Function conditional_bound_lifetime 

pub(crate) fn conditional_bound_lifetime() -> Lifetime

The binder of a conditional bound, for<'__mx_cond> FieldTy: Trait.

A where clause on a concrete type that does not hold is an error where the impl is written (E0277). Behind a higher-ranked binder the same bound is checked where the impl is used, so the impl just does not apply to that type. Derives use it for impls that exist only when the user’s field types allow them, such as a companion’s Debug. The name stays clear of user lifetimes, which a binder must not shadow (E0496).