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：
trait Foo {
type A;
fn foo(&self, a: Self::A);
}
trait Alias: Foo
where
<Self as Foo>::A: From<u8>,
{
}
impl<T> Alias for T
where
T: Foo,
<T as Foo>::A: From<u8>,
{
}
当我想要在函数 test 中使用此别名时：
fn test<T: Alias>(x: &T, y: u8) {
x.foo(y.into());
}
编译器会报错：
error[E0277]: the trait bound `<T as Foo>::A: From<u8>` is not satisfied
--> src/lib.rs:20:12
|
20 | fn test<T: Alias>(x: &T, y: u8) {
| ^^^^^ the trait `From<u8>` is not implemented for `<T as Foo>::A`
|
note: required by a bound in `Alias`
--> src/lib.rs:9:23
|
7 | trait Alias: Foo
| ----- required by a bound in this trait
8 | where
9 | <Self as Foo>::A: From<u8>,
| ^^^^^^^^ required by this bound in `Alias`
help: consider further restricting the associated type
|
20 | fn test<T: Alias>(x: &T, y: u8) where <T as Foo>::A: From<u8> {
| +++++++++++++++++++++++++++++
这意味着我应该添加约束 <T as Foo>::A: From<u8>，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:
trait Foo {
type A;
fn foo(&self, a: Self::A);
}
trait Alias: Foo
where
<Self as Foo>::A: From<u8>,
{
}
impl<T> Alias for T
where
T: Foo,
<T as Foo>::A: From<u8>,
{
}
When I want to use this alias in the function `test:
fn test<T: Alias>(x: &T, y: u8) {
x.foo(y.into());
}
the compiler complains:
error[E0277]: the trait bound `<T as Foo>::A: From<u8>` is not satisfied
--> src/lib.rs:20:12
|
20 | fn test<T: Alias>(x: &T, y: u8) {
| ^^^^^ the trait `From<u8>` is not implemented for `<T as Foo>::A`
|
note: required by a bound in `Alias`
--> src/lib.rs:9:23
|
7 | trait Alias: Foo
| ----- required by a bound in this trait
8 | where
9 | <Self as Foo>::A: From<u8>,
| ^^^^^^^^ required by this bound in `Alias`
help: consider further restricting the associated type
|
20 | fn test<T: Alias>(x: &T, y: u8) where <T as Foo>::A: From<u8> {
| +++++++++++++++++++++++++++++
that means I should add the constraint <T as Foo>::A: From<u8> 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。
trait Alias: Foo {
type AliasA: From<u8>;
}
impl<T> Alias for T
where
T: Foo,
<T as Foo>::A: From<u8>,
{
type AliasA = T::A;
}
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.
trait Alias: Foo {
type AliasA: From<u8>;
}
impl<T> Alias for T
where
T: Foo,
<T as Foo>::A: From<u8>,
{
type AliasA = T::A;
}
Then just replace any usage of T::A with T::AliasA.
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