Fix subslice length handling for slices

Author: bjorn3

src/base.rs

@@ -690,10 +690,11 @@ pub fn trans_place<'tcx>(
                     ty::Slice(elem_ty) => {
                         assert!(from_end, "slice subslices should be `from_end`");
                         let elem_layout = fx.layout_of(elem_ty);
-                        let (ptr, _len) = cplace.to_ptr_maybe_unsized(fx);
+                        let (ptr, len) = cplace.to_ptr_maybe_unsized(fx);
+                        let len = len.unwrap();
                         cplace = CPlace::for_ptr_with_extra(
                             ptr.offset_i64(fx, elem_layout.size.bytes() as i64 * from as i64),
-                            fx.bcx.ins().iconst(pointer_ty(fx.tcx), (to as u64 - from as u64) as i64),
+                            fx.bcx.ins().iadd_imm(len, -(from as i64 + to as i64)),
                             cplace.layout(),
                         );
                     }