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126 lines
3.5 KiB
Markdown
126 lines
3.5 KiB
Markdown
---
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title: "`#[derive(Clone)]` is broken"
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description: "Not just `#[derive(Clone)]`, but all of the standard trait derives are"
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tags:
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- rust
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---
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```rs
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use std::sync::Arc;
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struct NoClone;
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#[derive(Clone)]
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struct WrapArc<T>(Arc<T>);
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fn main() {
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let foo = WrapArc(Arc::new(NoClone));
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let foo_ = foo.clone();
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}
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```
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Do you think this code should compile?
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What about the following code:
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```rs
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struct AlwaysEq<T>(T);
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impl<T> PartialEq for AlwaysEq<T> {
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fn eq(&self, _other: &Self) -> bool {
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true
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}
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}
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impl<T> Eq for AlwaysEq<T> {}
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struct NotEq;
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#[derive(PartialEq, Eq)]
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struct WrapAlwaysEq<T>(AlwaysEq<T>);
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fn assert_is_eq(_: impl Eq) {}
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fn main() {
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let x = WrapAlwaysEq(AlwaysEq(NotEq));
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assert_is_eq(x);
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}
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```
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The second example is a bit far fetched, but you probably answered yes.
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But
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[neither](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=0d9a89eccb4773ce32e7a3cec5cf8603)
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[do](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=30233aea9cf20c3299cd5c1fc6e7ec8c).
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# Why not?
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The
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[implementation of `#[derive(Clone)] in the Rust compiler`](https://github.com/rust-lang/rust/blob/0c4fa2690de945f062668acfc36b3f8cfbd013e2/compiler/rustc_builtin_macros/src/deriving/clone.rs)
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generates a `Clone` implementation with the following requirements on the
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derived type:
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- All fields must be `Clone`.
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- All generic parameters must be `Clone`.
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Can you spot the issue here? It's the latter requirement: **we cannot just
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require all generic parameters to be `Clone`, as we cannot assume they are used
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in such a way that requires them to be cloned**.[^The reason this is the way it
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is is probably because Rust's type system wasn't powerful enough for this to be
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implemented back in the pre-1.0 days. Or it was just a simple oversight that got
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stabilized.]
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This applies to practically all builtin derive traits, such as `Clone`,
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`PartialEq`, `Eq`, or
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[even `Debug`](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b419e34c9f00d0fca92c40739f6c9fb2).
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# What can we do to fix this?
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There are two solutions to this. Both require deleting that second requirement.
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## The hard way
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We could create a Rust RFC, hopefully not bikeshed it to death, and get it
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stabilized in the next Rust edition as it is a breaking change.[^Surely it is a
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breaking change, or the compiler people would've fixed it already. Right?]
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This would take 4+ years to stabilize and be available to everyone. That sucks,
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but is the correct thing to do in the long-term.
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## The quick way
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We can just write our own macro that generates the following code:
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```rs
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/* input */
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#[derive(CustomClone)]
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struct WrapArc<T>(Arc<T>);
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/* generated code */
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impl<T> Clone for WrapArc<T>
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where
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Arc<T>: Clone,
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// and so on, `FieldType: DerivedTrait` for each field
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{
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// ...
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}
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```
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This does the job correctly.
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And it's not even hard to do. I know people who do this internally in their
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company codebases - it's not much code.
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So I've [opened an issue](https://github.com/JelteF/derive_more/issues/490)
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about replicating the builtin derive traits in a less restrictive and thus
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correct way in the `derive_more` crate's GitHub repository. The reason I chose
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this crate is because it already has a lot of users and is the main place for
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derive implementations.
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Replicating already-existing behaviour of the std may not be in the scope of the
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crate, which is a perfectly fine stance to take. If that doesn't get accepted,
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I'll probably create my own crate and release it on
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[crates.io](https://crates.io/).
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Stay tuned, I'll update this blog post.
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