Rust 通用特征实现的 E0119 错误

我有一个枚举:

enum Field {
    Str(String),
    Integer(i64),
}

我想做:

impl From<String> for Field {
    fn from(s: String) -> Field {
        Field::Str(s)
    }
}

impl<I> From<I> for Field where I: Into<i64> + Copy {
    fn from(i: I) -> Field {
        Field::Integer(Into::<i64>::into(i))
    }
}

上面的代码有一个错误:

error[E0119]: conflicting implementations of trait
`std::convert::From<std::string::String>` for type `Field`:
--> <anon>:12:5
   |
6  |          impl From<String> for Field {
   |  ________- starting here...
7  | |       fn from(s: String) -> Field {
8  | |         Field::Str(s)
9  | |       }
10 | |     }
   | |_____- ...ending here: first implementation here
11 | 
12 |       impl<I> From<I> for Field where I: Into<i64> + Copy {
   |  _____^ starting here...
13 | |       fn from(i: I) -> Field {
14 | |         Field::Integer(Into::<i64>::into(i))
15 | |       }
16 | |     }
   | |_____^ ...ending here: conflicting implementation for `Field`

String不是Into<i64>的实现者，那么为什么会发生错误E0119呢？