关于特质关联类型的别名边界

huangapple
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英文:

Aliasing bounds on associated types of traits

问题

我试图为一组具有关联类型的特质创建一个类型别名。但是，我的别名实际上是相反的，即它要求在使用它的任何地方都满足约束条件。

类似于这种情况的最小化示例如下。我想要创建一个别名，用于特质 Foo，只要其类型 A 可以从 u8 创建：

  1. trait Foo {
  2.     type A;
  4.     fn foo(&self, a: Self::A);
  5. }
  7. trait Alias: Foo
  8. where
  9.     <Self as Foo>::A: From<u8>,
  10. {
  11. }
  13. impl<T> Alias for T
  14. where
  15.     T: Foo,
  16.     <T as Foo>::A: From<u8>,
  17. {
  18. }

当我想要在函数 test 中使用此别名时：

  1. fn test<T: Alias>(x: &T, y: u8) {
  2.     x.foo(y.into());
  3. }

编译器会报错：

  1. error[E0277]: the trait bound `<T as Foo>::A: From<u8>` is not satisfied
  2.   --> src/lib.rs:20:12
  3.    |
  4. 20 | fn test<T: Alias>(x: &T, y: u8) {
  5.    |            ^^^^^ the trait `From<u8>` is not implemented for `<T as Foo>::A`
  6.    |
  7. note: required by a bound in `Alias`
  8.   --> src/lib.rs:9:23
  9.    |
  10. 7  | trait Alias: Foo
  11.    |       ----- required by a bound in this trait
  12. 8  | where
  13. 9  |     <Self as Foo>::A: From<u8>,
  14.    |                       ^^^^^^^^ required by this bound in `Alias`
  15. help: consider further restricting the associated type
  16.    |
  17. 20 | fn test<T: Alias>(x: &T, y: u8) where <T as Foo>::A: From<u8> {
  18.    |                                 +++++++++++++++++++++++++++++

这意味着我应该添加约束 &lt;T as Foo&gt;::A: From&lt;u8&gt;，但我想避免这样做。

我做错了什么？如果这是编译器的限制，有没有办法实现相同的行为？

英文:

I'm trying to make a type alias for a group of traits with associated types. However, my alias works in the opposite way, i.e., it requires the bounds to be satisfied wherever it's used.

A minimized example similar to the situation is as follows. I want to create an alias for the trait Foo whenever its type A can be created from a u8:

  1. trait Foo {
  2.     type A;
  4.     fn foo(&amp;self, a: Self::A);
  5. }
  7. trait Alias: Foo
  8. where
  9.     &lt;Self as Foo&gt;::A: From&lt;u8&gt;,
  10. {
  11. }
  13. impl&lt;T&gt; Alias for T
  14. where
  15.     T: Foo,
  16.     &lt;T as Foo&gt;::A: From&lt;u8&gt;,
  17. {
  18. }

When I want to use this alias in the function `test:

  1. fn test&lt;T: Alias&gt;(x: &amp;T, y: u8) {
  2.     x.foo(y.into());
  3. }

the compiler complains:

  1. error[E0277]: the trait bound `&lt;T as Foo&gt;::A: From&lt;u8&gt;` is not satisfied
  2.   --&gt; src/lib.rs:20:12
  3.    |
  4. 20 | fn test&lt;T: Alias&gt;(x: &amp;T, y: u8) {
  5.    |            ^^^^^ the trait `From&lt;u8&gt;` is not implemented for `&lt;T as Foo&gt;::A`
  6.    |
  7. note: required by a bound in `Alias`
  8.   --&gt; src/lib.rs:9:23
  9.    |
  10. 7  | trait Alias: Foo
  11.    |       ----- required by a bound in this trait
  12. 8  | where
  13. 9  |     &lt;Self as Foo&gt;::A: From&lt;u8&gt;,
  14.    |                       ^^^^^^^^ required by this bound in `Alias`
  15. help: consider further restricting the associated type
  16.    |
  17. 20 | fn test&lt;T: Alias&gt;(x: &amp;T, y: u8) where &lt;T as Foo&gt;::A: From&lt;u8&gt; {
  18.    |                                 +++++++++++++++++++++++++++++

that means I should add the constraint &lt;T as Foo&gt;::A: From&lt;u8&gt; as well, which I want to avoid.

Which part I'm doing wrong? If it is a limitation in the compiler, is there any way to achieve the same behavior?

答案1

得分: 1

这在你也在新的特质中进行类型更改时有效。

  1. trait Alias: Foo {
  2.     type AliasA: From<u8>;
  3. }
  5. impl<T> Alias for T
  6. where
  7.     T: Foo,
  8.     <T as Foo>::A: From<u8>,
  9. {
  10.     type AliasA = T::A;
  11. }

然后只需将任何对 T::A 的使用替换为 T::AliasA。

英文:

This works if you also make a type in the new trait.

  1. trait Alias: Foo {
  2.     type AliasA: From&lt;u8&gt;;
  3. }
  5. impl&lt;T&gt; Alias for T
  6. where
  7.     T: Foo,
  8.     &lt;T as Foo&gt;::A: From&lt;u8&gt;,
  9. {
  10.     type AliasA = T::A;
  11. }

Then just replace any usage of T::A with T::AliasA.

huangapple
  • 本文由 发表于 2023年5月30日 12:27:41
  • 转载请务必保留本文链接：https://go.coder-hub.com/76361626.html
  • rust
  • traits
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