rust SkipWhile error

   Compiling chell v0.1.0 (file:///home/chell/chell)
error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> sasl.rs:45:58
   |
45 | ) -> Result<(Option<Message>, futures::stream::SkipWhile<_, _, _>), ()> {
   |                                                          ^ not allowed in type signatures

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> sasl.rs:45:61
   |
45 | ) -> Result<(Option<Message>, futures::stream::SkipWhile<_, _, _>), ()> {
   |                                                             ^ not allowed in type signatures

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> sasl.rs:45:64
   |
45 | ) -> Result<(Option<Message>, futures::stream::SkipWhile<_, _, _>), ()> {
   |                                                                ^ not allowed in type signatures

error: aborting due to 3 previous errors

error: Could not compile `chell`.

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