# \`Aligned\` trait?
**URL:** https://internals.rust-lang.org/t/aligned-trait/17443
**Category:** language design
**Created:** [September 23, 2022, 8:29pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443 "2022-09-23T20:29:06Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@Jules-Bertholet](https://internals.rust-lang.org/u/Jules-Bertholet)
#### Post date: [September 23, 2022, 8:29pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/1 "2022-09-23T20:29:06Z")
[`core::mem::align_of`](https://doc.rust-lang.org/core/mem/fn.align_of.html) requires `Self: Sized`, even though it only needs to know the type's alignment. Notably, slices have an alignment known at compile time, even though they are not `Sized`. Should Rust have an `Aligned` trait (as a supertrait of `Sized`), so this bound can be loosened?
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### Author:  [@cuviper](https://internals.rust-lang.org/u/cuviper)
#### Post date: [September 23, 2022, 8:59pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/2 "2022-09-23T20:59:10Z")
It makes sense to me, but do you have a practical example where you would have used this?
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### Author:  [@Jules-Bertholet](https://internals.rust-lang.org/u/Jules-Bertholet)
#### Post date: [September 23, 2022, 9:05pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/3 "2022-09-23T21:05:15Z")
I'm trying to write a `Vec`-like type that can store unsized values (slices) as elements. I would prefer for my `Vec` to start out with an aligned dangling pointer, so that I don't need to change said pointer until I necessary. It's a minor annoyance, which I can easily work around, but still nice to have.
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### Author:  [@afetisov](https://internals.rust-lang.org/u/afetisov)
#### Post date: [September 23, 2022, 9:38pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/4 "2022-09-23T21:38:15Z")
Is there an example of `!Aligned` type, other than extern types?
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### Author:  [@Jules-Bertholet](https://internals.rust-lang.org/u/Jules-Bertholet)
#### Post date: [September 23, 2022, 9:38pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/5 "2022-09-23T21:38:35Z")
`dyn Trait`
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### Author:  [@cuviper](https://internals.rust-lang.org/u/cuviper)
#### Post date: [September 23, 2022, 9:44pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/6 "2022-09-23T21:44:58Z")
> [@Jules-Bertholet](#):
>
> I'm trying to write a `Vec`-like type that can store unsized values (slices) as elements. I would prefer for my `Vec` to start out with an aligned dangling pointer, so that I don't need to change said pointer until I necessary.
So if it's essentially `Vec<[T]>`, can't you use `align_of::()` for that pointer? You could also initialize that pointing to `[T; 0]`, not even dangling.
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### Author:  [@Jules-Bertholet](https://internals.rust-lang.org/u/Jules-Bertholet)
#### Post date: [September 23, 2022, 9:46pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/7 "2022-09-23T21:46:48Z")
> [@cuviper](#):
>
> can't you use `align_of::()` for that pointer
I could, hence why I stated it's "easy to work around". Though this becomes less fun if you also want to support structs with a slice as their last field.
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### Author:  [@cuviper](https://internals.rust-lang.org/u/cuviper)
#### Post date: [September 23, 2022, 9:55pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/8 "2022-09-23T21:55:03Z")
Yeah, custom slice-based DSTs are more interesting.
Language-wise, `?Sized` constraints are special, but I suppose that would also have to imply `?Aligned` for backward compatibility. And vice versa, `?Aligned` must imply `?Sized` for the supertrait. You would explicitly combine `T: Aligned + ?Sized` for dealing with the middle ground.
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### Author:  [@Jules-Bertholet](https://internals.rust-lang.org/u/Jules-Bertholet)
#### Post date: [September 24, 2022, 4:41pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/9 "2022-09-24T16:41:43Z")
[RFC posted](https://github.com/rust-lang/rfcs/pull/3319)
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### Author:  [@system](https://internals.rust-lang.org/u/system)
#### Post date: [December 23, 2022, 4:42pm UTC](https://internals.rust-lang.org/t/aligned-trait/17443/10 "2022-12-23T16:42:13Z")
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