allow empty field paths, but deny walking them early

Author: BennoLossin

compiler/rustc_middle/src/ty/field_path.rs

@@ -46,6 +46,7 @@ impl<'tcx> FieldPath<'tcx> {
         container: Ty<'tcx>,
         mut walker: impl FnMut(Ty<'tcx>, Symbol, Ty<'tcx>, bool) -> ControlFlow<T>,
     ) -> Option<T> {
+        assert!(!self.0.is_empty(), "tried walking empty field path");
         let mut cur = container;
         for (i, (variant, field)) in self.iter().enumerate() {
             let last = i == self.0.len() - 1;

compiler/rustc_middle/src/ty/sty.rs

@@ -673,7 +673,6 @@ impl<'tcx> Ty<'tcx> {
         container: Ty<'tcx>,
         field_path: FieldPath<'tcx>,
     ) -> Ty<'tcx> {
-        assert!(!field_path.0.is_empty());
         Ty::new(tcx, Field(container, field_path))
     }