Where bounds are ignored as part of trait type parameters on an impl

[richardwhiuk]

Where bounds are ignored, but suggested by the compiler, when implementing trait objects with second order types.

A full example is available at: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d61ff8acea97a202e00ea6fc32def211

The bound on line 21 is ignored:

    <T as hyper::service::MakeService<&'a SC>>::Service: 'static,

but suggested (with inaccurate markers):

error[E0310]: the associated type `<T as hyper::service::make_service::MakeService<&SC>>::Service` may not live long enough
  --> src/lib.rs:10:32
   |
10 | impl<'a, T, SC, RC, F, OB, ME> hyper::service::MakeService<&'a SC>
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider adding an explicit lifetime bound `<T as hyper::service::make_service::MakeService<&SC>>::Service: 'static`...
note: ...so that the type `<T as hyper::service::make_service::MakeService<&SC>>::Service` will meet its required lifetime bounds
  --> src/lib.rs:10:32
   |
10 | impl<'a, T, SC, RC, F, OB, ME> hyper::service::MakeService<&'a SC>
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In fact, in the example given, the error is repeated four times for good measure, in a standard cargo build.

The workaround is to add another type parameter to the function declaration - i.e. changing:

impl<'a, T, S, SC, RC, F, OB, ME> hyper::service::MakeService<&'a SC>
    for MakeService<T, RC>
where
    T: hyper::service::MakeService<
        &'a SC,
        ReqBody = hyper::Body,
        ResBody = OB,
        Error = hyper::Error,
        MakeError = ME,
        Future = F,
    >,
    <T as hyper::service::MakeService<&'a SC>>::Service: 'static,

to

impl<'a, T, SC, RC, F, OB, ME> hyper::service::MakeService<&'a SC>
    for MakeService<T, RC>
where
    T: hyper::service::MakeService<
        &'a SC,
        ReqBody = hyper::Body,
        ResBody = OB,
        Error = hyper::Error,
        Service = S,
        MakeError = ME,
        Future = F,
    >,
    S: 'static,

which additionally requires duplicating any bounds on the inner type (i.e,

    type Service: Service<
        ReqBody=Self::ReqBody,
        ResBody=Self::ResBody,
        Error=Self::Error,
    >;

specified at https://docs.rs/hyper/0.12.25/src/hyper/service/make_service.rs.html#10-44).

This results in function declarations with large numbers of type parameters, and more complex bounds than is strictly necessary.

[daboross]

This seems related to #20671?