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

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -10,7 +10,7 @@ use rustc_middle::bug;
 use rustc_middle::mir::interpret::{InterpResult, Scalar};
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
 use rustc_middle::mir::*;
-use rustc_middle::ty::layout::LayoutOf;
+use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_mir_dataflow::value_analysis::{
     Map, PlaceIndex, State, TrackElem, ValueAnalysis, ValueAnalysisWrapper, ValueOrPlace,
@@ -285,9 +285,11 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                 let val = match null_op {
                     NullOp::SizeOf if layout.is_sized() => layout.size.bytes(),
                     NullOp::AlignOf if layout.is_sized() => layout.align.abi.bytes(),
-                    NullOp::OffsetOf(fields) => {
-                        layout.offset_of_subfield(&self.ecx, fields.iter()).bytes()
-                    }
+                    NullOp::OffsetOf(fields) => self
+                        .ecx
+                        .tcx
+                        .offset_of_subfield(self.ecx.param_env(), layout, fields.iter())
+                        .bytes(),
                     _ => return ValueOrPlace::Value(FlatSet::Top),
                 };
                 FlatSet::Elem(Scalar::from_target_usize(val, &self.tcx))