make it more obvious that the size is not relevant

Author: RalfJung

src/librustc_mir/interpret/place.rs

@@ -353,9 +353,10 @@ where
         // Offset may need adjustment for unsized fields
         let (meta, offset) = if field_layout.is_unsized() {
             // re-use parent metadata to determine dynamic field layout
-            let (_, align) = self.size_and_align_of(base.meta, field_layout)?
-                // If this is an extern type, we fall back to its static size and alignment.
-                .unwrap_or_else(|| base.layout.size_and_align());
+            let align = self.size_and_align_of(base.meta, field_layout)?
+                .map(|(_, align)| align)
+                // If this is an extern type, we fall back to its static alignment.
+                .unwrap_or_else(|| base.layout.align);
             (base.meta, offset.abi_align(align))
         } else {
             // base.meta could be present; we might be accessing a sized field of an unsized