incorrect `FromEnv` predicates around supertraits

[nikomatsakis]

The predicates for supertraits look wrong, as you can see from this example:

#![feature(rustc_attrs)]
#![allow(dead_code)]

trait Foo { }

#[rustc_dump_program_clauses] //~ ERROR program clause dump
trait Bar: Foo { }

I see this:

error: program clause dump
  --> lower_env1.rs:16:1
   |
16 | #[rustc_dump_program_clauses] //~ ERROR program clause dump
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: Implemented(Self: Bar) :- FromEnv(Self: Bar).
   = note: FromEnv(Self: Bar) :- FromEnv(Self: Bar).

But I expect FromEnv(Self: Foo) :- FromEnv(Self: Bar).

cc @rust-lang/wg-traits

[scalexm]

Could it be because of the fact that for the moment, we unconditionally skip the first predicate (because it’s supposed to be Self: Bar)? Maybe in that case we are actually skipping the Self: Foo predicate?

[scalexm]

cc @tmandry

[tmandry]

Yeah I strongly suspect you are correct @scalexm. It looks like that hack doesn't work in all cases. I'm working to remove the Self: Foo from predicates_of so the hack isn't necessary, but if we want a fix now, it should be possible to run through all the predicates and filter out the one that matches Self: Foo. I'll try to submit one of those fixes tomorrow, if possible.

[scalexm]

Closed by #51895.