Allow unstable features under "dev" channel

This brings it in line with what's used by Rust and Rustfmt.

rls/src/config.rs

@@ -103,6 +103,17 @@ impl<T> AsRef<T> for Inferrable<T> {
     }
 }
 
+/// Returns whether RLS was built using nightly channel.
+///
+/// It is very similar to what rust and rustfmt uses [1] - it relies on
+/// CFG_RELEASE_CHANNEL being set by Rust bootstrap.
+/// In case the env var is missing, we assume that we're built by Cargo and are
+/// using nightly since that's the only channel supported right now.
+/// [1]: https://github.com/rust-lang/rustfmt/blob/dfa94d150555da40780413d7f1a1378565208c99/src/config/config_type.rs#L53-L67
+pub fn is_nightly() -> bool {
+    option_env!("CFG_RELEASE_CHANNEL").map_or(true, |c| c == "nightly" || c == "dev")
+}
+
 /// RLS configuration options.
 #[derive(Clone, Debug, Deserialize)]
 #[allow(missing_docs)]
@@ -251,10 +262,7 @@ impl Config {
     /// 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 !is_nightly() {
             if self.unstable_features {
                 eprintln!("`unstable_features` setting can only be used on nightly channel");
             }