post const args in type dependent paths cleanup

[lcnr]

#74113 has some open FIXMEs

• consider caching opt_const_param_of on disk (perf)
• consider first calling tcx.def_kind(def_id) in opt_const_param_of to improve incremental
• add a helper method for

if def.const_param_did.is_none() {
    if let const_param_did @ Some(_) = tcx.opt_const_param_of(def.did) {
        return tcx.query(ty::WithOptConstParam { const_param_did, ..def });
    }
}

[eddyb]

Copying a couple of comments here:

• Support const args in type dependent paths (Take 2) #74113 (comment) (src/librustc_typeck/collect/type_of.rs / fn opt_const_param_of)

  I think you can make this cheaper incremental-wise by checking tcx.def_kind(def_id) first, because that won't add a dependency to the entire definition pointed to by def_id, just the simple DefKind enum (but feel free to file this under further experimentation).

• Support const args in type dependent paths (Take 2) #74113 (comment) (referring to the if ... { if let ... { return ... } } pattern)

  I just realized that this two-step process could probably be a helper method on WithOptConstParam (and do e.g. if let Some(def) = def.upgrade_with_const_param(tcx) or something like that, with the method documentation describing this kind of use pattern).