Help with Default with generics not working.

[beckend]

#[derive(Educe, Debug)]
#[educe(Default(bound(T: Default), expression = Self { data: vec![1, 2, 3, 4, 5] }))]
struct Dummy<T> {
  data: T,
}

39 | #[educe(Default(bound(T: Default), expression = Self { data: vec![1, 2, 3, 4, 5] }))]
   |                                                              ^^^^^^^^^^^^^^^^^^^ expected type parameter `T`, found `Vec<{integer}>`

#[educe(Default)]
#[derive(Educe, Debug)]
struct DummyStructMusliGeneric<T> {
  #[educe(Default = true)]
  data: T,
}

43 |   #[educe(Default = true)]
   |                     ^^^^ the trait `From<bool>` is not implemented for `T`, which is required by `bool: std::convert::Into<_>`

[ijackson]

What do you expect

  #[educe(Default = true)]

to do ?

It looks like it's trying to say that when constructing a default value of DummyStructMusliGeneric<T>, the field data should be initialised to true.

That's fine for DummyStructMusliGeneric<bool> but it's obviously not going to work for DummyStructMusliGeneric<String>.

[linkenquydinhhexagon]

Ok got it, is there a way to annotate what T would be?

[ijackson]

Ok got it, is there a way to annotate what T would be?

I'm not sure I understand the question. In the original, you're setting the default to a vec of numbers. Why haven't you written:

struct Dummy {
   data: Vec<i32>,

? (Genuine question: there must be a reason.)

One option would be something like this:

trait HasMySpecialDefault {
   fn my_special_default() -> Self;
}
impl HasMySpecialDefault for Vec<i32> { ... }

#[derive(Educe, Debug)]
#[educe(Default(bound(T: HasMySpecialDefault))]
struct DummyStructMusliGeneric<T> {
  #[educe(Default = <T as HasMySpecialDefault>::my_special_default())]
  data: T,
}

I haven't tried that, the syntax may be wrong, but I hope you get the idea. You'd only be able to get a Dummy::default() if you'd provided the corresponding default value.