XXX: no chaining of Diagnostic methods.

that's what `DiagnosticBuilder` is for.

Author: nnethercote

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1291,7 +1291,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn suggest_slice_method_if_applicable(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'tcx>,
         place: Place<'tcx>,
         borrowed_place: Place<'tcx>,
     ) {