Hoist warning message from fill_from_parsed_config into helper function

Now that we're keeping track of all the unstable options we don't need
to emmit the warning when ``config.fill_from_parsed_config`` is called.

This also has the added benefit of removing a side-effect from calling
``config.fill_from_parsed_config``, and instead the entire warning
string is returned from a pure helper function, which made testing the
warning message easier.

Author: ytmimi

src/config/config_type.rs

@@ -163,6 +163,33 @@ macro_rules! create_config {
             pub fn using_unstable_options_on_stable_channel(&self) -> bool {
                 self.configured_unstable_options.is_some()
             }
+
+            /// Return a String warning users about all unstable options used on the stable channel
+            #[allow(unreachable_pub)]
+            pub fn unstable_options_warning_message(&self) -> Option<String> {
+                let unstable_options = self.unstable_options_set_on_stable_channel()?;
+                let upgrade_to_abort_message = "\nSet `abort_on_unrecognised_options = true` \
+                to convert this warning into an error\n\n";
+                // Capacity of 120 * len should be enough space for all options.
+                // the Warning string below is ~80 characters long so there is
+                // ~40 char buffer for option name and value
+                let mut output = String::with_capacity(
+                    upgrade_to_abort_message.len() + unstable_options.len() * 120
+                );
+
+                for option in unstable_options {
+                    output.push_str(
+                        &format!("Warning: can't set `{}`, unstable features are only \
+                            available in nightly channel.\n", option
+                        )
+                    );
+                }
+
+                output.push_str(upgrade_to_abort_message);
+
+                Some(output)
+            }
+
             fn fill_from_parsed_config(mut self, parsed: PartialConfig, dir: &Path) -> Config {
                 let mut unstable_options = vec![];
             $(
@@ -175,8 +202,6 @@ macro_rules! create_config {
                             self.$i.1 = true;
                             self.$i.2 = val;
                         } else {
-                            eprintln!("Warning: can't set `{} = {:?}`, unstable features are only \
-                                       available in nightly channel.", stringify!($i), val);
                             // only set abort_on_unrecognised_options, and store all other
                             // nightly only options
                             if stringify!($i) != "abort_on_unrecognised_options" {

src/config/mod.rs

@@ -578,6 +578,39 @@ mod test {
         assert!(config.using_unstable_options_on_stable_channel())
     }
 
+    #[test]
+    fn test_warning_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 warning = "\
+Warning: can't set `reorder_impl_items = true`, unstable features are only available in \
+nightly channel.
+
+Set `abort_on_unrecognised_options = true` to convert this warning into an error
+
+";
+        assert_eq!(warning, config.unstable_options_warning_message().unwrap())
+    }
+
+    #[test]
+    fn test_no_warning_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_warning_message().is_none())
+    }
+
     #[test]
     fn test_dump_default_config() {
         let default_config = format!(