Add #[must_use] to types and methods (#3395)

Co-authored-by: Theodore Bjernhed <[email protected]>

axum-core/src/error.rs

@@ -16,6 +16,7 @@ impl Error {
     }
 
     /// Convert an `Error` back into the underlying boxed trait object.
+    #[must_use]
     pub fn into_inner(self) -> BoxError {
         self.inner
     }