Rollup merge of #126090 - compiler-errors:supertrait-assoc-ty-unsoundness, r=lcnr

Fix supertrait associated type unsoundness

### What?

Object safety allows us to name `Self::Assoc` associated types in certain positions if they come from our trait or one of our supertraits. When this check was implemented, I think it failed to consider that supertraits can have different args, and it was only checking def-id equality.

This is problematic, since we can sneak different implementations in by implementing `Supertrait<NotActuallyTheSupertraitSubsts>` for a `dyn` type. This can be used to implement an unsound transmute function. See the committed test.

### How do we fix it?

We consider the whole trait ref when checking for supertraits. Right now, this is implemented using equality *without* normalization. We erase regions since those don't affect trait selection.

This is a limitation that could theoretically affect code that should be accepted, but doesn't matter in practice -- there are 0 crater regression. We could make this check stronger, but I would be worried about cycle issues. I assume that most people are writing `Self::Assoc` so they don't really care about the trait ref being normalized.

---

### What is up w the stacked commit

This is built on top of #122804 though that's really not related, it's just easier to make this modification with the changes to the object safety code that I did in that PR. The only thing is that PR may make this unsoundness slightly easier to abuse, since there are more positions that allow self-associated-types -- I am happy to stall that change until this PR merges.

---

Fixes #126079

r? lcnr

Author: matthiaskrgr

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -12,17 +12,16 @@ use super::elaborate;
 
 use crate::infer::TyCtxtInferExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
-use crate::traits::{self, Obligation, ObligationCause};
+use crate::traits::{util, Obligation, ObligationCause};
 use rustc_errors::FatalError;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{
-    self, EarlyBinder, ExistentialPredicateStableCmpExt as _, Ty, TyCtxt, TypeSuperVisitable,
-    TypeVisitable, TypeVisitor,
+    self, EarlyBinder, ExistentialPredicateStableCmpExt as _, GenericArgs, Ty, TyCtxt,
+    TypeFoldable, TypeFolder, TypeSuperFoldable, TypeSuperVisitable, TypeVisitable,
+    TypeVisitableExt, TypeVisitor, Upcast,
 };
-use rustc_middle::ty::{GenericArg, GenericArgs};
-use rustc_middle::ty::{TypeVisitableExt, Upcast};
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 use rustc_target::abi::Abi;
@@ -195,7 +194,13 @@ fn predicates_reference_self(
         .predicates
         .iter()
         .map(|&(predicate, sp)| (predicate.instantiate_supertrait(tcx, trait_ref), sp))
-        .filter_map(|predicate| predicate_references_self(tcx, predicate))
+        .filter_map(|(clause, sp)| {
+            // Super predicates cannot allow self projections, since they're
+            // impossible to make into existential bounds without eager resolution
+            // or something.
+            // e.g. `trait A: B<Item = Self::Assoc>`.
+            predicate_references_self(tcx, trait_def_id, clause, sp, AllowSelfProjections::No)
+        })
         .collect()
 }
 
@@ -204,20 +209,25 @@ fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span
         .in_definition_order()
         .filter(|item| item.kind == ty::AssocKind::Type)
         .flat_map(|item| tcx.explicit_item_bounds(item.def_id).iter_identity_copied())
-        .filter_map(|c| predicate_references_self(tcx, c))
+        .filter_map(|(clause, sp)| {
+            // Item bounds *can* have self projections, since they never get
+            // their self type erased.
+            predicate_references_self(tcx, trait_def_id, clause, sp, AllowSelfProjections::Yes)
+        })
         .collect()
 }
 
 fn predicate_references_self<'tcx>(
     tcx: TyCtxt<'tcx>,
-    (predicate, sp): (ty::Clause<'tcx>, Span),
+    trait_def_id: DefId,
+    predicate: ty::Clause<'tcx>,
+    sp: Span,
+    allow_self_projections: AllowSelfProjections,
 ) -> Option<Span> {
-    let self_ty = tcx.types.self_param;
-    let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
     match predicate.kind().skip_binder() {
         ty::ClauseKind::Trait(ref data) => {
             // In the case of a trait predicate, we can skip the "self" type.
-            data.trait_ref.args[1..].iter().any(has_self_ty).then_some(sp)
+            data.trait_ref.args[1..].iter().any(|&arg| contains_illegal_self_type_reference(tcx, trait_def_id, arg, allow_self_projections)).then_some(sp)
         }
         ty::ClauseKind::Projection(ref data) => {
             // And similarly for projections. This should be redundant with
@@ -235,9 +245,9 @@ fn predicate_references_self<'tcx>(
             //
             // This is ALT2 in issue #56288, see that for discussion of the
             // possible alternatives.
-            data.projection_term.args[1..].iter().any(has_self_ty).then_some(sp)
+            data.projection_term.args[1..].iter().any(|&arg| contains_illegal_self_type_reference(tcx, trait_def_id, arg, allow_self_projections)).then_some(sp)
         }
-        ty::ClauseKind::ConstArgHasType(_ct, ty) => has_self_ty(&ty.into()).then_some(sp),
+        ty::ClauseKind::ConstArgHasType(_ct, ty) => contains_illegal_self_type_reference(tcx, trait_def_id, ty, allow_self_projections).then_some(sp),
 
         ty::ClauseKind::WellFormed(..)
         | ty::ClauseKind::TypeOutlives(..)
@@ -383,7 +393,12 @@ fn virtual_call_violations_for_method<'tcx>(
     let mut errors = Vec::new();
 
     for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
-        if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
+        if contains_illegal_self_type_reference(
+            tcx,
+            trait_def_id,
+            sig.rebind(input_ty),
+            AllowSelfProjections::Yes,
+        ) {
             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
                 kind: hir::TraitItemKind::Fn(sig, _),
                 ..
@@ -396,7 +411,12 @@ fn virtual_call_violations_for_method<'tcx>(
             errors.push(MethodViolationCode::ReferencesSelfInput(span));
         }
     }
-    if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
+    if contains_illegal_self_type_reference(
+        tcx,
+        trait_def_id,
+        sig.output(),
+        AllowSelfProjections::Yes,
+    ) {
         errors.push(MethodViolationCode::ReferencesSelfOutput);
     }
     if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
@@ -482,7 +502,7 @@ fn virtual_call_violations_for_method<'tcx>(
             return false;
         }
 
-        contains_illegal_self_type_reference(tcx, trait_def_id, pred)
+        contains_illegal_self_type_reference(tcx, trait_def_id, pred, AllowSelfProjections::Yes)
     }) {
         errors.push(MethodViolationCode::WhereClauseReferencesSelf);
     }
@@ -711,121 +731,181 @@ fn receiver_is_dispatchable<'tcx>(
     infcx.predicate_must_hold_modulo_regions(&obligation)
 }
 
+#[derive(Copy, Clone)]
+enum AllowSelfProjections {
+    Yes,
+    No,
+}
+
+/// This is somewhat subtle. In general, we want to forbid
+/// references to `Self` in the argument and return types,
+/// since the value of `Self` is erased. However, there is one
+/// exception: it is ok to reference `Self` in order to access
+/// an associated type of the current trait, since we retain
+/// the value of those associated types in the object type
+/// itself.
+///
+/// ```rust,ignore (example)
+/// trait SuperTrait {
+///     type X;
+/// }
+///
+/// trait Trait : SuperTrait {
+///     type Y;
+///     fn foo(&self, x: Self) // bad
+///     fn foo(&self) -> Self // bad
+///     fn foo(&self) -> Option<Self> // bad
+///     fn foo(&self) -> Self::Y // OK, desugars to next example
+///     fn foo(&self) -> <Self as Trait>::Y // OK
+///     fn foo(&self) -> Self::X // OK, desugars to next example
+///     fn foo(&self) -> <Self as SuperTrait>::X // OK
+/// }
+/// ```
+///
+/// However, it is not as simple as allowing `Self` in a projected
+/// type, because there are illegal ways to use `Self` as well:
+///
+/// ```rust,ignore (example)
+/// trait Trait : SuperTrait {
+///     ...
+///     fn foo(&self) -> <Self as SomeOtherTrait>::X;
+/// }
+/// ```
+///
+/// Here we will not have the type of `X` recorded in the
+/// object type, and we cannot resolve `Self as SomeOtherTrait`
+/// without knowing what `Self` is.
 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
     tcx: TyCtxt<'tcx>,
     trait_def_id: DefId,
     value: T,
+    allow_self_projections: AllowSelfProjections,
 ) -> bool {
-    // This is somewhat subtle. In general, we want to forbid
-    // references to `Self` in the argument and return types,
-    // since the value of `Self` is erased. However, there is one
-    // exception: it is ok to reference `Self` in order to access
-    // an associated type of the current trait, since we retain
-    // the value of those associated types in the object type
-    // itself.
-    //
-    // ```rust
-    // trait SuperTrait {
-    //     type X;
-    // }
-    //
-    // trait Trait : SuperTrait {
-    //     type Y;
-    //     fn foo(&self, x: Self) // bad
-    //     fn foo(&self) -> Self // bad
-    //     fn foo(&self) -> Option<Self> // bad
-    //     fn foo(&self) -> Self::Y // OK, desugars to next example
-    //     fn foo(&self) -> <Self as Trait>::Y // OK
-    //     fn foo(&self) -> Self::X // OK, desugars to next example
-    //     fn foo(&self) -> <Self as SuperTrait>::X // OK
-    // }
-    // ```
-    //
-    // However, it is not as simple as allowing `Self` in a projected
-    // type, because there are illegal ways to use `Self` as well:
-    //
-    // ```rust
-    // trait Trait : SuperTrait {
-    //     ...
-    //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
-    // }
-    // ```
-    //
-    // Here we will not have the type of `X` recorded in the
-    // object type, and we cannot resolve `Self as SomeOtherTrait`
-    // without knowing what `Self` is.
-
-    struct IllegalSelfTypeVisitor<'tcx> {
-        tcx: TyCtxt<'tcx>,
-        trait_def_id: DefId,
-        supertraits: Option<Vec<DefId>>,
-    }
+    value
+        .visit_with(&mut IllegalSelfTypeVisitor {
+            tcx,
+            trait_def_id,
+            supertraits: None,
+            allow_self_projections,
+        })
+        .is_break()
+}
 
-    impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalSelfTypeVisitor<'tcx> {
-        type Result = ControlFlow<()>;
+struct IllegalSelfTypeVisitor<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    trait_def_id: DefId,
+    supertraits: Option<Vec<ty::TraitRef<'tcx>>>,
+    allow_self_projections: AllowSelfProjections,
+}
 
-        fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
-            match t.kind() {
-                ty::Param(_) => {
-                    if t == self.tcx.types.self_param {
-                        ControlFlow::Break(())
-                    } else {
-                        ControlFlow::Continue(())
-                    }
-                }
-                ty::Alias(ty::Projection, ref data)
-                    if self.tcx.is_impl_trait_in_trait(data.def_id) =>
-                {
-                    // We'll deny these later in their own pass
+impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalSelfTypeVisitor<'tcx> {
+    type Result = ControlFlow<()>;
+
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
+        match t.kind() {
+            ty::Param(_) => {
+                if t == self.tcx.types.self_param {
+                    ControlFlow::Break(())
+                } else {
                     ControlFlow::Continue(())
                 }
-                ty::Alias(ty::Projection, ref data) => {
-                    // This is a projected type `<Foo as SomeTrait>::X`.
-
-                    // Compute supertraits of current trait lazily.
-                    if self.supertraits.is_none() {
-                        let trait_ref =
-                            ty::Binder::dummy(ty::TraitRef::identity(self.tcx, self.trait_def_id));
-                        self.supertraits = Some(
-                            traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
-                        );
-                    }
+            }
+            ty::Alias(ty::Projection, ref data) if self.tcx.is_impl_trait_in_trait(data.def_id) => {
+                // We'll deny these later in their own pass
+                ControlFlow::Continue(())
+            }
+            ty::Alias(ty::Projection, ref data) => {
+                match self.allow_self_projections {
+                    AllowSelfProjections::Yes => {
+                        // This is a projected type `<Foo as SomeTrait>::X`.
+
+                        // Compute supertraits of current trait lazily.
+                        if self.supertraits.is_none() {
+                            self.supertraits = Some(
+                                util::supertraits(
+                                    self.tcx,
+                                    ty::Binder::dummy(ty::TraitRef::identity(
+                                        self.tcx,
+                                        self.trait_def_id,
+                                    )),
+                                )
+                                .map(|trait_ref| {
+                                    self.tcx.erase_regions(
+                                        self.tcx.instantiate_bound_regions_with_erased(trait_ref),
+                                    )
+                                })
+                                .collect(),
+                            );
+                        }
 
-                    // Determine whether the trait reference `Foo as
-                    // SomeTrait` is in fact a supertrait of the
-                    // current trait. In that case, this type is
-                    // legal, because the type `X` will be specified
-                    // in the object type. Note that we can just use
-                    // direct equality here because all of these types
-                    // are part of the formal parameter listing, and
-                    // hence there should be no inference variables.
-                    let is_supertrait_of_current_trait = self
-                        .supertraits
-                        .as_ref()
-                        .unwrap()
-                        .contains(&data.trait_ref(self.tcx).def_id);
-
-                    // only walk contained types if it's not a super trait
-                    if is_supertrait_of_current_trait {
-                        ControlFlow::Continue(())
-                    } else {
-                        t.super_visit_with(self) // POSSIBLY reporting an error
+                        // Determine whether the trait reference `Foo as
+                        // SomeTrait` is in fact a supertrait of the
+                        // current trait. In that case, this type is
+                        // legal, because the type `X` will be specified
+                        // in the object type. Note that we can just use
+                        // direct equality here because all of these types
+                        // are part of the formal parameter listing, and
+                        // hence there should be no inference variables.
+                        let is_supertrait_of_current_trait =
+                            self.supertraits.as_ref().unwrap().contains(
+                                &data.trait_ref(self.tcx).fold_with(
+                                    &mut EraseEscapingBoundRegions {
+                                        tcx: self.tcx,
+                                        binder: ty::INNERMOST,
+                                    },
+                                ),
+                            );
+
+                        // only walk contained types if it's not a super trait
+                        if is_supertrait_of_current_trait {
+                            ControlFlow::Continue(())
+                        } else {
+                            t.super_visit_with(self) // POSSIBLY reporting an error
+                        }
                     }
+                    AllowSelfProjections::No => t.super_visit_with(self),
                 }
-                _ => t.super_visit_with(self), // walk contained types, if any
             }
+            _ => t.super_visit_with(self),
         }
+    }
 
-        fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
-            // Constants can only influence object safety if they are generic and reference `Self`.
-            // This is only possible for unevaluated constants, so we walk these here.
-            self.tcx.expand_abstract_consts(ct).super_visit_with(self)
-        }
+    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
+        // Constants can only influence object safety if they are generic and reference `Self`.
+        // This is only possible for unevaluated constants, so we walk these here.
+        self.tcx.expand_abstract_consts(ct).super_visit_with(self)
     }
+}
 
-    value
-        .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
-        .is_break()
+struct EraseEscapingBoundRegions<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    binder: ty::DebruijnIndex,
+}
+
+impl<'tcx> TypeFolder<TyCtxt<'tcx>> for EraseEscapingBoundRegions<'tcx> {
+    fn cx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_binder<T>(&mut self, t: ty::Binder<'tcx, T>) -> ty::Binder<'tcx, T>
+    where
+        T: TypeFoldable<TyCtxt<'tcx>>,
+    {
+        self.binder.shift_in(1);
+        let result = t.super_fold_with(self);
+        self.binder.shift_out(1);
+        result
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        if let ty::ReBound(debruijn, _) = *r
+            && debruijn < self.binder
+        {
+            r
+        } else {
+            self.tcx.lifetimes.re_erased
+        }
+    }
 }
 
 pub fn contains_illegal_impl_trait_in_trait<'tcx>(