Use tcx.require_lang_item instead of unwrapping

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -1,7 +1,7 @@
 //! Concrete error types for all operations which may be invalid in a certain const context.
 
 use hir::def_id::LocalDefId;
-use hir::ConstContext;
+use hir::{ConstContext, LangItem};
 use rustc_errors::{
     error_code, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
 };
@@ -304,7 +304,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     err.span_note(deref_target, "deref defined here");
                 }
 
-                diag_trait(&mut err, self_ty, tcx.lang_items().deref_trait().unwrap());
+                diag_trait(&mut err, self_ty, tcx.require_lang_item(LangItem::Deref, Some(span)));
                 err
             }
             _ if tcx.opt_parent(callee) == tcx.get_diagnostic_item(sym::ArgumentV1Methods) => {