> Is the syntax suggestion not applicable?

Lets look at proposed syntax

  fn list_items<lifetime life0, lifetime life1, lifetime async_trait>(
         &lifetime life0 self,
         collection_href: &lifetime life1 str,
     ) -> Pin<Box<dyn Future<Output = Result<Vec<ItemRef>, Error>> + Send + async_trait>>
     where
         Self: life0,
         life0: async_trait,
         life1: async_trait,

I'm not going to pretend I understood what mrweasel meant fully, so I assume we can either omit generic or in parameter declaration (so I went with omitting lifetime keyword in parameters):

  fn list_items<lifetime life0, lifetime life1, lifetime async_trait>(
         &life0 self,
         collection_href: &life1 str,
     ) -> Pin<Box<dyn Future<Output = Result<Vec<ItemRef>, Error>> + Send + async_trait>>
     where
         Self: life0,
         life0: async_trait,
         life1: async_trait,

I guess you might be able to omit the "generic part" like so (it might be impossible, lifetime are just generics useful for lifetime tracking):

  fn list_items(
         &lifetime life0 self,
         collection_href: &lifetime life1 str,
     ) -> Pin<Box<dyn Future<Output = Result<Vec<ItemRef>, Error>> + Send + async_trait>>
     where
         Self: life0,
         life0: async_trait,
         life1: async_trait,

In both cases, you get a huge verbosity increase, and mix between not knowing if a value like `Self: x` is a trait with lower case or a lifetime.

So you trade verbosity for more ambiguity and programmer confusion, and possibly worse error reporting (is &a b a lifetime or a missing comma e.g. &a, b).