Idea: impl chains ("else impls") for when other impls don't match

[Boscop]

A lot of the possible impl surface of a trait can't be impld in today's Rust because of the restrictions placed on impls.
I think it would be useful to have a way to express if THIS impl doesn't match, try THAT impl, in a chain, like in PureScript: purescript/purescript#2315

E.g. in Rust:

impl<.. /* some bounds */> Trait for Foo { ... }
else
impl<.. /* other bounds */> Trait for Foo { ... }
else
impl ...

---

It could even be possible to specify else impls for impls defined in dependency crates, if we can refer to them by a name.
That would require named impls, which there is already some demand for:

rust-lang/rfcs#2251

https://internals.rust-lang.org/t/looking-for-rfc-coauthors-on-named-impls/6275

[oli-obk]

This is a feature request to change the language and as such needs an RFC. I suggest you discuss this in the internals forum to figure out the caveats and then open an rfc