Only run test on nightly

src/test/mod.rs

@@ -259,9 +259,9 @@ fn assert_output(source: &Path, expected_filename: &Path) {
 #[test]
 fn idempotence_tests() {
     run_test_with(&TestSetting::default(), || {
-        match option_env!("CFG_RELEASE_CHANNEL") {
-            None | Some("nightly") => {}
-            _ => return, // these tests require nightly
+        // these tests require nightly
+        if !is_nightly() {
+            return;
         }
         // Get all files in the tests/target directory.
         let files = get_test_files(Path::new("tests/target"), true);
@@ -277,9 +277,9 @@ fn idempotence_tests() {
 // no warnings are emitted.
 #[test]
 fn self_tests() {
-    match option_env!("CFG_RELEASE_CHANNEL") {
-        None | Some("nightly") => {}
-        _ => return, // Issue-3443: these tests require nightly
+    // Issue-3443: these tests require nightly
+    if !is_nightly() {
+        return;
     }
     let mut files = get_test_files(Path::new("tests"), false);
     let bin_directories = vec!["cargo-fmt", "git-rustfmt", "bin", "format-diff"];
@@ -313,6 +313,11 @@ fn self_tests() {
     );
 }
 
+fn is_nightly() -> bool {
+    let release_channel = option_env!("CFG_RELEASE_CHANNEL");
+    release_channel.is_none() || release_channel == Some("nightly")
+}
+
 #[test]
 fn stdin_formatting_smoke_test() {
     let input = Input::Text("fn main () {}".to_owned());
@@ -426,6 +431,16 @@ fn check_files(files: Vec<PathBuf>, opt_config: &Option<PathBuf>) -> (Vec<Format
     let mut reports = vec![];
 
     for file_name in files {
+        let sig_comments = read_significant_comments(&file_name);
+        if sig_comments.contains_key("unstable") && !is_nightly() {
+            debug!(
+                "Skipping '{}' because it requires unstable \
+                 features which are only available on nightly...",
+                file_name.display()
+            );
+            continue;
+        }
+
         debug!("Testing '{}'...", file_name.display());
 
         match idempotent_check(&file_name, &opt_config) {

tests/source/normalize_multiline_doc_attribute.rs

@@ -1,3 +1,5 @@
+// rustfmt-unstable: true
+
 #[doc = "This comment
 is split
 on multiple lines"]

tests/target/normalize_multiline_doc_attribute.rs

@@ -1,3 +1,5 @@
+// rustfmt-unstable: true
+
 ///This comment
 ///is split
 ///on multiple lines