1 files changed, 327 insertions, 0 deletions
diff --git a/compiler/rustc_typeck/src/check/dropck.rs b/compiler/rustc_typeck/src/check/dropck.rs
new file mode 100644
index 000000000..321064ec0
--- /dev/null
+++ b/compiler/rustc_typeck/src/check/dropck.rs
@@ -0,0 +1,327 @@
+// FIXME(@lcnr): Move this module out of `rustc_typeck`.
+//
+// We don't do any drop checking during hir typeck.
+use crate::hir::def_id::{DefId, LocalDefId};
+use rustc_errors::{struct_span_err, ErrorGuaranteed};
+use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::util::IgnoreRegions;
+use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
+
+/// This function confirms that the `Drop` implementation identified by
+/// `drop_impl_did` is not any more specialized than the type it is
+/// attached to (Issue #8142).
+///
+/// This means:
+///
+/// 1. The self type must be nominal (this is already checked during
+///    coherence),
+///
+/// 2. The generic region/type parameters of the impl's self type must
+///    all be parameters of the Drop impl itself (i.e., no
+///    specialization like `impl Drop for Foo<i32>`), and,
+///
+/// 3. Any bounds on the generic parameters must be reflected in the
+///    struct/enum definition for the nominal type itself (i.e.
+///    cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
+///
+pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorGuaranteed> {
+    let dtor_self_type = tcx.type_of(drop_impl_did);
+    let dtor_predicates = tcx.predicates_of(drop_impl_did);
+    match dtor_self_type.kind() {
+        ty::Adt(adt_def, self_to_impl_substs) => {
+            ensure_drop_params_and_item_params_correspond(
+                tcx,
+                drop_impl_did.expect_local(),
+                adt_def.did(),
+                self_to_impl_substs,
+            )?;
+
+            ensure_drop_predicates_are_implied_by_item_defn(
+                tcx,
+                dtor_predicates,
+                adt_def.did().expect_local(),
+                self_to_impl_substs,
+            )
+        }
+        _ => {
+            // Destructors only work on nominal types.  This was
+            // already checked by coherence, but compilation may
+            // not have been terminated.
+            let span = tcx.def_span(drop_impl_did);
+            let reported = tcx.sess.delay_span_bug(
+                span,
+                &format!("should have been rejected by coherence check: {dtor_self_type}"),
+            );
+            Err(reported)
+        }
+    }
+}
+
+fn ensure_drop_params_and_item_params_correspond<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    drop_impl_did: LocalDefId,
+    self_type_did: DefId,
+    drop_impl_substs: SubstsRef<'tcx>,
+) -> Result<(), ErrorGuaranteed> {
+    let Err(arg) = tcx.uses_unique_generic_params(drop_impl_substs, IgnoreRegions::No) else {
+        return Ok(())
+    };
+
+    let drop_impl_span = tcx.def_span(drop_impl_did);
+    let item_span = tcx.def_span(self_type_did);
+    let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
+    let mut err =
+        struct_span_err!(tcx.sess, drop_impl_span, E0366, "`Drop` impls cannot be specialized");
+    match arg {
+        ty::util::NotUniqueParam::DuplicateParam(arg) => {
+            err.note(&format!("`{arg}` is mentioned multiple times"))
+        }
+        ty::util::NotUniqueParam::NotParam(arg) => {
+            err.note(&format!("`{arg}` is not a generic parameter"))
+        }
+    };
+    err.span_note(
+        item_span,
+        &format!(
+            "use the same sequence of generic lifetime, type and const parameters \
+                     as the {self_descr} definition",
+        ),
+    );
+    Err(err.emit())
+}
+
+/// Confirms that every predicate imposed by dtor_predicates is
+/// implied by assuming the predicates attached to self_type_did.
+fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    dtor_predicates: ty::GenericPredicates<'tcx>,
+    self_type_did: LocalDefId,
+    self_to_impl_substs: SubstsRef<'tcx>,
+) -> Result<(), ErrorGuaranteed> {
+    let mut result = Ok(());
+
+    // Here is an example, analogous to that from
+    // `compare_impl_method`.
+    //
+    // Consider a struct type:
+    //
+    //     struct Type<'c, 'b:'c, 'a> {
+    //         x: &'a Contents            // (contents are irrelevant;
+    //         y: &'c Cell<&'b Contents>, //  only the bounds matter for our purposes.)
+    //     }
+    //
+    // and a Drop impl:
+    //
+    //     impl<'z, 'y:'z, 'x:'y> Drop for P<'z, 'y, 'x> {
+    //         fn drop(&mut self) { self.y.set(self.x); } // (only legal if 'x: 'y)
+    //     }
+    //
+    // We start out with self_to_impl_substs, that maps the generic
+    // parameters of Type to that of the Drop impl.
+    //
+    //     self_to_impl_substs = {'c => 'z, 'b => 'y, 'a => 'x}
+    //
+    // Applying this to the predicates (i.e., assumptions) provided by the item
+    // definition yields the instantiated assumptions:
+    //
+    //     ['y : 'z]
+    //
+    // We then check all of the predicates of the Drop impl:
+    //
+    //     ['y:'z, 'x:'y]
+    //
+    // and ensure each is in the list of instantiated
+    // assumptions. Here, `'y:'z` is present, but `'x:'y` is
+    // absent. So we report an error that the Drop impl injected a
+    // predicate that is not present on the struct definition.
+
+    // We can assume the predicates attached to struct/enum definition
+    // hold.
+    let generic_assumptions = tcx.predicates_of(self_type_did);
+
+    let assumptions_in_impl_context = generic_assumptions.instantiate(tcx, &self_to_impl_substs);
+    let assumptions_in_impl_context = assumptions_in_impl_context.predicates;
+
+    let self_param_env = tcx.param_env(self_type_did);
+
+    // An earlier version of this code attempted to do this checking
+    // via the traits::fulfill machinery. However, it ran into trouble
+    // since the fulfill machinery merely turns outlives-predicates
+    // 'a:'b and T:'b into region inference constraints. It is simpler
+    // just to look for all the predicates directly.
+
+    assert_eq!(dtor_predicates.parent, None);
+    for &(predicate, predicate_sp) in dtor_predicates.predicates {
+        // (We do not need to worry about deep analysis of type
+        // expressions etc because the Drop impls are already forced
+        // to take on a structure that is roughly an alpha-renaming of
+        // the generic parameters of the item definition.)
+
+        // This path now just checks *all* predicates via an instantiation of
+        // the `SimpleEqRelation`, which simply forwards to the `relate` machinery
+        // after taking care of anonymizing late bound regions.
+        //
+        // However, it may be more efficient in the future to batch
+        // the analysis together via the fulfill (see comment above regarding
+        // the usage of the fulfill machinery), rather than the
+        // repeated `.iter().any(..)` calls.
+
+        // This closure is a more robust way to check `Predicate` equality
+        // than simple `==` checks (which were the previous implementation).
+        // It relies on `ty::relate` for `TraitPredicate`, `ProjectionPredicate`,
+        // `ConstEvaluatable` and `TypeOutlives` (which implement the Relate trait),
+        // while delegating on simple equality for the other `Predicate`.
+        // This implementation solves (Issue #59497) and (Issue #58311).
+        // It is unclear to me at the moment whether the approach based on `relate`
+        // could be extended easily also to the other `Predicate`.
+        let predicate_matches_closure = |p: Predicate<'tcx>| {
+            let mut relator: SimpleEqRelation<'tcx> = SimpleEqRelation::new(tcx, self_param_env);
+            let predicate = predicate.kind();
+            let p = p.kind();
+            match (predicate.skip_binder(), p.skip_binder()) {
+                (ty::PredicateKind::Trait(a), ty::PredicateKind::Trait(b)) => {
+                    // Since struct predicates cannot have ~const, project the impl predicate
+                    // onto one that ignores the constness. This is equivalent to saying that
+                    // we match a `Trait` bound on the struct with a `Trait` or `~const Trait`
+                    // in the impl.
+                    let non_const_a =
+                        ty::TraitPredicate { constness: ty::BoundConstness::NotConst, ..a };
+                    relator.relate(predicate.rebind(non_const_a), p.rebind(b)).is_ok()
+                }
+                (ty::PredicateKind::Projection(a), ty::PredicateKind::Projection(b)) => {
+                    relator.relate(predicate.rebind(a), p.rebind(b)).is_ok()
+                }
+                (
+                    ty::PredicateKind::ConstEvaluatable(a),
+                    ty::PredicateKind::ConstEvaluatable(b),
+                ) => tcx.try_unify_abstract_consts(self_param_env.and((a, b))),
+                (
+                    ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_a, lt_a)),
+                    ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_b, lt_b)),
+                ) => {
+                    relator.relate(predicate.rebind(ty_a), p.rebind(ty_b)).is_ok()
+                        && relator.relate(predicate.rebind(lt_a), p.rebind(lt_b)).is_ok()
+                }
+                (ty::PredicateKind::WellFormed(arg_a), ty::PredicateKind::WellFormed(arg_b)) => {
+                    relator.relate(predicate.rebind(arg_a), p.rebind(arg_b)).is_ok()
+                }
+                _ => predicate == p,
+            }
+        };
+
+        if !assumptions_in_impl_context.iter().copied().any(predicate_matches_closure) {
+            let item_span = tcx.def_span(self_type_did);
+            let self_descr = tcx.def_kind(self_type_did).descr(self_type_did.to_def_id());
+            let reported = struct_span_err!(
+                tcx.sess,
+                predicate_sp,
+                E0367,
+                "`Drop` impl requires `{predicate}` but the {self_descr} it is implemented for does not",
+            )
+            .span_note(item_span, "the implementor must specify the same requirement")
+            .emit();
+            result = Err(reported);
+        }
+    }
+
+    result
+}
+
+// This is an implementation of the TypeRelation trait with the
+// aim of simply comparing for equality (without side-effects).
+// It is not intended to be used anywhere else other than here.
+pub(crate) struct SimpleEqRelation<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> SimpleEqRelation<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> SimpleEqRelation<'tcx> {
+        SimpleEqRelation { tcx, param_env }
+    }
+}
+
+impl<'tcx> TypeRelation<'tcx> for SimpleEqRelation<'tcx> {
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+
+    fn tag(&self) -> &'static str {
+        "dropck::SimpleEqRelation"
+    }
+
+    fn a_is_expected(&self) -> bool {
+        true
+    }
+
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        _: ty::Variance,
+        _info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        // Here we ignore variance because we require drop impl's types
+        // to be *exactly* the same as to the ones in the struct definition.
+        self.relate(a, b)
+    }
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        debug!("SimpleEqRelation::tys(a={:?}, b={:?})", a, b);
+        ty::relate::super_relate_tys(self, a, b)
+    }
+
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        debug!("SimpleEqRelation::regions(a={:?}, b={:?})", a, b);
+
+        // We can just equate the regions because LBRs have been
+        // already anonymized.
+        if a == b {
+            Ok(a)
+        } else {
+            // I'm not sure is this `TypeError` is the right one, but
+            // it should not matter as it won't be checked (the dropck
+            // will emit its own, more informative and higher-level errors
+            // in case anything goes wrong).
+            Err(TypeError::RegionsPlaceholderMismatch)
+        }
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        debug!("SimpleEqRelation::consts(a={:?}, b={:?})", a, b);
+        ty::relate::super_relate_consts(self, a, b)
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        debug!("SimpleEqRelation::binders({:?}: {:?}", a, b);
+
+        // Anonymizing the LBRs is necessary to solve (Issue #59497).
+        // After we do so, it should be totally fine to skip the binders.
+        let anon_a = self.tcx.anonymize_bound_vars(a);
+        let anon_b = self.tcx.anonymize_bound_vars(b);
+        self.relate(anon_a.skip_binder(), anon_b.skip_binder())?;
+
+        Ok(a)
+    }
+}