Rollup merge of #126232 - RalfJung:dyn-trait-equality, r=oli-obk

interpret: dyn trait metadata check: equate traits in a proper way

Hopefully fixes https://github.com/rust-lang/miri/issues/3541... unfortunately we don't have a testcase.

The first commit is just a refactor without functional change.

r? `@oli-obk`

1 files changed, 83 insertions, 18 deletions

diff --git a/compiler/rustc_const_eval/src/interpret/traits.rs b/compiler/rustc_const_eval/src/interpret/traits.rs
index 244a6ba48a4..bd2c6519421 100644
--- a/compiler/rustc_const_eval/src/interpret/traits.rs
+++ b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -1,11 +1,14 @@
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::ObligationCause;
 use rustc_middle::mir::interpret::{InterpResult, Pointer};
 use rustc_middle::ty::layout::LayoutOf;
-use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_middle::ty::{self, Ty};
 use rustc_target::abi::{Align, Size};
+use rustc_trait_selection::traits::ObligationCtxt;
 use tracing::trace;
 
 use super::util::ensure_monomorphic_enough;
-use super::{InterpCx, Machine};
+use super::{throw_ub, InterpCx, MPlaceTy, Machine, MemPlaceMeta, OffsetMode, Projectable};
 
 impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     /// Creates a dynamic vtable for the given type and vtable origin. This is used only for
@@ -33,28 +36,90 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         Ok(vtable_ptr.into())
     }
 
-    /// Returns a high-level representation of the entries of the given vtable.
-    pub fn get_vtable_entries(
-        &self,
-        vtable: Pointer<Option<M::Provenance>>,
-    ) -> InterpResult<'tcx, &'tcx [ty::VtblEntry<'tcx>]> {
-        let (ty, poly_trait_ref) = self.get_ptr_vtable(vtable)?;
-        Ok(if let Some(poly_trait_ref) = poly_trait_ref {
-            let trait_ref = poly_trait_ref.with_self_ty(*self.tcx, ty);
-            let trait_ref = self.tcx.erase_regions(trait_ref);
-            self.tcx.vtable_entries(trait_ref)
-        } else {
-            TyCtxt::COMMON_VTABLE_ENTRIES
-        })
-    }
-
     pub fn get_vtable_size_and_align(
         &self,
         vtable: Pointer<Option<M::Provenance>>,
+        expected_trait: Option<&'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>>,
     ) -> InterpResult<'tcx, (Size, Align)> {
-        let (ty, _trait_ref) = self.get_ptr_vtable(vtable)?;
+        let ty = self.get_ptr_vtable_ty(vtable, expected_trait)?;
         let layout = self.layout_of(ty)?;
         assert!(layout.is_sized(), "there are no vtables for unsized types");
         Ok((layout.size, layout.align.abi))
     }
+
+    /// Check that the given vtable trait is valid for a pointer/reference/place with the given
+    /// expected trait type.
+    pub(super) fn check_vtable_for_type(
+        &self,
+        vtable_trait: Option<ty::PolyExistentialTraitRef<'tcx>>,
+        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+    ) -> InterpResult<'tcx> {
+        // Fast path: if they are equal, it's all fine.
+        if expected_trait.principal() == vtable_trait {
+            return Ok(());
+        }
+        if let (Some(expected_trait), Some(vtable_trait)) =
+            (expected_trait.principal(), vtable_trait)
+        {
+            // Slow path: spin up an inference context to check if these traits are sufficiently equal.
+            let infcx = self.tcx.infer_ctxt().build();
+            let ocx = ObligationCtxt::new(&infcx);
+            let cause = ObligationCause::dummy_with_span(self.cur_span());
+            // equate the two trait refs after normalization
+            let expected_trait = ocx.normalize(&cause, self.param_env, expected_trait);
+            let vtable_trait = ocx.normalize(&cause, self.param_env, vtable_trait);
+            if ocx.eq(&cause, self.param_env, expected_trait, vtable_trait).is_ok() {
+                if ocx.select_all_or_error().is_empty() {
+                    // All good.
+                    return Ok(());
+                }
+            }
+        }
+        throw_ub!(InvalidVTableTrait { expected_trait, vtable_trait });
+    }
+
+    /// Turn a place with a `dyn Trait` type into a place with the actual dynamic type.
+    pub(super) fn unpack_dyn_trait(
+        &self,
+        mplace: &MPlaceTy<'tcx, M::Provenance>,
+        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
+        assert!(
+            matches!(mplace.layout.ty.kind(), ty::Dynamic(_, _, ty::Dyn)),
+            "`unpack_dyn_trait` only makes sense on `dyn*` types"
+        );
+        let vtable = mplace.meta().unwrap_meta().to_pointer(self)?;
+        let ty = self.get_ptr_vtable_ty(vtable, Some(expected_trait))?;
+        // This is a kind of transmute, from a place with unsized type and metadata to
+        // a place with sized type and no metadata.
+        let layout = self.layout_of(ty)?;
+        let mplace = mplace.offset_with_meta(
+            Size::ZERO,
+            OffsetMode::Wrapping,
+            MemPlaceMeta::None,
+            layout,
+            self,
+        )?;
+        Ok(mplace)
+    }
+
+    /// Turn a `dyn* Trait` type into an value with the actual dynamic type.
+    pub(super) fn unpack_dyn_star<P: Projectable<'tcx, M::Provenance>>(
+        &self,
+        val: &P,
+        expected_trait: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+    ) -> InterpResult<'tcx, P> {
+        assert!(
+            matches!(val.layout().ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
+            "`unpack_dyn_star` only makes sense on `dyn*` types"
+        );
+        let data = self.project_field(val, 0)?;
+        let vtable = self.project_field(val, 1)?;
+        let vtable = self.read_pointer(&vtable.to_op(self)?)?;
+        let ty = self.get_ptr_vtable_ty(vtable, Some(expected_trait))?;
+        // `data` is already the right thing but has the wrong type. So we transmute it.
+        let layout = self.layout_of(ty)?;
+        let data = data.transmute(layout, self)?;
+        Ok(data)
+    }
 }