I want to implement Serialize on Vec<u8>, but got an error:
Only traits defined in the current crate can be implemented for arbitrary types
Could somebody explain if there any good solution for this except defining my own type:
struct MyVec(Vec<u8>);
impl Serialize for MyVec { ... }
That's pretty much the recommendation in the general case, yeah. You can implement extra traits to make this less disruptive, in particular Deref/Mut let's you call Vec methods.
In the specific case of serde, though, you can also use #[serde(remote)], though it does require additional annotations on the use sites.
It's also important to note that Vec<T> already implements Serialize whenever T does, so you need to wrap it into a new type or add some annotation if you want to override the default behaviour of just serialising every element in a collection.
And, to complete the line of reasoning, u8 also implements Serialize, so this case isn’t just a forward-compatibility issue: There’s already a working implementation of Serialize for Vec<u8> which would conflict with OP’s new implementation.
The solution is to use #[serde(serialize_with=...)]
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