src/librustc_typeck/check/dropck.rs

   1 use crate::check::regionck::RegionCtxt;
   2 use crate::hir;
   3 use crate::hir::def_id::{DefId, LocalDefId};
   4 use rustc_errors::{struct_span_err, ErrorReported};
   5 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
   6 use rustc_infer::infer::{InferOk, RegionckMode, TyCtxtInferExt};
   7 use rustc_infer::traits::TraitEngineExt as _;
   8 use rustc_middle::ty::error::TypeError;
   9 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
  10 use rustc_middle::ty::subst::{Subst, SubstsRef};
  11 use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
  12 use rustc_span::Span;
  13 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  14 use rustc_trait_selection::traits::query::dropck_outlives::AtExt;
  15 use rustc_trait_selection::traits::{ObligationCause, TraitEngine, TraitEngineExt};
  16
  17 /// This function confirms that the `Drop` implementation identified by
  18 /// `drop_impl_did` is not any more specialized than the type it is
  19 /// attached to (Issue #8142).
  20 ///
  21 /// This means:
  22 ///
  23 /// 1. The self type must be nominal (this is already checked during
  24 ///    coherence),
  25 ///
  26 /// 2. The generic region/type parameters of the impl's self type must
  27 ///    all be parameters of the Drop impl itself (i.e., no
  28 ///    specialization like `impl Drop for Foo<i32>`), and,
  29 ///
  30 /// 3. Any bounds on the generic parameters must be reflected in the
  31 ///    struct/enum definition for the nominal type itself (i.e.
  32 ///    cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
  33 ///
  34 pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorReported> {
  35     let dtor_self_type = tcx.type_of(drop_impl_did);
  36     let dtor_predicates = tcx.predicates_of(drop_impl_did);
  37     match dtor_self_type.kind {
  38         ty::Adt(adt_def, self_to_impl_substs) => {
  39             ensure_drop_params_and_item_params_correspond(
  40                 tcx,
  41                 drop_impl_did.expect_local(),
  42                 dtor_self_type,
  43                 adt_def.did,
  44             )?;
  45
  46             ensure_drop_predicates_are_implied_by_item_defn(
  47                 tcx,
  48                 dtor_predicates,
  49                 adt_def.did.expect_local(),
  50                 self_to_impl_substs,
  51             )
  52         }
  53         _ => {
  54             // Destructors only work on nominal types.  This was
  55             // already checked by coherence, but compilation may
  56             // not have been terminated.
  57             let span = tcx.def_span(drop_impl_did);
  58             tcx.sess.delay_span_bug(
  59                 span,
  60                 &format!("should have been rejected by coherence check: {}", dtor_self_type),
  61             );
  62             Err(ErrorReported)
  63         }
  64     }
  65 }
  66
  67 fn ensure_drop_params_and_item_params_correspond<'tcx>(
  68     tcx: TyCtxt<'tcx>,
  69     drop_impl_did: LocalDefId,
  70     drop_impl_ty: Ty<'tcx>,
  71     self_type_did: DefId,
  72 ) -> Result<(), ErrorReported> {
  73     let drop_impl_hir_id = tcx.hir().as_local_hir_id(drop_impl_did);
  74
  75     // check that the impl type can be made to match the trait type.
  76
  77     tcx.infer_ctxt().enter(|ref infcx| {
  78         let impl_param_env = tcx.param_env(self_type_did);
  79         let tcx = infcx.tcx;
  80         let mut fulfillment_cx = TraitEngine::new(tcx);
  81
  82         let named_type = tcx.type_of(self_type_did);
  83
  84         let drop_impl_span = tcx.def_span(drop_impl_did);
  85         let fresh_impl_substs =
  86             infcx.fresh_substs_for_item(drop_impl_span, drop_impl_did.to_def_id());
  87         let fresh_impl_self_ty = drop_impl_ty.subst(tcx, fresh_impl_substs);
  88
  89         let cause = &ObligationCause::misc(drop_impl_span, drop_impl_hir_id);
  90         match infcx.at(cause, impl_param_env).eq(named_type, fresh_impl_self_ty) {
  91             Ok(InferOk { obligations, .. }) => {
  92                 fulfillment_cx.register_predicate_obligations(infcx, obligations);
  93             }
  94             Err(_) => {
  95                 let item_span = tcx.def_span(self_type_did);
  96                 let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
  97                 struct_span_err!(
  98                     tcx.sess,
  99                     drop_impl_span,
 100                     E0366,
 101                     "`Drop` impls cannot be specialized"
 102                 )
 103                 .span_note(
 104                     item_span,
 105                     &format!(
 106                         "use the same sequence of generic type, lifetime and const parameters \
 107                         as the {} definition",
 108                         self_descr,
 109                     ),
 110                 )
 111                 .emit();
 112                 return Err(ErrorReported);
 113             }
 114         }
 115
 116         if let Err(ref errors) = fulfillment_cx.select_all_or_error(&infcx) {
 117             // this could be reached when we get lazy normalization
 118             infcx.report_fulfillment_errors(errors, None, false);
 119             return Err(ErrorReported);
 120         }
 121
 122         // NB. It seems a bit... suspicious to use an empty param-env
 123         // here. The correct thing, I imagine, would be
 124         // `OutlivesEnvironment::new(impl_param_env)`, which would
 125         // allow region solving to take any `a: 'b` relations on the
 126         // impl into account. But I could not create a test case where
 127         // it did the wrong thing, so I chose to preserve existing
 128         // behavior, since it ought to be simply more
 129         // conservative. -nmatsakis
 130         let outlives_env = OutlivesEnvironment::new(ty::ParamEnv::empty());
 131
 132         infcx.resolve_regions_and_report_errors(
 133             drop_impl_did.to_def_id(),
 134             &outlives_env,
 135             RegionckMode::default(),
 136         );
 137         Ok(())
 138     })
 139 }
 140
 141 /// Confirms that every predicate imposed by dtor_predicates is
 142 /// implied by assuming the predicates attached to self_type_did.
 143 fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
 144     tcx: TyCtxt<'tcx>,
 145     dtor_predicates: ty::GenericPredicates<'tcx>,
 146     self_type_did: LocalDefId,
 147     self_to_impl_substs: SubstsRef<'tcx>,
 148 ) -> Result<(), ErrorReported> {
 149     let mut result = Ok(());
 150
 151     // Here is an example, analogous to that from
 152     // `compare_impl_method`.
 153     //
 154     // Consider a struct type:
 155     //
 156     //     struct Type<'c, 'b:'c, 'a> {
 157     //         x: &'a Contents            // (contents are irrelevant;
 158     //         y: &'c Cell<&'b Contents>, //  only the bounds matter for our purposes.)
 159     //     }
 160     //
 161     // and a Drop impl:
 162     //
 163     //     impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
 164     //         fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
 165     //     }
 166     //
 167     // We start out with self_to_impl_substs, that maps the generic
 168     // parameters of Type to that of the Drop impl.
 169     //
 170     //     self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
 171     //
 172     // Applying this to the predicates (i.e., assumptions) provided by the item
 173     // definition yields the instantiated assumptions:
 174     //
 175     //     ['y : 'z]
 176     //
 177     // We then check all of the predicates of the Drop impl:
 178     //
 179     //     ['y:'z, 'x:'y]
 180     //
 181     // and ensure each is in the list of instantiated
 182     // assumptions. Here, `'y:'z` is present, but `'x:'y` is
 183     // absent. So we report an error that the Drop impl injected a
 184     // predicate that is not present on the struct definition.
 185
 186     let self_type_hir_id = tcx.hir().as_local_hir_id(self_type_did);
 187
 188     // We can assume the predicates attached to struct/enum definition
 189     // hold.
 190     let generic_assumptions = tcx.predicates_of(self_type_did);
 191
 192     let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
 193     let assumptions_in_impl_context = assumptions_in_impl_context.predicates;
 194
 195     let self_param_env = tcx.param_env(self_type_did);
 196
 197     // An earlier version of this code attempted to do this checking
 198     // via the traits::fulfill machinery. However, it ran into trouble
 199     // since the fulfill machinery merely turns outlives-predicates
 200     // 'a:'b and T:'b into region inference constraints. It is simpler
 201     // just to look for all the predicates directly.
 202
 203     assert_eq!(dtor_predicates.parent, None);
 204     for &(predicate, predicate_sp) in dtor_predicates.predicates {
 205         // (We do not need to worry about deep analysis of type
 206         // expressions etc because the Drop impls are already forced
 207         // to take on a structure that is roughly an alpha-renaming of
 208         // the generic parameters of the item definition.)
 209
 210         // This path now just checks *all* predicates via an instantiation of
 211         // the `SimpleEqRelation`, which simply forwards to the `relate` machinery
 212         // after taking care of anonymizing late bound regions.
 213         //
 214         // However, it may be more efficient in the future to batch
 215         // the analysis together via the fulfill (see comment above regarding
 216         // the usage of the fulfill machinery), rather than the
 217         // repeated `.iter().any(..)` calls.
 218
 219         // This closure is a more robust way to check `Predicate` equality
 220         // than simple `==` checks (which were the previous implementation).
 221         // It relies on `ty::relate` for `TraitPredicate` and `ProjectionPredicate`
 222         // (which implement the Relate trait), while delegating on simple equality
 223         // for the other `Predicate`.
 224         // This implementation solves (Issue #59497) and (Issue #58311).
 225         // It is unclear to me at the moment whether the approach based on `relate`
 226         // could be extended easily also to the other `Predicate`.
 227         let predicate_matches_closure = |p: Predicate<'tcx>| {
 228             let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
 229             match (predicate.kind(), p.kind()) {
 230                 (&ty::PredicateKind::Trait(a, _), &ty::PredicateKind::Trait(b, _)) => {
 231                     relator.relate(a, b).is_ok()
 232                 }
 233                 (&ty::PredicateKind::Projection(a), &ty::PredicateKind::Projection(b)) => {
 234                     relator.relate(a, b).is_ok()
 235                 }
 236                 _ => predicate == p,
 237             }
 238         };
 239
 240         if !assumptions_in_impl_context.iter().copied().any(predicate_matches_closure) {
 241             let item_span = tcx.hir().span(self_type_hir_id);
 242             let self_descr = tcx.def_kind(self_type_did).descr(self_type_did.to_def_id());
 243             struct_span_err!(
 244                 tcx.sess,
 245                 predicate_sp,
 246                 E0367,
 247                 "`Drop` impl requires `{}` but the {} it is implemented for does not",
 248                 predicate,
 249                 self_descr,
 250             )
 251             .span_note(item_span, "the implementor must specify the same requirement")
 252             .emit();
 253             result = Err(ErrorReported);
 254         }
 255     }
 256
 257     result
 258 }
 259
 260 /// This function is not only checking that the dropck obligations are met for
 261 /// the given type, but it's also currently preventing non-regular recursion in
 262 /// types from causing stack overflows (dropck_no_diverge_on_nonregular_*.rs).
 263 crate fn check_drop_obligations<'a, 'tcx>(
 264     rcx: &mut RegionCtxt<'a, 'tcx>,
 265     ty: Ty<'tcx>,
 266     span: Span,
 267     body_id: hir::HirId,
 268 ) -> Result<(), ErrorReported> {
 269     debug!("check_drop_obligations typ: {:?}", ty);
 270
 271     let cause = &ObligationCause::misc(span, body_id);
 272     let infer_ok = rcx.infcx.at(cause, rcx.fcx.param_env).dropck_outlives(ty);
 273     debug!("dropck_outlives = {:#?}", infer_ok);
 274     rcx.fcx.register_infer_ok_obligations(infer_ok);
 275
 276     Ok(())
 277 }
 278
 279 // This is an implementation of the TypeRelation trait with the
 280 // aim of simply comparing for equality (without side-effects).
 281 // It is not intended to be used anywhere else other than here.
 282 crate struct SimpleEqRelation<'tcx> {
 283     tcx: TyCtxt<'tcx>,
 284     param_env: ty::ParamEnv<'tcx>,
 285 }
 286
 287 impl<'tcx> SimpleEqRelation<'tcx> {
 288     fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
 289         SimpleEqRelation { tcx, param_env }
 290     }
 291 }
 292
 293 impl TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
 294     fn tcx(&self) -> TyCtxt<'tcx> {
 295         self.tcx
 296     }
 297
 298     fn param_env(&self) -> ty::ParamEnv<'tcx> {
 299         self.param_env
 300     }
 301
 302     fn tag(&self) -> &'static str {
 303         "dropck::SimpleEqRelation"
 304     }
 305
 306     fn a_is_expected(&self) -> bool {
 307         true
 308     }
 309
 310     fn relate_with_variance<T: Relate<'tcx>>(
 311         &mut self,
 312         _: ty::Variance,
 313         a: T,
 314         b: T,
 315     ) -> RelateResult<'tcx, T> {
 316         // Here we ignore variance because we require drop impl's types
 317         // to be *exactly* the same as to the ones in the struct definition.
 318         self.relate(a, b)
 319     }
 320
 321     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
 322         debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
 323         ty::relate::super_relate_tys(self, a, b)
 324     }
 325
 326     fn regions(
 327         &mut self,
 328         a: ty::Region<'tcx>,
 329         b: ty::Region<'tcx>,
 330     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
 331         debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);
 332
 333         // We can just equate the regions because LBRs have been
 334         // already anonymized.
 335         if a == b {
 336             Ok(a)
 337         } else {
 338             // I'm not sure is this `TypeError` is the right one, but
 339             // it should not matter as it won't be checked (the dropck
 340             // will emit its own, more informative and higher-level errors
 341             // in case anything goes wrong).
 342             Err(TypeError::RegionsPlaceholderMismatch)
 343         }
 344     }
 345
 346     fn consts(
 347         &mut self,
 348         a: &'tcx ty::Const<'tcx>,
 349         b: &'tcx ty::Const<'tcx>,
 350     ) -> RelateResult<'tcx, &'tcx ty::Const<'tcx>> {
 351         debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
 352         ty::relate::super_relate_consts(self, a, b)
 353     }
 354
 355     fn binders<T>(
 356         &mut self,
 357         a: ty::Binder<T>,
 358         b: ty::Binder<T>,
 359     ) -> RelateResult<'tcx, ty::Binder<T>>
 360     where
 361         T: Relate<'tcx>,
 362     {
 363         debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);
 364
 365         // Anonymizing the LBRs is necessary to solve (Issue #59497).
 366         // After we do so, it should be totally fine to skip the binders.
 367         let anon_a = self.tcx.anonymize_late_bound_regions(&a);
 368         let anon_b = self.tcx.anonymize_late_bound_regions(&b);
 369         self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;
 370
 371         Ok(a.clone())
 372     }
 373 }