如何表达闭包的生命周期限制以匹配特征有界生命周期？

debugcn 发表于 Dev

恶棍

我有一个特征，该特征会返回附加到其自身生命的借项：

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

如何在闭包的where子句中表达相同的限制，以便SomeTrait可以impl From<Closure>？

例

场景的一个最小的，可复制的示例（操场）：

// 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)
    }
}

无法编译：

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，我未能在封闭范围内表达特征施加的限制，并且编译器不满意。
编译器没有帮助我使用应使用的语法，这将使我将两个生命周期锁定在一起。

您可以将函数包装在另一个结构（此处称为FnWrapper）中：

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(&()));
}

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