add helper methods for accessing struct tail

compiler/rustc_middle/src/ty/mod.rs

@@ -2028,6 +2028,22 @@ impl VariantDef {
 
         &self.fields[FieldIdx::from_u32(0)]
     }
+
+    /// Returns the last field in this variant, if present.
+    #[inline]
+    pub fn tail_opt(&self) -> Option<&FieldDef> {
+        self.fields.raw.last()
+    }
+
+    /// Returns the last field in this variant.
+    ///
+    /// # Panics
+    ///
+    /// Panics, if the variant has no fields.
+    #[inline]
+    pub fn tail(&self) -> &FieldDef {
+        self.tail_opt().expect("expected unsized ADT to have a tail field")
+    }
 }
 
 impl PartialEq for VariantDef {