如何表达绑定在另一个泛型类型的特征绑定的类型参数上的特征？

Question

我想添加类型参数给我已经有的代码。但我有原来的代码看起来像这样的麻烦

：

fn parse_row(text: String) -> Result<Vec<u32>> { 
    text.split(" ") 
     .map(|s| s.to_owned() 
      .parse::<u32>() 
      .map_err(|e| err(&e.to_string()[..])) 
     ) 
     .collect::<Result<Vec<u32>>>() 
} 

这里是通用版本：

fn parse_row<Item>(text: String) -> Result<Vec<Item>> 
    where Item: Copy + Debug + FromStr + Display 
{ 
    text.split(" ") 
     .map(|s| s.to_owned() 
      .parse::<Item>() 
      .map_err(|e| err(&e.to_string()[..])) 
     ) 
     .collect::<Result<Vec<Item>>>() 
} 

我得到的错误是：

src/grid.rs:108:35: 108:46 error: no method named `to_string` found for type `<Item as core::str::FromStr>::Err` in the current scope 
src/grid.rs:108    .map_err(|e| err(&e.to_string()[..])) 
               ^~~~~~~~~~~ 
note: in expansion of closure expansion 
src/grid.rs:108:23: 108:52 note: expansion site 
note: in expansion of closure expansion 
src/grid.rs:106:14: 108:53 note: expansion site 
src/grid.rs:108:35: 108:46 note: the method `to_string` exists but the following trait bounds were not satisfied: `<Item as core::str::FromStr>::Err : core::fmt::Display` 

<Item as core::str::FromStr>::Err指的是与Item的相关的类型参数执行（我花了一段时间才弄清楚！）。但我怎么能表达这种我实际上不知道的类型具有Display特质？

Answer 1

这最初是因为我不明白它指的是哪个Err - 并且认为它是Result的错误类型参数。一旦我发现FromStr有自己的Err类型参数，我只需要弄清楚如何表达这个约束。这里，它是：

fn parse_row<Item, E>(text: String) -> Result<Vec<Item>> 
    where Item: Copy + Debug + FromStr<Err = E>, 
      E: Display 
{ 
    text.split(" ") 
     .map(|s| s.to_owned() 
      .parse::<Item>() 
      .map_err(|e| err(&e.to_string()[..])) 
     ) 
     .collect::<Result<Vec<Item>>>() 
}