Function rustc_resolve::late::lifetimes::resolve_lifetimes_trait_definition

fn resolve_lifetimes_trait_definition(
    tcx: TyCtxt<'_>,
    local_def_id: LocalDefId
) -> ResolveLifetimes

Like resolve_lifetimes, but does not resolve lifetimes for trait items. Also does not generate any diagnostics.

This is ultimately a subset of the resolve_lifetimes work. It effectively resolves lifetimes only within the trait “header” – that is, the trait and supertrait list. In contrast, resolve_lifetimes resolves all the lifetimes within the trait and its items. There is room to refactor this, for example to resolve lifetimes for each trait item in separate queries, but it’s convenient to do the entire trait at once because the lifetimes from the trait definition are in scope within the trait items as well.

The reason for this separate call is to resolve what would otherwise be a cycle. Consider this example:

trait Base<'a> {
    type BaseItem;
}
trait Sub<'b>: for<'a> Base<'a> {
   type SubItem: Sub<BaseItem = &'b u32>;
}

When we resolve Sub and all its items, we also have to resolve Sub<BaseItem = &'b u32>. To figure out the index of 'b, we have to know about the supertraits of Sub so that we can determine that the for<'a> will be in scope. (This is because we – currently at least – flatten all the late-bound lifetimes into a single binder.) This requires us to resolve the trait definition of Sub; basically just enough lifetime information to look at the supertraits.