Add debug assertion for custom feature namespaces

src/wrapper/clap/features.rs

@@ -85,7 +85,19 @@ impl ClapFeature {
             ClapFeature::Stereo => "stereo",
             ClapFeature::Surround => "surround",
             ClapFeature::Ambisonic => "ambisonic",
-            ClapFeature::Custom(s) => s,
+            ClapFeature::Custom(s) => {
+                // Custom features must be prefixed with a namespace. We'll use `.split(':').all()`
+                // here instead of `.split_once()` in case the user for whatever reason uses more
+                // than one colon (which the docs don't say anything about, but uh yeah).
+                nih_debug_assert!(
+                    s.contains(':') && s.split(':').all(|x| !x.is_empty()),
+                    "'{s}' is not a valid feature, custom features must be namespaced (e.g. \
+                     'nih:{s}')",
+                    s = s
+                );
+
+                s
+            }
         }
     }
 }