Don't emit an error if the custom_code_classes_in_docs feature is disabled when its syntax is used.

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -208,7 +208,14 @@ impl<'a, 'b> DocVisitor for CoverageCalculator<'a, 'b> {
                 let has_docs = !i.attrs.doc_strings.is_empty();
                 let mut tests = Tests { found_tests: 0 };
 
-                find_testable_code(&i.doc_value(), &mut tests, ErrorCodes::No, false, None);
+                find_testable_code(
+                    &i.doc_value(),
+                    &mut tests,
+                    ErrorCodes::No,
+                    false,
+                    None,
+                    self.ctx.tcx.features().custom_code_classes_in_docs,
+                );
 
                 let has_doc_example = tests.found_tests != 0;
                 let hir_id = DocContext::as_local_hir_id(self.ctx.tcx, i.item_id).unwrap();