Auto traits do not get de-duplicated properly when principal trait is auto trait #57057

This was an issue that came up during work on #55994.

Basically, a bug in the de-duplication code means that dyn Send is a different type from dyn Send + Send, and likewise dyn Sync + Send is different from dyn Sync + Send + Sync. However, this clearly should not be the case, and indeed dyn Send + Send is the same type as dyn Send + Send + Send. (Note, this is only tangentially related to the lint #56522.)

Apart from potentially surprising behaviour, the issue with the above lies with trait impls and coherence. I am unaware of a specific ICE that can arise because of it, but it is probably UB in general.

trait Foo {}

impl Foo for dyn Send {}

impl Foo for dyn Send + Send {}

playground

trait Foo {}

impl Foo for dyn Send + Send {}

impl Foo for dyn Send + Send + Send {}

playground

CC @nikomatsakis @arielb1