error: return type captures more lifetimes than trait definition
  --> $DIR/signature-mismatch.rs:17:47
   |
LL |     fn async_fn(&self, buff: &[u8]) -> impl Future<Output = Vec<u8>>;
   |                              - this lifetime was captured
...
LL |     fn async_fn<'a>(&self, buff: &'a [u8]) -> impl Future<Output = Vec<u8>> + 'a {
   |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: hidden type must only reference lifetimes captured by this impl trait
  --> $DIR/signature-mismatch.rs:11:40
   |
LL |     fn async_fn(&self, buff: &[u8]) -> impl Future<Output = Vec<u8>>;
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: hidden type inferred to be `impl Future<Output = Vec<u8>> + '_`

error: aborting due to previous error