Function eagerly_handle_placeholders_in_universe 

pub fn eagerly_handle_placeholders_in_universe<Infcx, I>(
    infcx: &Infcx,
    constraint: RegionConstraint<I>,
    u: UniverseIndex,
) -> RegionConstraint<I>
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,

Takes any constraints involving placeholders from the current universe and eagerly checks them. This can be done a few ways:

• There’s an assumption on the binder introducing the placeholder which means the constraint is satisfied (true)
• There’s assumptions on the binder introducing the placeholder which allow us to rewrite the constraint in terms of lower universe variables. For example given for<'a> where('b: 'a) { prove(T: '!a_u1) } we can convert this constraint to T: 'b which no longer references anything from u1.
• There are no relevant assumptions so we can neither rewrite the constraint nor consider it satisfied (false)
• We failed to compute the full set of assumptions when entering the binder corresponding to u. (ambiguity)

After handling all of the region constraints in u we then evaluate the entire constraint as much as possible, propagating true/false/ambiguity as close to the root of the constraint as we can. The returned constraint should be checked for whether it is true/false/ambiguous as that should affect the result of whatever operation required entering the binder corresponding to u.