Function extend_cause_with_original_assoc_item_obligation

fn extend_cause_with_original_assoc_item_obligation<'tcx>(
    tcx: TyCtxt<'tcx>,
    item: Option<&Item<'tcx>>,
    cause: &mut ObligationCause<'tcx>,
    pred: Predicate<'tcx>,
)

Points the cause span of a super predicate at the relevant associated type.

Given a trait impl item:

impl TargetTrait for TargetType {
   type Assoc = SomeType;
}

And a super predicate of TargetTrait that has any of the following forms:

1. <OtherType as OtherTrait>::Assoc == <TargetType as TargetTrait>::Assoc
2. <<TargetType as TargetTrait>::Assoc as OtherTrait>::Assoc == OtherType
3. <TargetType as TargetTrait>::Assoc: OtherTrait

Replace the span of the cause with the span of the associated item:

impl TargetTrait for TargetType {
    type Assoc = SomeType;
//               ^^^^^^^^ this span
}

Note that bounds that can be expressed as associated item bounds are not super predicates. This means that form 2 and 3 from above are only relevant if the GenericArgsRef of the projection type are not its identity arguments.