Remove allow(rustc::potential_query_instability) in rustc_trait_selection

[CastilloDel]

Related to #84447, #103723 and indexmap-rs/indexmap#242

rustc_trait_selection still has some potential query instabilities, however, trying to remove them caused regressions. This issue tracks this problem after #103723 was merged. The idea is to wait until the drain_filter method is eventually added to IndexMap which would make the change easier and hopefully solve the regression.

[ndrewxie]

Correct me if I'm wrong, but the only place I see drain_filter being used on the map field of ProvisionalEvaluationCache is the following piece of code:

for (fresh_trait_pred, eval) in
    self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
{
    debug!(?fresh_trait_pred, ?eval, "on_completion");
}

This looks like it could just be replaced with a .retain without any performance penalty, right? Because debug doesn't need to own the value

I'll try it out later and see if it works

[ndrewxie]

Alright this appears to be resolved - can someone please close this issue? Thanks! (I messed up the wording of the PR and didn't link the issues properly)

[jyn514]

Sure thing, glad you got this working :)