Add `Default` implementation for `Features` so backends need only specify features they actually support.

src/backends/appkit.rs

@@ -95,9 +95,9 @@ impl Backend for AppKit {
         Features {
             stop: true,
             rate: true,
-            pitch: false,
             volume: true,
             is_speaking: true,
+            ..Default::default(),
         }
     }
 

src/backends/tolk.rs

@@ -22,10 +22,7 @@ impl Backend for Tolk {
     fn supported_features(&self) -> Features {
         Features {
             stop: true,
-            rate: false,
+            ..Default::default(),
-            pitch: false,
-            volume: false,
-            is_speaking: false,
         }
     }
 

src/lib.rs

@@ -48,6 +48,18 @@ pub struct Features {
     pub is_speaking: bool,
 }
 
+impl Default for Features {
+    fn default() -> Self {
+        Self {
+            stop: false,
+            rate: false,
+            pitch: false,
+            volume: false,
+            is_speaking: false,
+        }
+    }
+}
+
 #[derive(Debug, Error)]
 pub enum Error {
     #[error("IO error: {0}")]