Add helper function to generate abort message

This helper function has a similiar responsibiltiy as the helper
function used to generate the warning message.

Because it's a pure function that returns Option<String> it's easy to
test.

Author: ytmimi

src/config/config_type.rs

@@ -190,6 +190,27 @@ macro_rules! create_config {
                 Some(output)
             }
 
+            /// Returns a String explaining why rustfmt exited early when using unstable options
+            /// on the stable channel
+            #[allow(unreachable_pub)]
+            pub fn unstable_options_abort_message(&self) -> Option<String> {
+                let unstable_options = self.unstable_options_set_on_stable_channel()?;
+                let first_line = "Can't set nightly options when using stable rustfmt\n";
+                let to_warning_message = "\nSet `abort_on_unrecognised_options = false` \
+                to convert this error into a warning\n\n";
+                // 40 = buffer we'll use for all options. Most will be less than 40 char long
+                let mut output = String::with_capacity(
+                    to_warning_message.len() + first_line.len() + unstable_options.len() * 40
+                );
+                output.push_str(first_line);
+
+                for option in unstable_options {
+                    output.push_str(&format!("    - `{}`\n", option));
+                }
+                output.push_str(to_warning_message);
+                Some(output)
+            }
+
             fn fill_from_parsed_config(mut self, parsed: PartialConfig, dir: &Path) -> Config {
                 let mut unstable_options = vec![];
             $(

src/config/mod.rs

@@ -611,6 +611,42 @@ Set `abort_on_unrecognised_options = true` to convert this warning into an error
         assert!(config.unstable_options_warning_message().is_none())
     }
 
+    #[test]
+    fn test_abort_message_when_using_unstable_options_on_stable_channel() {
+        if crate::is_nightly_channel!() {
+            // This test requires non-nightly
+            return;
+        }
+        let toml = r#"
+            reorder_impl_items = true
+        "#;
+        let config = Config::from_toml(toml, Path::new("")).unwrap();
+        let abort_message = "\
+Can't set nightly options when using stable rustfmt
+    - `reorder_impl_items = true`
+
+Set `abort_on_unrecognised_options = false` to convert this error into a warning
+
+";
+        assert_eq!(
+            abort_message,
+            config.unstable_options_abort_message().unwrap()
+        )
+    }
+
+    #[test]
+    fn test_no_abort_message_when_using_unstable_options_on_nightly() {
+        if !crate::is_nightly_channel!() {
+            // This test requires nightly
+            return;
+        }
+        let toml = r#"
+            reorder_impl_items = true
+        "#;
+        let config = Config::from_toml(toml, Path::new("")).unwrap();
+        assert!(config.unstable_options_abort_message().is_none())
+    }
+
     #[test]
     fn test_dump_default_config() {
         let default_config = format!(