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blog(derive-broken): init

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---
title: "`#[derive(Clone)]` is broken"
description: "Not just `#[derive(Clone)]`, but all of the standard trait derives"
tags:
- rust
---
```rs
use std::sync::Arc;
struct NoClone;
#[derive(Clone)]
struct WrapArc<T>(Arc<T>);
fn main() {
let foo = WrapArc(Arc::new(NoClone));
let foo_ = foo.clone();
}
```
Do you think this code should compile?
What about the following code:
```rs
struct AlwaysEq<T>(T);
impl<T> PartialEq for AlwaysEq<T> {
fn eq(&self, _other: &Self) -> bool {
true
}
}
impl<T> Eq for AlwaysEq<T> {}
struct NotEq;
#[derive(PartialEq, Eq)]
struct WrapAlwaysEq<T>(AlwaysEq<T>);
fn assert_is_eq(_: impl Eq) {}
fn main() {
let x = WrapAlwaysEq(AlwaysEq(NotEq));
assert_is_eq(x);
}
```
The second example is a bit far fetched, but you probably answered yes.
But neither do.
# Why not?
The
[implementation of `#[derive(Clone)] in the Rust compiler`](https://github.com/rust-lang/rust/blob/0c4fa2690de945f062668acfc36b3f8cfbd013e2/compiler/rustc_builtin_macros/src/deriving/clone.rs)
generates a `Clone` implementation with the following requirements on the
derived type:
- All fields must be `Clone`.
- All generic parameters must be `Clone`.
Can you spot the issue here? It's the latter requirement: **we cannot just
require all generic parameters to be `Clone`, as we cannot assume they are used
in such a way that requires them to be cloned**.[^The reason this is the way it
is is probably because Rust's type system wasn't powerful enough for this to be
implemented back in the pre-1.0 days. Or it was just a simple oversight that got
stabilized.]
This applies to practically all builtin derive traits, such as `Clone`,
`PartialEq`, `Eq`, or
[even `Debug`](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b419e34c9f00d0fca92c40739f6c9fb2).
# What can we do to fix this?
There are two solutions to this. Both require deleting that second requirement.
## The hard way
We could create a Rust RFC, hopefully not bikeshed it to death, and get it
stabilized in the next Rust edition as it is a breaking change.[^Surely it is a
breaking change, or the compiler people would've fixed it already. Right?]
This would take 4+ years to stabilize and be available to everyone. That sucks,
but is the correct thing to do in the long-term.
## The quick way
We can just write our own macro that generates the following code:
```rs
/* input */
#[derive(CustomClone)]
struct WrapArc<T>(Arc<T>);
/* generated code */
impl<T> Clone for WrapArc<T>
where
Arc<T>: Clone,
// and so on, `FieldType: DerivedTrait` for each field
{
// ...
}
```
This does the job correctly.
And it's not even hard to do. I know people who do this internally in their
company codebases - it's not much code.
So I've [opened an issue](https://github.com/JelteF/derive_more/issues/490)
about replicating the builtin derive traits in a less restrictive and thus
correct way in the `derive_more` crate's GitHub repository. The reason I chose
this crate is because it already has a lot of users and is the main place for
derive implementations.
Replicating already-existing behaviour of the std may not be in the scope of the
crate, which is a perfectly fine stance to take. If that doesn't get accepted,
I'll probably create my own crate and release it on
[crates.io](https://crates.io/).
Stay tuned, I'll update this blog post.