fix ICE due to empty span and empty suggestions

Author: RalfJung

compiler/rustc_resolve/src/diagnostics.rs

@@ -2596,7 +2596,9 @@ fn show_candidates(
             );
             if let [first, .., last] = &path[..] {
                 let sp = first.ident.span.until(last.ident.span);
-                if sp.can_be_used_for_suggestions() {
+                // Our suggestion is empty, so make sure the span is not empty (or we'd ICE).
+                // Can happen for derive-generated spans.
+                if sp.can_be_used_for_suggestions() && !sp.is_empty() {
                     err.span_suggestion_verbose(
                         sp,
                         format!("if you import `{}`, refer to it directly", last.ident),