interpret: refactor dyn trait handling

We can check that the vtable is for the right trait very early, and then just pass the type around.

1 files changed, 48 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..1df9f5f7b2a 100644
--- a/compiler/rustc_const_eval/src/interpret/traits.rs
+++ b/compiler/rustc_const_eval/src/interpret/traits.rs
@@ -1,11 +1,11 @@
 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 tracing::trace;
 
 use super::util::ensure_monomorphic_enough;
-use super::{InterpCx, Machine};
+use super::{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 +33,58 @@ 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>>,
     ) -> InterpResult<'tcx, (Size, Align)> {
-        let (ty, _trait_ref) = self.get_ptr_vtable(vtable)?;
+        let ty = self.get_ptr_vtable_ty(vtable, None)?;
         let layout = self.layout_of(ty)?;
         assert!(layout.is_sized(), "there are no vtables for unsized types");
         Ok((layout.size, layout.align.abi))
     }
+
+    /// 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)
+    }
 }