1 use crate::check::regionck::RegionCtxt;
3 use crate::hir::def_id::DefId;
4 use crate::util::common::ErrorReported;
5 use rustc::infer::outlives::env::OutlivesEnvironment;
6 use rustc::infer::{InferOk, SuppressRegionErrors};
7 use rustc::middle::region;
8 use rustc::traits::{ObligationCause, TraitEngine, TraitEngineExt};
9 use rustc::ty::error::TypeError;
10 use rustc::ty::relate::{Relate, RelateResult, TypeRelation};
11 use rustc::ty::subst::{Subst, SubstsRef};
12 use rustc::ty::{self, Predicate, Ty, TyCtxt};
13 use rustc_errors::struct_span_err;
17 use rustc_error_codes::*;
19 /// This function confirms that the `Drop` implementation identified by
20 /// `drop_impl_did` is not any more specialized than the type it is
21 /// attached to (Issue #8142).
25 /// 1. The self type must be nominal (this is already checked during
28 /// 2. The generic region/type parameters of the impl's self type must
29 /// all be parameters of the Drop impl itself (i.e., no
30 /// specialization like `impl Drop for Foo<i32>`), and,
32 /// 3. Any bounds on the generic parameters must be reflected in the
33 /// struct/enum definition for the nominal type itself (i.e.
34 /// cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
36 pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorReported> {
37 let dtor_self_type = tcx.type_of(drop_impl_did);
38 let dtor_predicates = tcx.predicates_of(drop_impl_did);
39 match dtor_self_type.kind {
40 ty::Adt(adt_def, self_to_impl_substs) => {
41 ensure_drop_params_and_item_params_correspond(
48 ensure_drop_predicates_are_implied_by_item_defn(
56 // Destructors only work on nominal types. This was
57 // already checked by coherence, but compilation may
58 // not have been terminated.
59 let span = tcx.def_span(drop_impl_did);
60 tcx.sess.delay_span_bug(
62 &format!("should have been rejected by coherence check: {}", dtor_self_type),
69 fn ensure_drop_params_and_item_params_correspond<'tcx>(
72 drop_impl_ty: Ty<'tcx>,
74 ) -> Result<(), ErrorReported> {
75 let drop_impl_hir_id = tcx.hir().as_local_hir_id(drop_impl_did).unwrap();
77 // check that the impl type can be made to match the trait type.
79 tcx.infer_ctxt().enter(|ref infcx| {
80 let impl_param_env = tcx.param_env(self_type_did);
82 let mut fulfillment_cx = TraitEngine::new(tcx);
84 let named_type = tcx.type_of(self_type_did);
86 let drop_impl_span = tcx.def_span(drop_impl_did);
87 let fresh_impl_substs = infcx.fresh_substs_for_item(drop_impl_span, drop_impl_did);
88 let fresh_impl_self_ty = drop_impl_ty.subst(tcx, fresh_impl_substs);
90 let cause = &ObligationCause::misc(drop_impl_span, drop_impl_hir_id);
91 match infcx.at(cause, impl_param_env).eq(named_type, fresh_impl_self_ty) {
92 Ok(InferOk { obligations, .. }) => {
93 fulfillment_cx.register_predicate_obligations(infcx, obligations);
96 let item_span = tcx.def_span(self_type_did);
98 .def_kind(self_type_did)
99 .map(|kind| kind.descr(self_type_did))
105 "`Drop` impls cannot be specialized"
110 "use the same sequence of generic type, lifetime and const parameters \
111 as the {} definition",
116 return Err(ErrorReported);
120 if let Err(ref errors) = fulfillment_cx.select_all_or_error(&infcx) {
121 // this could be reached when we get lazy normalization
122 infcx.report_fulfillment_errors(errors, None, false);
123 return Err(ErrorReported);
126 let region_scope_tree = region::ScopeTree::default();
128 // NB. It seems a bit... suspicious to use an empty param-env
129 // here. The correct thing, I imagine, would be
130 // `OutlivesEnvironment::new(impl_param_env)`, which would
131 // allow region solving to take any `a: 'b` relations on the
132 // impl into account. But I could not create a test case where
133 // it did the wrong thing, so I chose to preserve existing
134 // behavior, since it ought to be simply more
135 // conservative. -nmatsakis
136 let outlives_env = OutlivesEnvironment::new(ty::ParamEnv::empty());
138 infcx.resolve_regions_and_report_errors(
142 SuppressRegionErrors::default(),
148 /// Confirms that every predicate imposed by dtor_predicates is
149 /// implied by assuming the predicates attached to self_type_did.
150 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
152 dtor_predicates: ty::GenericPredicates<'tcx>,
153 self_type_did: DefId,
154 self_to_impl_substs: SubstsRef<'tcx>,
155 ) -> Result<(), ErrorReported> {
156 let mut result = Ok(());
158 // Here is an example, analogous to that from
159 // `compare_impl_method`.
161 // Consider a struct type:
163 // struct Type<'c, 'b:'c, 'a> {
164 // x: &'a Contents // (contents are irrelevant;
165 // y: &'c Cell<&'b Contents>, // only the bounds matter for our purposes.)
170 // impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
171 // fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
174 // We start out with self_to_impl_substs, that maps the generic
175 // parameters of Type to that of the Drop impl.
177 // self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
179 // Applying this to the predicates (i.e., assumptions) provided by the item
180 // definition yields the instantiated assumptions:
184 // We then check all of the predicates of the Drop impl:
188 // and ensure each is in the list of instantiated
189 // assumptions. Here, `'y:'z` is present, but `'x:'y` is
190 // absent. So we report an error that the Drop impl injected a
191 // predicate that is not present on the struct definition.
193 let self_type_hir_id = tcx.hir().as_local_hir_id(self_type_did).unwrap();
195 // We can assume the predicates attached to struct/enum definition
197 let generic_assumptions = tcx.predicates_of(self_type_did);
199 let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
200 let assumptions_in_impl_context = assumptions_in_impl_context.predicates;
202 let self_param_env = tcx.param_env(self_type_did);
204 // An earlier version of this code attempted to do this checking
205 // via the traits::fulfill machinery. However, it ran into trouble
206 // since the fulfill machinery merely turns outlives-predicates
207 // 'a:'b and T:'b into region inference constraints. It is simpler
208 // just to look for all the predicates directly.
210 assert_eq!(dtor_predicates.parent, None);
211 for (predicate, predicate_sp) in dtor_predicates.predicates {
212 // (We do not need to worry about deep analysis of type
213 // expressions etc because the Drop impls are already forced
214 // to take on a structure that is roughly an alpha-renaming of
215 // the generic parameters of the item definition.)
217 // This path now just checks *all* predicates via an instantiation of
218 // the `SimpleEqRelation`, which simply forwards to the `relate` machinery
219 // after taking care of anonymizing late bound regions.
221 // However, it may be more efficient in the future to batch
222 // the analysis together via the fulfill (see comment above regarding
223 // the usage of the fulfill machinery), rather than the
224 // repeated `.iter().any(..)` calls.
226 // This closure is a more robust way to check `Predicate` equality
227 // than simple `==` checks (which were the previous implementation).
228 // It relies on `ty::relate` for `TraitPredicate` and `ProjectionPredicate`
229 // (which implement the Relate trait), while delegating on simple equality
230 // for the other `Predicate`.
231 // This implementation solves (Issue #59497) and (Issue #58311).
232 // It is unclear to me at the moment whether the approach based on `relate`
233 // could be extended easily also to the other `Predicate`.
234 let predicate_matches_closure = |p: &'_ Predicate<'tcx>| {
235 let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
236 match (predicate, p) {
237 (Predicate::Trait(a), Predicate::Trait(b)) => relator.relate(a, b).is_ok(),
238 (Predicate::Projection(a), Predicate::Projection(b)) => {
239 relator.relate(a, b).is_ok()
245 if !assumptions_in_impl_context.iter().any(predicate_matches_closure) {
246 let item_span = tcx.hir().span(self_type_hir_id);
248 tcx.def_kind(self_type_did).map(|kind| kind.descr(self_type_did)).unwrap_or("type");
253 "`Drop` impl requires `{}` but the {} it is implemented for does not",
257 .span_note(item_span, "the implementor must specify the same requirement")
259 result = Err(ErrorReported);
266 /// This function is not only checking that the dropck obligations are met for
267 /// the given type, but it's also currently preventing non-regular recursion in
268 /// types from causing stack overflows (dropck_no_diverge_on_nonregular_*.rs).
269 crate fn check_drop_obligations<'a, 'tcx>(
270 rcx: &mut RegionCtxt<'a, 'tcx>,
274 ) -> Result<(), ErrorReported> {
275 debug!("check_drop_obligations typ: {:?}", ty);
277 let cause = &ObligationCause::misc(span, body_id);
278 let infer_ok = rcx.infcx.at(cause, rcx.fcx.param_env).dropck_outlives(ty);
279 debug!("dropck_outlives = {:#?}", infer_ok);
280 rcx.fcx.register_infer_ok_obligations(infer_ok);
285 // This is an implementation of the TypeRelation trait with the
286 // aim of simply comparing for equality (without side-effects).
287 // It is not intended to be used anywhere else other than here.
288 crate struct SimpleEqRelation<'tcx> {
290 param_env: ty::ParamEnv<'tcx>,
293 impl<'tcx> SimpleEqRelation<'tcx> {
294 fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
295 SimpleEqRelation { tcx, param_env }
299 impl TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
300 fn tcx(&self) -> TyCtxt<'tcx> {
304 fn param_env(&self) -> ty::ParamEnv<'tcx> {
308 fn tag(&self) -> &'static str {
309 "dropck::SimpleEqRelation"
312 fn a_is_expected(&self) -> bool {
316 fn relate_with_variance<T: Relate<'tcx>>(
321 ) -> RelateResult<'tcx, T> {
322 // Here we ignore variance because we require drop impl's types
323 // to be *exactly* the same as to the ones in the struct definition.
327 fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
328 debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
329 ty::relate::super_relate_tys(self, a, b)
336 ) -> RelateResult<'tcx, ty::Region<'tcx>> {
337 debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);
339 // We can just equate the regions because LBRs have been
340 // already anonymized.
344 // I'm not sure is this `TypeError` is the right one, but
345 // it should not matter as it won't be checked (the dropck
346 // will emit its own, more informative and higher-level errors
347 // in case anything goes wrong).
348 Err(TypeError::RegionsPlaceholderMismatch)
354 a: &'tcx ty::Const<'tcx>,
355 b: &'tcx ty::Const<'tcx>,
356 ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
357 debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
358 ty::relate::super_relate_consts(self, a, b)
365 ) -> RelateResult<'tcx, ty::Binder<T>>
369 debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);
371 // Anonymizing the LBRs is necessary to solve (Issue #59497).
372 // After we do so, it should be totally fine to skip the binders.
373 let anon_a = self.tcx.anonymize_late_bound_regions(a);
374 let anon_b = self.tcx.anonymize_late_bound_regions(b);
375 self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;