Also emit future compatibility warning if there is a dot in the codeblock tag

Author: GuillaumeGomez

src/librustdoc/passes/check_custom_code_classes.rs

@@ -21,6 +21,10 @@ pub(crate) const CHECK_CUSTOM_CODE_CLASSES: Pass = Pass {
 };
 
 pub(crate) fn check_custom_code_classes(krate: Crate, cx: &mut DocContext<'_>) -> Crate {
+    if cx.tcx.features().custom_code_classes_in_docs {
+        // Nothing to check here if the feature is enabled.
+        return krate;
+    }
     let mut coll = CustomCodeClassLinter { cx };
 
     coll.fold_crate(krate)
@@ -40,49 +44,56 @@ impl<'a, 'tcx> DocFolder for CustomCodeClassLinter<'a, 'tcx> {
 #[derive(Debug)]
 struct TestsWithCustomClasses {
     custom_classes_found: Vec<String>,
+    contains_dot: bool,
 }
 
 impl crate::doctest::Tester for TestsWithCustomClasses {
     fn add_test(&mut self, _: String, config: LangString, _: usize) {
+        self.contains_dot |= config.original.contains('.');
         self.custom_classes_found.extend(config.added_classes.into_iter());
     }
 }
 
+fn emit_warning(cx: &DocContext<'_>, item: &Item, custom_classes: Option<&[String]>) {
+    let span = item.attr_span(cx.tcx);
+    let sess = &cx.tcx.sess.parse_sess;
+    let mut err = sess
+        .span_diagnostic
+        .struct_span_warn(span, "custom classes in code blocks will change behaviour");
+    add_feature_diagnostics_for_issue(
+        &mut err,
+        sess,
+        sym::custom_code_classes_in_docs,
+        GateIssue::Language,
+        false,
+    );
+
+    if let Some(custom_classes) = custom_classes {
+        err.note(
+            // This will list the wrong items to make them more easily searchable.
+            // To ensure the most correct hits, it adds back the 'class:' that was stripped.
+            format!("found these custom classes: class={}", custom_classes.join(",class=")),
+        );
+    }
+
+    // A later feature_err call can steal and cancel this warning.
+    err.stash(span, StashKey::EarlySyntaxWarning);
+}
+
 pub(crate) fn look_for_custom_classes<'tcx>(cx: &DocContext<'tcx>, item: &Item) {
     if !item.item_id.is_local() {
         // If non-local, no need to check anything.
         return;
     }
 
-    let mut tests = TestsWithCustomClasses { custom_classes_found: vec![] };
+    let mut tests = TestsWithCustomClasses { custom_classes_found: vec![], contains_dot: false };
 
     let dox = item.attrs.doc_value();
     find_codes(&dox, &mut tests, ErrorCodes::No, false, None, true, true);
 
-    if !tests.custom_classes_found.is_empty() && !cx.tcx.features().custom_code_classes_in_docs {
-        let span = item.attr_span(cx.tcx);
-        let sess = &cx.tcx.sess.parse_sess;
-        let mut err = sess
-            .span_diagnostic
-            .struct_span_warn(span, "custom classes in code blocks will change behaviour");
-        add_feature_diagnostics_for_issue(
-            &mut err,
-            sess,
-            sym::custom_code_classes_in_docs,
-            GateIssue::Language,
-            false,
-        );
-
-        err.note(
-            // This will list the wrong items to make them more easily searchable.
-            // To ensure the most correct hits, it adds back the 'class:' that was stripped.
-            format!(
-                "found these custom classes: class={}",
-                tests.custom_classes_found.join(",class=")
-            ),
-        );
-
-        // A later feature_err call can steal and cancel this warning.
-        err.stash(span, StashKey::EarlySyntaxWarning);
+    if !tests.custom_classes_found.is_empty() {
+        emit_warning(cx, item, Some(&tests.custom_classes_found));
+    } else if tests.contains_dot {
+        emit_warning(cx, item, None);
     }
 }