Only permit unstable features on nightly

And make workspace mode unstable

Author: nrc

src/actions/mod.rs

@@ -802,7 +802,10 @@ impl ActionHandler {
                          .and_then(|value| Config::deserialize(value));
 
         let new_config = match config {
-            Ok(value) => value,
+            Ok(mut value) => {
+                value.normalise();
+                value
+            }
             Err(err) => {
                 debug!("Received unactionable config: {:?} (error: {:?})", params.settings, err);
                 return;

src/config.rs

@@ -111,7 +111,7 @@ pub struct Config {
 
 impl Default for Config {
     fn default() -> Config {
-        Config {
+        let mut result = Config {
             sysroot: None,
             target: None,
             rustflags: None,
@@ -127,7 +127,9 @@ impl Default for Config {
             clear_env_rust_log: true,
             build_on_save: false,
             use_crate_blacklist: true,
-        }
+        };
+        result.normalise();
+        result
     }
 }
 
@@ -139,6 +141,27 @@ impl Config {
         *self = new;
     }
 
+    /// Ensures that unstable options are only allowed if `unstable_features` is
+    /// true and that is not allowed on stable release channels.
+    pub fn normalise(&mut self) {
+        let allow_unstable = option_env!("CFG_RELEASE_CHANNEL").map(|c| c == "nightly").unwrap_or(true);
+
+        if !allow_unstable {
+            if self.unstable_features {
+                eprintln!("`unstable_features` setting can only be used on nightly channel");
+            }
+            self.unstable_features = false;
+        }
+
+        if !self.unstable_features {
+            if self.workspace_mode {
+                eprintln!("`workspace_mode` setting is unstable; ignored");
+            }
+            self.workspace_mode = false;
+            self.analyze_package = None;
+        }
+    }
+
     pub fn needs_inference(&self) -> bool {
         match (&self.build_lib, &self.build_bin) {
             (&Inferrable::None, _) |