[chalkify] duplicate predicates with `predicates_defined_on`

[scalexm]

It seems like I missed this one when reviewing #51895:

rust/src/test/ui/chalkify/lower_env1.stderr

Lines 7 to 8 in c6807bb

	= note: FromEnv(Self: Foo) :- FromEnv(Self: Bar).
	= note: FromEnv(Self: Foo) :- FromEnv(Self: Bar).

(same duplicate just below in that same file)

I'm just opening this issue for reference, I'll take some time to investigate later.

cc @nikomatsakis if it's still there when you get back, cc @tmandry

[scalexm]

Hint: this duplication is only triggered when using the following form for a super trait:

trait Bar where Self: Foo {}

For the alternative syntax trait Bar: Foo {} as well as any where clause not of the form Self: ..., the output seems fine.

[scalexm]

Ok found the culprit: explicit_predicates_of collects the where clauses explicitly written on the trait, and then also adds those returned by super_predicates_of. But super_predicates_of collects both the « inline » super trait bounds e.g. trait Bar: Foo and the ones included as where clauses e.g. where Self: Foo, but the where Self: Foo was already collected before calling super_predicates_of, hence the duplicate.

Overall, this is not a big problem apart from giving chalk (and maybe some parts of rustc not using e.g. a HashMap or HashSet) the same clause twice. However for clean outputs, I’d just suggest having an additional explicit_super_predicates_of or something like that (which would not need to be added as a query), which would only return super trait bounds written in the form trait Bar: Foo and which would be used by explicit_predicates_of.

cc @nikomatsakis

[nikomatsakis]

Sounds about right to me