Rust 直接调用函数时可变引用的寿命足够长，但通过中间函数调用时可变引用的寿命不够长

对于以下防锈代码：

fn facing_of_mut<'a>(domain: &'a mut Domain, entity: Entity) -> Option<&'a mut Direction> {
    component_of_mut(&mut domain, entity)
}

---

…我可以在单元测试中直接调用它，在编译过程中不会出现生命周期错误。只有当facing of mut调用它时，我才会收到错误。

您正在引用域。域是参数的局部变量，并且已经是引用。域仅在函数调用期间持续，因此对该值的引用可以存在多长时间

若要修复此问题，请不要尝试引用引用：

type Domain = String;
type Entity = String;
type Direction = String;

pub fn component_of_mut<'a, C>(domain: &'a mut Domain, entity: Entity) -> Option<&'a mut C>
    where C: 'static
{
    unimplemented!()
}

fn facing_of_mut<'a>(domain: &'a mut Domain, entity: Entity) -> Option<&'a mut Direction> {
    component_of_mut(domain, entity)
}

fn main() {}

type Domain = String;
type Entity = String;
type Direction = String;

pub fn component_of_mut<'a, C>(domain: &'a mut Domain, entity: Entity) -> Option<&'a mut C>
    where C: 'static
{
    unimplemented!()
}

fn facing_of_mut<'a>(domain: &'a mut Domain, entity: Entity) -> Option<&'a mut Direction> {
    component_of_mut(domain, entity)
}

fn main() {}