offset_of: allow (unstably) taking the offset of slice tail fields

compiler/rustc_middle/src/ty/layout.rs

@@ -1351,3 +1351,37 @@ pub trait FnAbiOf<'tcx>: FnAbiOfHelpers<'tcx> {
 }
 
 impl<'tcx, C: FnAbiOfHelpers<'tcx>> FnAbiOf<'tcx> for C {}
+
+impl<'tcx> TyCtxt<'tcx> {
+    pub fn offset_of_subfield<I>(
+        self,
+        param_env: ty::ParamEnv<'tcx>,
+        mut layout: TyAndLayout<'tcx>,
+        indices: I,
+    ) -> Size
+    where
+        I: Iterator<Item = (VariantIdx, FieldIdx)>,
+    {
+        let cx = LayoutCx { tcx: self, param_env };
+        let mut offset = Size::ZERO;
+
+        for (variant, field) in indices {
+            layout = layout.for_variant(&cx, variant);
+            let index = field.index();
+            offset += layout.fields.offset(index);
+            layout = layout.field(&cx, index);
+            if !layout.is_sized() {
+                // If it is not sized, then the tail must still have at least a known static alignment.
+                let tail = self.struct_tail_erasing_lifetimes(layout.ty, param_env);
+                if !matches!(tail.kind(), ty::Slice(..)) {
+                    bug!(
+                        "offset of not-statically-aligned field (type {:?}) cannot be computed statically",
+                        layout.ty
+                    );
+                }
+            }
+        }
+
+        offset
+    }
+}