{"ast_node_type":"function","tokens":[{"end":69,"kind":"keyword","start":0,"text":"fn"}]}
                fn fn_ctr() fn() -> Option<Foo<T>> [type_could_unify]
                fn fn_builder() fn() -> FooBuilder [type_could_unify]
                fn fn_ctr2() fn() -> Result<Foo<T>, u32> [type_could_unify]