Construct dyn* during const interp

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -907,7 +907,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         llargs.push(data_ptr);
                         continue;
                     }
-                    _ => span_bug!(span, "can't codegen a virtual call on {:?}", op),
+                    _ => span_bug!(span, "can't codegen a virtual call on {:#?}", op),
                 }
             }
 

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -110,7 +110,18 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             DynStar => {
-                unimplemented!()
+                if let ty::Dynamic(data, _, ty::TraitObjectRepresentation::Sized) = cast_ty.kind() {
+                    // Initial cast from sized to dyn trait
+                    let vtable = self.get_vtable_ptr(src.layout.ty, data.principal())?;
+                    let ptr = self.read_immediate(src)?.to_scalar();
+                    // FIXME(dyn-star): This should not use new_dyn_trait, but
+                    // it does exactly the same thing (makes a scalar pair)...
+                    // so maybe we should just duplicate/rename the function.
+                    let val = Immediate::new_dyn_trait(ptr, vtable, &*self.tcx);
+                    self.write_immediate(val, dest)?;
+                } else {
+                    bug!()
+                }
             }
         }
         Ok(())

compiler/rustc_middle/src/ty/layout.rs

@@ -2545,15 +2545,9 @@ where
                     }
                 }
 
-                // dyn*
+                // dyn* (both fields are usize-sized)
                 ty::Dynamic(_, _, TraitObjectRepresentation::Sized) => {
-                    TyMaybeWithLayout::TyAndLayout(
-                        tcx.layout_of(
-                            ty::ParamEnv::reveal_all()
-                                .and(tcx.mk_tup([tcx.types.usize, tcx.types.usize].into_iter())),
-                        )
-                        .unwrap(),
-                    )
+                    TyMaybeWithLayout::Ty(tcx.types.usize)
                 }
 
                 ty::Projection(_)

src/test/ui/async-await/dyn-star-trait-const.rs

@@ -1,6 +1,6 @@
 // run-pass
-// ignore-test
 #![feature(async_fn_in_traits)]
+#![allow(unused)]
 
 use std::fmt::Debug;