Require build_on_save for build_command config option

Author: Xanewok

src/config.rs

@@ -159,8 +159,10 @@ pub struct Config {
     pub show_hover_context: bool,
     /// EXPERIMENTAL (needs unstable features)
     /// If set, executes a given program responsible for rebuilding save-analysis
-    /// to be loaded by the RLS. The program given should output a list of 
+    /// to be loaded by the RLS. The program given should output a list of
     /// resulting .json files on stdout.
+    /// Currently also requires `build_on_save` to be set to true, since external
+    /// commands have no insight into in-memory text buffers (unsaved files).
     pub build_command: Option<String>,
 }
 
@@ -228,6 +230,9 @@ impl Config {
             self.build_lib = Inferrable::Inferred(false);
             self.cfg_test = false;
             self.build_command = None;
+        } else if !self.build_on_save {
+            self.build_command = None;
+            eprintln!("`build_command` also requires enabled `build_on_save` option");
         }
     }