1 use crate::check::regionck::RegionCtxt;
3 use crate::hir::def_id::{DefId, LocalDefId};
4 use rustc_errors::{struct_span_err, ErrorGuaranteed};
5 use rustc_middle::ty::error::TypeError;
6 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
7 use rustc_middle::ty::subst::SubstsRef;
8 use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
10 use rustc_trait_selection::traits::query::dropck_outlives::AtExt;
11 use rustc_trait_selection::traits::ObligationCause;
13 /// This function confirms that the `Drop` implementation identified by
14 /// `drop_impl_did` is not any more specialized than the type it is
15 /// attached to (Issue #8142).
19 /// 1. The self type must be nominal (this is already checked during
22 /// 2. The generic region/type parameters of the impl's self type must
23 /// all be parameters of the Drop impl itself (i.e., no
24 /// specialization like `impl Drop for Foo<i32>`), and,
26 /// 3. Any bounds on the generic parameters must be reflected in the
27 /// struct/enum definition for the nominal type itself (i.e.
28 /// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
30 pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorGuaranteed> {
31 let dtor_self_type = tcx.type_of(drop_impl_did);
32 let dtor_predicates = tcx.predicates_of(drop_impl_did);
33 match dtor_self_type.kind() {
34 ty::Adt(adt_def, self_to_impl_substs) => {
35 ensure_drop_params_and_item_params_correspond(
37 drop_impl_did.expect_local(),
42 ensure_drop_predicates_are_implied_by_item_defn(
45 adt_def.did().expect_local(),
50 // Destructors only work on nominal types. This was
51 // already checked by coherence, but compilation may
52 // not have been terminated.
53 let span = tcx.def_span(drop_impl_did);
54 let reported = tcx.sess.delay_span_bug(
56 &format!("should have been rejected by coherence check: {dtor_self_type}"),
63 fn ensure_drop_params_and_item_params_correspond<'tcx>(
65 drop_impl_did: LocalDefId,
67 drop_impl_substs: SubstsRef<'tcx>,
68 ) -> Result<(), ErrorGuaranteed> {
69 let Err(arg) = tcx.uses_unique_generic_params(drop_impl_substs, false) else {
73 let drop_impl_span = tcx.def_span(drop_impl_did);
74 let item_span = tcx.def_span(self_type_did);
75 let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
77 struct_span_err!(tcx.sess, drop_impl_span, E0366, "`Drop` impls cannot be specialized");
79 ty::util::NotUniqueParam::DuplicateParam(arg) => {
80 err.note(&format!("`{arg}` is mentioned multiple times"))
82 ty::util::NotUniqueParam::NotParam(arg) => {
83 err.note(&format!("`{arg}` is not a generic parameter"))
89 "use the same sequence of generic lifetime, type and const parameters \
90 as the {self_descr} definition",
96 /// Confirms that every predicate imposed by dtor_predicates is
97 /// implied by assuming the predicates attached to self_type_did.
98 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
100 dtor_predicates: ty::GenericPredicates<'tcx>,
101 self_type_did: LocalDefId,
102 self_to_impl_substs: SubstsRef<'tcx>,
103 ) -> Result<(), ErrorGuaranteed> {
104 let mut result = Ok(());
106 // Here is an example, analogous to that from
107 // `compare_impl_method`.
109 // Consider a struct type:
111 // struct Type<'c, 'b:'c, 'a> {
112 // x: &'a Contents // (contents are irrelevant;
113 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
118 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
119 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
122 // We start out with self_to_impl_substs, that maps the generic
123 // parameters of Type to that of the Drop impl.
125 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
127 // Applying this to the predicates (i.e., assumptions) provided by the item
128 // definition yields the instantiated assumptions:
132 // We then check all of the predicates of the Drop impl:
136 // and ensure each is in the list of instantiated
137 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
138 // absent. So we report an error that the Drop impl injected a
139 // predicate that is not present on the struct definition.
141 // We can assume the predicates attached to struct/enum definition
143 let generic_assumptions = tcx.predicates_of(self_type_did);
145 let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
146 let assumptions_in_impl_context = assumptions_in_impl_context.predicates;
148 let self_param_env = tcx.param_env(self_type_did);
150 // An earlier version of this code attempted to do this checking
151 // via the traits::fulfill machinery. However, it ran into trouble
152 // since the fulfill machinery merely turns outlives-predicates
153 // 'a:'b and T:'b into region inference constraints. It is simpler
154 // just to look for all the predicates directly.
156 assert_eq!(dtor_predicates.parent, None);
157 for &(predicate, predicate_sp) in dtor_predicates.predicates {
158 // (We do not need to worry about deep analysis of type
159 // expressions etc because the Drop impls are already forced
160 // to take on a structure that is roughly an alpha-renaming of
161 // the generic parameters of the item definition.)
163 // This path now just checks *all* predicates via an instantiation of
164 // the `SimpleEqRelation`, which simply forwards to the `relate` machinery
165 // after taking care of anonymizing late bound regions.
167 // However, it may be more efficient in the future to batch
168 // the analysis together via the fulfill (see comment above regarding
169 // the usage of the fulfill machinery), rather than the
170 // repeated `.iter().any(..)` calls.
172 // This closure is a more robust way to check `Predicate` equality
173 // than simple `==` checks (which were the previous implementation).
174 // It relies on `ty::relate` for `TraitPredicate`, `ProjectionPredicate`,
175 // `ConstEvaluatable` and `TypeOutlives` (which implement the Relate trait),
176 // while delegating on simple equality for the other `Predicate`.
177 // This implementation solves (Issue #59497) and (Issue #58311).
178 // It is unclear to me at the moment whether the approach based on `relate`
179 // could be extended easily also to the other `Predicate`.
180 let predicate_matches_closure = |p: Predicate<'tcx>| {
181 let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
182 let predicate = predicate.kind();
184 match (predicate.skip_binder(), p.skip_binder()) {
185 (ty::PredicateKind::Trait(a), ty::PredicateKind::Trait(b)) => {
186 // Since struct predicates cannot have ~const, project the impl predicate
187 // onto one that ignores the constness. This is equivalent to saying that
188 // we match a `Trait` bound on the struct with a `Trait` or `~const Trait`
191 ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..a };
192 relator.relate(predicate.rebind(non_const_a), p.rebind(b)).is_ok()
194 (ty::PredicateKind::Projection(a), ty::PredicateKind::Projection(b)) => {
195 relator.relate(predicate.rebind(a), p.rebind(b)).is_ok()
198 ty::PredicateKind::ConstEvaluatable(a),
199 ty::PredicateKind::ConstEvaluatable(b),
200 ) => tcx.try_unify_abstract_consts(self_param_env.and((a, b))),
202 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_a, lt_a)),
203 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_b, lt_b)),
205 relator.relate(predicate.rebind(ty_a), p.rebind(ty_b)).is_ok()
206 && relator.relate(predicate.rebind(lt_a), p.rebind(lt_b)).is_ok()
212 if !assumptions_in_impl_context.iter().copied().any(predicate_matches_closure) {
213 let item_span = tcx.def_span(self_type_did);
214 let self_descr = tcx.def_kind(self_type_did).descr(self_type_did.to_def_id());
215 let reported = struct_span_err!(
219 "`Drop` impl requires `{predicate}` but the {self_descr} it is implemented for does not",
221 .span_note(item_span, "the implementor must specify the same requirement")
223 result = Err(reported);
230 /// This function is not only checking that the dropck obligations are met for
231 /// the given type, but it's also currently preventing non-regular recursion in
232 /// types from causing stack overflows (dropck_no_diverge_on_nonregular_*.rs).
233 crate fn check_drop_obligations<'a, 'tcx>(
234 rcx: &mut RegionCtxt<'a, 'tcx>,
239 debug!("check_drop_obligations typ: {:?}", ty);
241 let cause = &ObligationCause::misc(span, body_id);
242 let infer_ok = rcx.infcx.at(cause, rcx.fcx.param_env).dropck_outlives(ty);
243 debug!("dropck_outlives = {:#?}", infer_ok);
244 rcx.fcx.register_infer_ok_obligations(infer_ok);
247 // This is an implementation of the TypeRelation trait with the
248 // aim of simply comparing for equality (without side-effects).
249 // It is not intended to be used anywhere else other than here.
250 crate struct SimpleEqRelation<'tcx> {
252 param_env: ty::ParamEnv<'tcx>,
255 impl<'tcx> SimpleEqRelation<'tcx> {
256 fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
257 SimpleEqRelation { tcx, param_env }
261 impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
262 fn tcx(&self) -> TyCtxt<'tcx> {
266 fn param_env(&self) -> ty::ParamEnv<'tcx> {
270 fn tag(&self) -> &'static str {
271 "dropck::SimpleEqRelation"
274 fn a_is_expected(&self) -> bool {
278 fn relate_with_variance<T: Relate<'tcx>>(
281 _info: ty::VarianceDiagInfo<'tcx>,
284 ) -> RelateResult<'tcx, T> {
285 // Here we ignore variance because we require drop impl's types
286 // to be *exactly* the same as to the ones in the struct definition.
290 fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
291 debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
292 ty::relate::super_relate_tys(self, a, b)
299 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
300 debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);
302 // We can just equate the regions because LBRs have been
303 // already anonymized.
307 // I'm not sure is this `TypeError` is the right one, but
308 // it should not matter as it won't be checked (the dropck
309 // will emit its own, more informative and higher-level errors
310 // in case anything goes wrong).
311 Err(TypeError::RegionsPlaceholderMismatch)
319 ) -> RelateResult<'tcx, ty::Const<'tcx>> {
320 debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
321 ty::relate::super_relate_consts(self, a, b)
326 a: ty::Binder<'tcx, T>,
327 b: ty::Binder<'tcx, T>,
328 ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
332 debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);
334 // Anonymizing the LBRs is necessary to solve (Issue #59497).
335 // After we do so, it should be totally fine to skip the binders.
336 let anon_a = self.tcx.anonymize_late_bound_regions(a);
337 let anon_b = self.tcx.anonymize_late_bound_regions(b);
338 self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;