relate trait),
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// while delegating on simple equality for the other `Predicate`.
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// This implementation solves (Issue #59497) and (Issue #58311).
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// It is unclear to me at the moment whether the approach based on `relate`
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// could be extended easily also to the other `Predicate`.
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let predicate_matches_closure = |p: Predicate<'tcx>| {
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let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
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let predicate = predicate.kind();
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let p = p.kind();
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match (predicate.skip_binder(), p.skip_binder()) {
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(
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ty::PredicateKind::Clause(ty::Clause::Trait(a)),
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ty::PredicateKind::Clause(ty::Clause::Trait(b)),
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) => relator.relate(predicate.rebind(a), p.rebind(b)).is_ok(),
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(
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ty::PredicateKind::Clause(ty::Clause::Projection(a)),
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ty::PredicateKind::Clause(ty::Clause::Projection(b)),
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) => relator.relate(predicate.rebind(a), p.rebind(b)).is_ok(),
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(
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ty::PredicateKind::ConstEvaluatable(a),
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ty::PredicateKind::ConstEvaluatable(b),
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) => relator.relate(predicate.rebind(a), predicate.rebind(b)).is_ok(),
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(
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ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
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ty_a,
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lt_a,
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))),
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ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
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ty_b,
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lt_b,
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))),
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) => {
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relator.relate(predicate.rebind(ty_a), p.rebind(ty_b)).is_ok()
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&& relator.relate(predicate.rebind(lt_a), p.rebind(lt_b)).is_ok()
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}
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(ty::PredicateKind::WellFormed(arg_a), ty::PredicateKind::WellFormed(arg_b)) => {
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relator.relate(predicate.rebind(arg_a), p.rebind(arg_b)).is_ok()
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}
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_ => predicate == p,
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}
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};
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if !assumptions_in_impl_context.iter().copied().any(predicate_matches_closure) {
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let item_span = tcx.def_span(self_type_did);
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let self_descr = tcx.def_kind(self_type_did).descr(self_type_did.to_def_id());
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let reported = struct_span_err!(
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tcx.sess,
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predicate_sp,
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E0367,
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"`Drop` impl requires `{predicate}` but the {self_descr} it is implemented for does not",
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)
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.span_note(item_span, "the implementor must specify the same requirement")
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.emit();
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result = Err(reported);
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}
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}
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result
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}
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/// This is an implementation of the [`TypeRelation`] trait with the
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/// aim of simply comparing for equality (without side-effects).
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///
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/// It is not intended to be used anywhere else other than here.
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pub(crate) struct SimpleEqRelation<'tcx> {
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tcx: TyCtxt<'tcx>,
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param_env: ty::ParamEnv<'tcx>,
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}
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impl<'tcx> SimpleEqRelation<'tcx> {
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fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
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SimpleEqRelation { tcx, param_env }
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}
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}
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impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
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fn tcx(&self) -> TyCtxt<'tcx> {
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self.tcx
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}
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fn intercrate(&self) -> bool {
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false
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}
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fn param_env(&self) -> ty::ParamEnv<'tcx> {
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self.param_env
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}
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fn tag(&self) -> &'static str {
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"dropck::SimpleEqRelation"
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}
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fn a_is_expected(&self) -> bool {
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true
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}
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fn mark_ambiguous(&mut self) {
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bug!()
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}
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fn relate_with_variance<T: Relate<'tcx>>(
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&mut self,
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_: ty::Variance,
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_info: ty::VarianceDiagInfo<'tcx>,
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a: T,
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b: T,
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) -> RelateResult<'tcx, T> {
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// Here we ignore variance because we require drop impl's types
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// to be *exactly* the same as to the ones in the struct definition.
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self.relate(a, b)
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}
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fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
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debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
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ty::relate::super_relate_tys(self, a, b)
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}
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fn regions(
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&mut self,
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a: ty::Region<'tcx>,
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b: ty::Region<'tcx>,
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) -> RelateResult<'tcx, ty::Region<'tcx>> {
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debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);
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// We can just equate the regions because LBRs have been
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// already anonymized.
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if a == b {
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Ok(a)
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} else {
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// I'm not sure is this `TypeError` is the right one, but
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// it should not matter as it won't be checked (the dropck
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// will emit its own, more informative and higher-level errors
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// in case anything goes wrong).
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Err(TypeError::RegionsPlaceholderMismatch)
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}
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}
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fn consts(
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&mut self,
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a: ty::Const<'tcx>,
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b: ty::Const<'tcx>,
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) -> RelateResult<'tcx, ty::Const<'tcx>> {
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debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
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ty::relate::super_relate_consts(self, a, b)
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}
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fn binders<T>(
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&mut self,
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a: ty::Binder<'tcx, T>,
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b: ty::Binder<'tcx, T>,
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) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
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where
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T: Relate<'tcx>,
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{
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debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);
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// Anonymizing the LBRs is necessary to solve (Issue #59497).
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// After we do so, it should be totally fine to skip the binders.
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let anon_a = self.tcx.anonymize_bound_vars(a);
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let anon_b = self.tcx.anonymize_bound_vars(b);
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self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;
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Ok(a)
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}
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}
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