Add comments to remind everyone to keep the get_vtable impls in sync

Author: oli-obk

Diff for: src/librustc_codegen_llvm/meth.rs

@@ -87,6 +87,10 @@ pub fn get_vtable(
     let nullptr = C_null(Type::i8p(cx));
 
     let (size, align) = cx.size_and_align_of(ty);
+    // /////////////////////////////////////////////////////////////////////////////////////////////
+    // If you touch this code, be sure to also make the corresponding changes to
+    // `get_vtable` in rust_mir/interpret/traits.rs
+    // /////////////////////////////////////////////////////////////////////////////////////////////
     let mut components: Vec<_> = [
         callee::get_fn(cx, monomorphize::resolve_drop_in_place(cx.tcx, ty)),
         C_usize(cx, size.bytes()),

Diff for: src/librustc_mir/interpret/traits.rs

@@ -48,6 +48,10 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
 
         let ptr_size = self.pointer_size();
         let ptr_align = self.tcx.data_layout.pointer_align;
+        // /////////////////////////////////////////////////////////////////////////////////////////
+        // If you touch this code, be sure to also make the corresponding changes to
+        // `get_vtable` in rust_codegen_llvm/meth.rs
+        // /////////////////////////////////////////////////////////////////////////////////////////
         let vtable = self.memory.allocate(
             ptr_size * (3 + methods.as_ref().map_or(0, |m| m.len() as u64)),
             ptr_align,