Auto merge of #56837 - arielb1:nonprincipal-trait-objects, r=nikomatsakis

Add support for trait-objects without a principal

The hard-error version of #56481 - should be merged after we do something about the `traitobject` crate.

Fixes #33140.
Fixes #57057.

r? @nikomatsakis

2 files changed, 10 insertions, 5 deletions

diff --git a/src/librustc_mir/interpret/eval_context.rs b/src/librustc_mir/interpret/eval_context.rs
index 9a76a36ef22..19362b6cfdb 100644
--- a/src/librustc_mir/interpret/eval_context.rs
+++ b/src/librustc_mir/interpret/eval_context.rs
@@ -42,7 +42,7 @@ pub struct EvalContext<'a, 'mir, 'tcx: 'a + 'mir, M: Machine<'a, 'mir, 'tcx>> {
     pub(crate) stack: Vec<Frame<'mir, 'tcx, M::PointerTag, M::FrameExtra>>,
 
     /// A cache for deduplicating vtables
-    pub(super) vtables: FxHashMap<(Ty<'tcx>, ty::PolyExistentialTraitRef<'tcx>), AllocId>,
+    pub(super) vtables: FxHashMap<(Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>), AllocId>,
 }
 
 /// A stack frame.
diff --git a/src/librustc_mir/interpret/traits.rs b/src/librustc_mir/interpret/traits.rs
index 69a52b002ce..642bbc114f5 100644
--- a/src/librustc_mir/interpret/traits.rs
+++ b/src/librustc_mir/interpret/traits.rs
@@ -1,3 +1,4 @@
+use rustc_data_structures::sync::Lrc;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{Size, Align, LayoutOf};
 use rustc::mir::interpret::{Scalar, Pointer, EvalResult, PointerArithmetic};
@@ -14,7 +15,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     pub fn get_vtable(
         &mut self,
         ty: Ty<'tcx>,
-        poly_trait_ref: ty::PolyExistentialTraitRef<'tcx>,
+        poly_trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
     ) -> EvalResult<'tcx, Pointer<M::PointerTag>> {
         trace!("get_vtable(trait_ref={:?})", poly_trait_ref);
 
@@ -24,10 +25,14 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
             return Ok(Pointer::from(vtable).with_default_tag());
         }
 
-        let trait_ref = poly_trait_ref.with_self_ty(*self.tcx, ty);
-        let trait_ref = self.tcx.erase_regions(&trait_ref);
+        let methods = 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);
 
-        let methods = self.tcx.vtable_methods(trait_ref);
+            self.tcx.vtable_methods(trait_ref)
+        } else {
+            Lrc::new(Vec::new())
+        };
 
         let layout = self.layout_of(ty)?;
         assert!(!layout.is_unsized(), "can't create a vtable for an unsized type");