如何表达闭包的生命周期限制以匹配特征有界生命周期？

Question

我有一个特性，它返回一个附加到它自己生命周期的借用：

trait SomeTrait {
    fn do<'a>(&'a self, other: &AnonymousLifetime) -> &'a Output;
}

如何在where clause 中表达相同的限制，以便SomeTrait 可以impl From<Closure>？

示例

场景的minimal, reproducible example (playground)：

// The traits
trait Context {
    fn give(&self) -> usize;
}
trait ContextDecider {
    fn decide<'a>(&'a self, context: &dyn Context) -> &'a str;
}

// A concrete implementation example
// As expected, works OK
struct SomeDecider(Vec<String>);
impl ContextDecider for SomeDecider {
    fn decide<'a>(&'a self, context: &dyn Context) -> &'a str {
        let some_context = context.give();
        if some_context > self.0.len() {
            panic!("Oh no!");
        }

        &self.0[some_context]
    }
}

// An implemetation for a closure
// Help here!!
impl<'a, F> ContextDecider for F
where
    F: 'a + Fn(&dyn Context) -> &'a str,
{
    fn decide<'b>(&'b self, giver: &dyn Context) -> &'b str {
        self(giver)
    }
}

编译失败：

error[E0312]: lifetime of reference outlives lifetime of borrowed content...
  --> src/lib.rs:30:9
   |
30 |         self(giver)
   |         ^^^^^^^^^^^
   |
note: ...the reference is valid for the lifetime `'b` as defined on the method body at 29:15...
  --> src/lib.rs:29:15
   |
29 |     fn decide<'b>(&'b self, giver: &dyn Context) -> &'b str {
   |               ^^
note: ...but the borrowed content is only valid for the lifetime `'a` as defined on the impl at 25:6
  --> src/lib.rs:25:6
   |
25 | impl<'a, F> ContextDecider for F
   |      ^^

在示例中，我未能在闭包边界中表达特征所施加的限制并且编译器不满意。
编译器并没有帮助我使用什么语法来让我将两个生命周期锁定在一起。

Answer 1

你能在ContextDecider 上应用一个终身特征吗？ （这样你就可以得到 ContextDeciderinstead of having the lifetime only ondecide`。）

这会导致：

trait Context {
    fn give(&self) -> usize;
}
trait ContextDecider<'a> {
    fn decide(&'a self, context: &dyn Context) -> &'a str;
}

struct SomeDecider(Vec<String>);
impl<'a> ContextDecider<'a> for SomeDecider {
    fn decide(&'a self, context: &dyn Context) -> &'a str {
        let some_context = context.give();
        if some_context > self.0.len() {
            panic!("Oh no!");
        }

        &self.0[some_context]
    }
}

impl<'f, F> ContextDecider<'f> for F
where
    F: 'f,
    for<'ctx>F: Fn(&'ctx dyn Context) -> &'f str,
{
    fn decide(&'f self, giver: &dyn Context) -> &'f str {
        self(giver)
    }
}

Answer 2

您可以将函数包装在另一个结构中（这里称为FnWrapper）：

trait Context {
    fn give(&self) -> usize;
}
impl Context for () {
    fn give(&self) -> usize {0}
}
trait ContextDecider {
    fn decide<'a>(&'a self, context: &dyn Context) -> &'a str;
}

struct SomeDecider(Vec<String>);
impl ContextDecider for SomeDecider {
    fn decide<'a>(&'a self, context: &dyn Context) -> &'a str {
        let some_context = context.give();
        if some_context > self.0.len() {
            panic!("Oh no!");
        }

        &self.0[some_context]
    }
}

struct FnWrapper<'a, F> {
    f: F,
    _phantom: std::marker::PhantomData<& 'a ()>,
}
impl<'a, F> FnWrapper<'a, F> 
    where F: 'a + Fn(&dyn Context) -> &'a str,
{
    fn new(f: F) -> Self {
        Self {
            f,
            _phantom: Default::default(),
        }
    }
    fn f_decide<'b>(&'b self, giver: &dyn Context) -> &'b str {
        (self.f)(giver)
    }
}

impl<'a, F> ContextDecider for FnWrapper<'a, F>
where
    F: 'a + Fn(&dyn Context) -> &'a str,
{
    fn decide<'b>(&'b self, giver: &dyn Context) -> &'b str {
        self.f_decide(giver)
    }
}

fn main() {
    println!("{}", FnWrapper::new(|giver| vec!["1", "2", "3"][giver.give()]).decide(&()));
}