rust - `` 和 `where T: Trait` 有什么区别？

Question

在the docs对于 Send 特性，我看到了两者

impl<T> Send for LinkedList<T> 
where
    T: Send, 

和

impl<T: Send> Send for LinkedList<T>

这两种语法之间有什么区别，如果我为自己的特征编写 impl 声明，它会如何影响我的代码？

Answer 1

在 where 中定义的特征边界子句是内联声明的特征边界的超集。内联样式存在于 where 之前条款; where子句是 introduced in RFC 135 :

Add where clauses, which provide a more expressive means ofspecifying trait parameter bounds. [...] The existing bounds notationwould remain as syntactic sugar for where clauses.

Here is a list of limitations with the current bounds syntax that areovercome with the where syntax:

• It cannot express bounds on anything other than type parameters. Therefore, if you have a function generic in T, you can writeT:MyTrait to declare that T must implement MyTrait, but you can'twrite Option<T> : MyTrait or (int, T) : MyTrait. These forms are lesscommonly required but still important.

• It does not work well with associated types. This is because there is no space to specify the value of an associated type. Otherlanguages use where clauses (or something analogous) for this purpose.

• It's just plain hard to read. Experience has shown that as the number of bounds grows, the current syntax becomes hard to read andformat.

从那时起，您还可以在 for <'a> ... 中使用 higher-ranked trait bounds ( where )子句:

fn foo<T, U>()
where
    // higher-ranked trait bounds
    for<'a> T: SomethingElse<'a>,
    // Bound not directly on the generic type
    i32: From<U>,
    T: Iterator,
    // Bound on an associated type
    T::Item: Clone,
    // Just really long
    U: ReallyLong + AnotherReallyLong + WowReallyLong,
{}

如果内联特征边界可以满足您的需求，那么对您的代码没有任何影响。如果你需要额外的权力，只有 where启用，那么你需要使用where .

另见:

• What are the differences between an impl trait argument and generic function parameter?

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我的个人风格是总是使用where形式。具有单一形状也更容易git diff添加新边界对我来说值得额外的代码行。