Remove needless alloc_slice

Don't invoke alloc_slice.

Arenas are temporary,
empty slices are eternal!

compiler/rustc_mir/src/const_eval/mod.rs

@@ -50,7 +50,7 @@ pub(crate) fn destructure_const<'tcx>(
     let (field_count, variant, down) = match val.ty.kind() {
         ty::Array(_, len) => (usize::try_from(len.eval_usize(tcx, param_env)).unwrap(), None, op),
         ty::Adt(def, _) if def.variants.is_empty() => {
-            return mir::DestructuredConst { variant: None, fields: tcx.arena.alloc_slice(&[]) };
+            return mir::DestructuredConst { variant: None, fields: &[] };
         }
         ty::Adt(def, _) => {
             let variant = ecx.read_discriminant(op).unwrap().1;