Add docs, remove code, change subtyper code

src/base.rs

@@ -872,13 +872,11 @@ pub(crate) fn codegen_place<'tcx>(
 
     for elem in place.projection {
         match elem {
-            PlaceElem::Subtype(_) => {
-                continue;
-            }
             PlaceElem::Deref => {
                 cplace = cplace.place_deref(fx);
             }
             PlaceElem::OpaqueCast(ty) => bug!("encountered OpaqueCast({ty}) in codegen"),
+            PlaceElem::Subtype(ty) => cplace = cplace.place_transmute_type(fx, ty),
             PlaceElem::Field(field, _ty) => {
                 cplace = cplace.place_field(fx, field);
             }

src/value_and_place.rs

@@ -674,6 +674,14 @@ fn transmute_scalar<'tcx>(
         }
     }
 
+    pub(crate) fn place_transmute_type(
+        self,
+        fx: &mut FunctionCx<'_, '_, 'tcx>,
+        ty: Ty<'tcx>,
+    ) -> CPlace<'tcx> {
+        CPlace { inner: self.inner, layout: fx.layout_of(fx.monomorphize(ty)) }
+    }
+
     pub(crate) fn place_field(
         self,
         fx: &mut FunctionCx<'_, '_, 'tcx>,