Trait 实现冲突与孤儿规则

  外部 trait + 外部类型(Display for Vec<u8>)
      → 失败:error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate
  本地 trait + 外部类型(MyTr for Vec<u8>)
      → 编译通过
  外部 trait + 本地类型(Display for Local)
      → 编译通过
  外部 trait + 外部类型,但 trait 的类型参数是本地类型(From<Local> for Vec<u8>)
      → 编译通过
  外部 trait + Box<本地类型>(Box 是 #[fundamental])
      → 编译通过
  外部 trait + Vec<本地类型>(Vec 不是 fundamental)
      → 失败:error[E0117]: only traits defined in the current crate can be implemented for types defined outside of the crate

  源码:
      1 | trait Describe { fn d(&self) -> String; }
      2 | impl<T: std::fmt::Display> Describe for T { fn d(&self) -> String { format!("显示:{}", self) } }
      3 | impl Describe for i32 { fn d(&self) -> String { "整数".into() } }
      4 | fn main() {}
  编译:失败
    error[E0119]: conflicting implementations of trait `Describe` for type `i32`
     --> <实验目录>/src/u47a.rs:3:1
      |
    2 | impl<T: std::fmt::Display> Describe for T { fn d(&self) -> String { format!("显示:{}", self) } }
      | ----------------------------------------- first implementation here
    3 | impl Describe for i32 { fn d(&self) -> String { "整数".into() } }
      | ^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `i32`
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0119`.

  源码:
      1 | trait Describe { fn d(&self) -> String; }
      2 | impl<T: std::fmt::Display> Describe for T { fn d(&self) -> String { format!("显示:{}", self) } }
      3 | impl Describe for Vec<u8> { fn d(&self) -> String { "字节向量".into() } }
      4 | fn main() {}
  编译:失败
    error[E0119]: conflicting implementations of trait `Describe` for type `Vec<u8>`
     --> <实验目录>/src/u47b.rs:3:1
      |
    2 | impl<T: std::fmt::Display> Describe for T { fn d(&self) -> String { format!("显示:{}", self) } }
      | ----------------------------------------- first implementation here
    3 | impl Describe for Vec<u8> { fn d(&self) -> String { "字节向量".into() } }
      | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Vec<u8>`
      |
      = note: upstream crates may add a new impl of trait `std::fmt::Display` for type `std::vec::Vec<u8>` in future versions
    
    error: aborting due to 1 previous error
    
    For more information about this error, try `rustc --explain E0119`.
  源码:
      1 | trait Describe { fn d(&self) -> String; }
      2 | impl<T: std::fmt::Display> Describe for T { fn d(&self) -> String { format!("显示:{}", self) } }
      3 | struct Mine;                                            // 本地类型:上游永远不会给它实现 Display
      4 | impl Describe for Mine { fn d(&self) -> String { "我的类型".into() } }
      5 | fn main() { println!("{} {}", Mine.d(), 5.d()); }
  编译:成功
  运行(退出码 0):
    我的类型 显示:5

  源码:
      1 | use std::fmt;
      2 | struct Bytes(Vec<u8>);                                                      // newtype
      3 | impl fmt::Display for Bytes { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "{} 字节", self.0.len()) } }
      4 | trait Hex { fn hex(&self) -> String; }                                      // 自己的 trait,可以为外部类型实现
      5 | impl Hex for Vec<u8> { fn hex(&self) -> String { self.iter().map(|b| format!("{b:02x}")).collect() } }
      6 | fn main() { println!("{} / {}", Bytes(vec![1, 2, 3]), vec![0xde, 0xad].hex()); }
  编译:成功
  运行(退出码 0):
    3 字节 / dead