Rust 覆盖类型的要求到底是什么为什么单个元素元组满足它

假设存在以下代码

use core::any::Any;

enum Value {
    Any(Box<dyn Any>),
    Other, // placeholder, this code is adapted from mine
}

此代码引发了一个我不太理解的诊断

impl<T: Any> TryFrom<Value> for T {
    type Error = &'static str;

    fn try_from(val: Value) -> Result<Self, Self::Error> {
        if let Value::Any(any) = val {
            if let Ok(down) = any.downcast::<T>() {
                Ok(*down)
            } else {
                Err("incorrect type")
            }
        } else { Err("not an any") }
    }
}

fn main() {
    let res: Result<usize, &'static str> = Value::Any(Box::new(1usize)).try_into();
    dbg!(res);
}

error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Value`)
 --> src/main.rs:9:6
  |
9 | impl<T: Any> TryFrom<Value> for T {
  |      ^ type parameter `T` must be covered by another type when it appears before the first local type (`Value`)
  |
  = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
  = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last

我仍然不太明白"必须由另一种类型覆盖"是什么意思，也不太明白"当它出现在第一个本地类型之前时"是什么意思.

但是，如果我将IMPL签名修改为以包含T的单元素元组为目标，则IMPL不会引发错误，并且代码可以正确运行:

impl<T: Any> TryFrom<Value> for (T,) {
    type Error = &'static str;

    fn try_from(val: Value) -> Result<Self, Self::Error> {
        if let Value::Any(any) = val {
            if let Ok(down) = any.downcast::<T>() {
                Ok((*down,))
            } else {
                Err("incorrect type")
            }
        } else { Err("not an any") }
    }
}

fn main() {
    let res: Result<(usize,), &'static str> = Value::Any(Box::new(1usize)).try_into();
    dbg!(res);
}

单元素元组的实际用途是什么？

(Playground Link)