Can we automatically derive a user-defined trait?

Question

I have a struct like this

#[derive(CustomTrait)]
struct Sample {
    v: Vec<u8>,
}

and my trait goes like this

trait CustomTrait {...}

Can I do the above stuff? It threw an error for me.

I want something similar to the Clone trait. Is this possible with Rust?

Answer 1

#[derive(Foo, Bar)] is sugar for #[derive_Foo] #[derive_Bar], so it is possible to implement your own decorator attribute in the same way as #[derive_Clone] is, but this requires you to write a compiler plugin, which is not a stable part of Rust and will not be stable in 1.0 (and will thus be unavailable in the stable and beta channels).

There is a little documentation on such matters in the book, but not much; you’re largely on your own with it.

Bear in mind that what you can actually do at that stage is limited; you have access to the struct definition only, and know nothing about the actual types mentioned. This is a good fit for all of the traits for which #[derive] support is built in, but is not for many other traits.

Answer 2

No, you can't. derive instructs the compiler to provide a basic implementation of the trait. You can't expect the compiler to magically know how to implement a user-defined trait.

You can only use derive with these traits (taken from http://rustbyexample.com/trait/derive.html):

• Comparison traits: Eq, PartialEq, Ord, PartialOrd
• Serialization: Encodable, Decodable
• Clone, to create T from &T via a copy.
• Hash, to compute a hash from &T.
• Rand, to create a random instance of a data type.
• Default, to create an empty instance of a data type.
• Zero, to create a zero instance of a numeric data type.
• FromPrimitive, to create an instance from a numeric primitive.
• Debug, to format a value using the {:?} formatter.

NOTE: Apparently this was proposed and is being discussed here