1 // FIXME(@lcnr): Move this module out of `rustc_typeck`.
3 // We don't do any drop checking during hir typeck.
4 use crate::hir::def_id::{DefId, LocalDefId};
5 use rustc_errors::{struct_span_err, ErrorGuaranteed};
6 use rustc_middle::ty::error::TypeError;
7 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
8 use rustc_middle::ty::subst::SubstsRef;
9 use rustc_middle::ty::util::IgnoreRegions;
10 use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
12 /// This function confirms that the `Drop` implementation identified by
13 /// `drop_impl_did` is not any more specialized than the type it is
14 /// attached to (Issue #8142).
18 /// 1. The self type must be nominal (this is already checked during
21 /// 2. The generic region/type parameters of the impl's self type must
22 /// all be parameters of the Drop impl itself (i.e., no
23 /// specialization like `impl Drop for Foo<i32>`), and,
25 /// 3. Any bounds on the generic parameters must be reflected in the
26 /// struct/enum definition for the nominal type itself (i.e.
27 /// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
29 pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorGuaranteed> {
30 let dtor_self_type = tcx.type_of(drop_impl_did);
31 let dtor_predicates = tcx.predicates_of(drop_impl_did);
32 match dtor_self_type.kind() {
33 ty::Adt(adt_def, self_to_impl_substs) => {
34 ensure_drop_params_and_item_params_correspond(
36 drop_impl_did.expect_local(),
41 ensure_drop_predicates_are_implied_by_item_defn(
44 adt_def.did().expect_local(),
49 // Destructors only work on nominal types. This was
50 // already checked by coherence, but compilation may
51 // not have been terminated.
52 let span = tcx.def_span(drop_impl_did);
53 let reported = tcx.sess.delay_span_bug(
55 &format!("should have been rejected by coherence check: {dtor_self_type}"),
62 fn ensure_drop_params_and_item_params_correspond<'tcx>(
64 drop_impl_did: LocalDefId,
66 drop_impl_substs: SubstsRef<'tcx>,
67 ) -> Result<(), ErrorGuaranteed> {
68 let Err(arg) = tcx.uses_unique_generic_params(drop_impl_substs, IgnoreRegions::No) else {
72 let drop_impl_span = tcx.def_span(drop_impl_did);
73 let item_span = tcx.def_span(self_type_did);
74 let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
76 struct_span_err!(tcx.sess, drop_impl_span, E0366, "`Drop` impls cannot be specialized");
78 ty::util::NotUniqueParam::DuplicateParam(arg) => {
79 err.note(&format!("`{arg}` is mentioned multiple times"))
81 ty::util::NotUniqueParam::NotParam(arg) => {
82 err.note(&format!("`{arg}` is not a generic parameter"))
88 "use the same sequence of generic lifetime, type and const parameters \
89 as the {self_descr} definition",
95 /// Confirms that every predicate imposed by dtor_predicates is
96 /// implied by assuming the predicates attached to self_type_did.
97 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
99 dtor_predicates: ty::GenericPredicates<'tcx>,
100 self_type_did: LocalDefId,
101 self_to_impl_substs: SubstsRef<'tcx>,
102 ) -> Result<(), ErrorGuaranteed> {
103 let mut result = Ok(());
105 // Here is an example, analogous to that from
106 // `compare_impl_method`.
108 // Consider a struct type:
110 // struct Type<'c, 'b:'c, 'a> {
111 // x: &'a Contents // (contents are irrelevant;
112 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
117 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
118 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
121 // We start out with self_to_impl_substs, that maps the generic
122 // parameters of Type to that of the Drop impl.
124 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
126 // Applying this to the predicates (i.e., assumptions) provided by the item
127 // definition yields the instantiated assumptions:
131 // We then check all of the predicates of the Drop impl:
135 // and ensure each is in the list of instantiated
136 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
137 // absent. So we report an error that the Drop impl injected a
138 // predicate that is not present on the struct definition.
140 // We can assume the predicates attached to struct/enum definition
142 let generic_assumptions = tcx.predicates_of(self_type_did);
144 let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
145 let assumptions_in_impl_context = assumptions_in_impl_context.predicates;
147 let self_param_env = tcx.param_env(self_type_did);
149 // An earlier version of this code attempted to do this checking
150 // via the traits::fulfill machinery. However, it ran into trouble
151 // since the fulfill machinery merely turns outlives-predicates
152 // 'a:'b and T:'b into region inference constraints. It is simpler
153 // just to look for all the predicates directly.
155 assert_eq!(dtor_predicates.parent, None);
156 for &(predicate, predicate_sp) in dtor_predicates.predicates {
157 // (We do not need to worry about deep analysis of type
158 // expressions etc because the Drop impls are already forced
159 // to take on a structure that is roughly an alpha-renaming of
160 // the generic parameters of the item definition.)
162 // This path now just checks *all* predicates via an instantiation of
163 // the `SimpleEqRelation`, which simply forwards to the `relate` machinery
164 // after taking care of anonymizing late bound regions.
166 // However, it may be more efficient in the future to batch
167 // the analysis together via the fulfill (see comment above regarding
168 // the usage of the fulfill machinery), rather than the
169 // repeated `.iter().any(..)` calls.
171 // This closure is a more robust way to check `Predicate` equality
172 // than simple `==` checks (which were the previous implementation).
173 // It relies on `ty::relate` for `TraitPredicate`, `ProjectionPredicate`,
174 // `ConstEvaluatable` and `TypeOutlives` (which implement the Relate trait),
175 // while delegating on simple equality for the other `Predicate`.
176 // This implementation solves (Issue #59497) and (Issue #58311).
177 // It is unclear to me at the moment whether the approach based on `relate`
178 // could be extended easily also to the other `Predicate`.
179 let predicate_matches_closure = |p: Predicate<'tcx>| {
180 let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
181 let predicate = predicate.kind();
183 match (predicate.skip_binder(), p.skip_binder()) {
184 (ty::PredicateKind::Trait(a), ty::PredicateKind::Trait(b)) => {
185 // Since struct predicates cannot have ~const, project the impl predicate
186 // onto one that ignores the constness. This is equivalent to saying that
187 // we match a `Trait` bound on the struct with a `Trait` or `~const Trait`
190 ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..a };
191 relator.relate(predicate.rebind(non_const_a), p.rebind(b)).is_ok()
193 (ty::PredicateKind::Projection(a), ty::PredicateKind::Projection(b)) => {
194 relator.relate(predicate.rebind(a), p.rebind(b)).is_ok()
197 ty::PredicateKind::ConstEvaluatable(a),
198 ty::PredicateKind::ConstEvaluatable(b),
199 ) => tcx.try_unify_abstract_consts(self_param_env.and((a, b))),
201 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_a, lt_a)),
202 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_b, lt_b)),
204 relator.relate(predicate.rebind(ty_a), p.rebind(ty_b)).is_ok()
205 && relator.relate(predicate.rebind(lt_a), p.rebind(lt_b)).is_ok()
207 (ty::PredicateKind::WellFormed(arg_a), ty::PredicateKind::WellFormed(arg_b)) => {
208 relator.relate(predicate.rebind(arg_a), p.rebind(arg_b)).is_ok()
214 if !assumptions_in_impl_context.iter().copied().any(predicate_matches_closure) {
215 let item_span = tcx.def_span(self_type_did);
216 let self_descr = tcx.def_kind(self_type_did).descr(self_type_did.to_def_id());
217 let reported = struct_span_err!(
221 "`Drop` impl requires `{predicate}` but the {self_descr} it is implemented for does not",
223 .span_note(item_span, "the implementor must specify the same requirement")
225 result = Err(reported);
232 // This is an implementation of the TypeRelation trait with the
233 // aim of simply comparing for equality (without side-effects).
234 // It is not intended to be used anywhere else other than here.
235 pub(crate) struct SimpleEqRelation<'tcx> {
237 param_env: ty::ParamEnv<'tcx>,
240 impl<'tcx> SimpleEqRelation<'tcx> {
241 fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
242 SimpleEqRelation { tcx, param_env }
246 impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
247 fn tcx(&self) -> TyCtxt<'tcx> {
251 fn param_env(&self) -> ty::ParamEnv<'tcx> {
255 fn tag(&self) -> &'static str {
256 "dropck::SimpleEqRelation"
259 fn a_is_expected(&self) -> bool {
263 fn relate_with_variance<T: Relate<'tcx>>(
266 _info: ty::VarianceDiagInfo<'tcx>,
269 ) -> RelateResult<'tcx, T> {
270 // Here we ignore variance because we require drop impl's types
271 // to be *exactly* the same as to the ones in the struct definition.
275 fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
276 debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
277 ty::relate::super_relate_tys(self, a, b)
284 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
285 debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);
287 // We can just equate the regions because LBRs have been
288 // already anonymized.
292 // I'm not sure is this `TypeError` is the right one, but
293 // it should not matter as it won't be checked (the dropck
294 // will emit its own, more informative and higher-level errors
295 // in case anything goes wrong).
296 Err(TypeError::RegionsPlaceholderMismatch)
304 ) -> RelateResult<'tcx, ty::Const<'tcx>> {
305 debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
306 ty::relate::super_relate_consts(self, a, b)
311 a: ty::Binder<'tcx, T>,
312 b: ty::Binder<'tcx, T>,
313 ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
317 debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);
319 // Anonymizing the LBRs is necessary to solve (Issue #59497).
320 // After we do so, it should be totally fine to skip the binders.
321 let anon_a = self.tcx.anonymize_bound_vars(a);
322 let anon_b = self.tcx.anonymize_bound_vars(b);
323 self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;