feat(core): remove useless impls of the callback traits, add a useful one

Author: yvt

src/core/mod.rs

@@ -139,14 +139,8 @@ pub trait UnlockCallback<InProgress> {
     fn complete(&mut self, in_progress: InProgress);
 }
 
-impl LockCallback<()> for () {
-    type Output = ();
-    fn in_progress(self) -> (Self::Output, ()) {
-        ((), ())
+impl UnlockCallback<!> for () {
+    fn complete(&mut self, in_progress: !) {
+        match in_progress {}
     }
-    fn complete(self) -> Self::Output {}
-}
-
-impl UnlockCallback<()> for () {
-    fn complete(&mut self, _in_progress: ()) {}
 }