CFI: Support provided methods on traits

Provided methods currently don't get type erasure performed on them
because they are not in an `impl` block. If we are instantiating a
method that is an associated item, but *not* in an impl block, treat it
as a provided method instead.

compiler/rustc_sanitizers/src/cfi/typeid/itanium_cxx_abi/transform.rs

@@ -9,10 +9,10 @@
 use rustc_middle::bug;
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{
-    self, ExistentialPredicateStableCmpExt as _, Instance, IntTy, List, Ty, TyCtxt, TypeFoldable,
+    self, ExistentialPredicateStableCmpExt as _, Instance, InstanceKind, IntTy, List, TraitRef, Ty,
-    TypeVisitableExt, UintTy,
+    TyCtxt, TypeFoldable, TypeVisitableExt, UintTy,
 };
-use rustc_span::sym;
+use rustc_span::{def_id::DefId, sym};
 use rustc_trait_selection::traits;
 use std::iter;
 use tracing::{debug, instrument};
@@ -360,18 +360,7 @@ pub fn transform_instance<'tcx>(
     if !options.contains(TransformTyOptions::USE_CONCRETE_SELF) {
         // Perform type erasure for calls on trait objects by transforming self into a trait object
         // of the trait that defines the method.
-        if let Some(impl_id) = tcx.impl_of_method(instance.def_id())
+        if let Some((trait_ref, method_id, ancestor)) = implemented_method(tcx, instance) {
-            && let Some(trait_ref) = tcx.impl_trait_ref(impl_id)
-        {
-            let impl_method = tcx.associated_item(instance.def_id());
-            let method_id = impl_method
-                .trait_item_def_id
-                .expect("Part of a trait implementation, but not linked to the def_id?");
-            let trait_method = tcx.associated_item(method_id);
-            let trait_id = trait_ref.skip_binder().def_id;
-            if traits::is_vtable_safe_method(tcx, trait_id, trait_method)
-                && tcx.is_object_safe(trait_id)
-            {
             // Trait methods will have a Self polymorphic parameter, where the concreteized
             // implementatation will not. We need to walk back to the more general trait method
             let trait_ref = tcx.instantiate_and_normalize_erasing_regions(
@@ -393,8 +382,7 @@ pub fn transform_instance<'tcx>(
             instance.def = ty::InstanceKind::Virtual(method_id, 0);
             let abstract_trait_args =
                 tcx.mk_args_trait(invoke_ty, trait_ref.args.into_iter().skip(1));
-                instance.args = instance.args.rebase_onto(tcx, impl_id, abstract_trait_args);
+            instance.args = instance.args.rebase_onto(tcx, ancestor, abstract_trait_args);
-            }
         } else if tcx.is_closure_like(instance.def_id()) {
             // We're either a closure or a coroutine. Our goal is to find the trait we're defined on,
             // instantiate it, and take the type of its only method as our own.
@@ -452,3 +440,36 @@ pub fn transform_instance<'tcx>(
 
     instance
 }
+
+fn implemented_method<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    instance: Instance<'tcx>,
+) -> Option<(ty::EarlyBinder<'tcx, TraitRef<'tcx>>, DefId, DefId)> {
+    let trait_ref;
+    let method_id;
+    let trait_id;
+    let trait_method;
+    let ancestor = if let Some(impl_id) = tcx.impl_of_method(instance.def_id()) {
+        // Implementation in an `impl` block
+        trait_ref = tcx.impl_trait_ref(impl_id)?;
+        let impl_method = tcx.associated_item(instance.def_id());
+        method_id = impl_method.trait_item_def_id?;
+        trait_method = tcx.associated_item(method_id);
+        trait_id = trait_ref.skip_binder().def_id;
+        impl_id
+    } else if let InstanceKind::Item(def_id) = instance.def
+        && let Some(trait_method_bound) = tcx.opt_associated_item(def_id)
+    {
+        // Provided method in a `trait` block
+        trait_method = trait_method_bound;
+        method_id = instance.def_id();
+        trait_id = tcx.trait_of_item(method_id)?;
+        trait_ref = ty::EarlyBinder::bind(TraitRef::from_method(tcx, trait_id, instance.args));
+        trait_id
+    } else {
+        return None;
+    };
+    let vtable_possible =
+        traits::is_vtable_safe_method(tcx, trait_id, trait_method) && tcx.is_object_safe(trait_id);
+    vtable_possible.then_some((trait_ref, method_id, ancestor))
+}

tests/ui/sanitizer/cfi-supertraits.rs

@@ -16,6 +16,9 @@
 trait Parent1 {
     type P1;
     fn p1(&self) -> Self::P1;
+    fn d(&self) -> i32 {
+        42
+    }
 }
 
 trait Parent2 {
@@ -60,14 +63,17 @@ fn main() {
     x.c();
     x.p1();
     x.p2();
+    x.d();
     // Parents can be created and access their methods.
     let y = &Foo as &dyn Parent1<P1=u16>;
     y.p1();
+    y.d();
     let z = &Foo as &dyn Parent2<P2=u32>;
     z.p2();
     // Trait upcasting works
     let x1 = x as &dyn Parent1<P1=u16>;
     x1.p1();
+    x1.d();
     let x2 = x as &dyn Parent2<P2=u32>;
     x2.p2();
 }