{"ast_node_type":"other","tokens":[{"end":65,"kind":"keyword","start":0,"text":"///"}]}
    /// should only push exactly 3 `GenericArg`s before building.
    /// This method prepopulates the builder with placeholder substitution of `parent`, so you
    pub fn subst_for_coroutine(db: &dyn HirDatabase, parent: DefWithBodyId) -> TyBuilder<()> {