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# [derive(Clone)] Is Broken

(rgbcu.be)
128 points RGBCube | 2 comments | 04 Jul 25 20:31 UTC | HN request time: 0s | source
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jhugo ◴[08 Jul 25 09:16 UTC] No.44498376[source]▶
> 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.

No, this is backwards. We have to require all generic parameters are Clone, as we cannot assume that any are not used in a way that requires them to be Clone.

> 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.

The type system can't know whether you call `T::clone()` in a method somewhere.

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berkes ◴[08 Jul 25 09:21 UTC] No.44498401[source]▶
> The type system can't know whether you call `T::clone()` in a method somewhere.

Why not?

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jhugo ◴[08 Jul 25 10:26 UTC] No.44498701[source]▶
Types don't carry behavioral information about what the method does internally. Everything about a method is known from its signature.

The compiler doesn't introspect the code inside the method and add additional hidden information to its signature (and it would be difficult to reason about a compiler that did).

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1. ninkendo ◴[08 Jul 25 10:56 UTC] No.44498864[source]▶
> Types don't carry behavioral information about what the method does internally.

I don’t remember specifics, but I very distinctly remember changing a method in some way and Rust determining that my type is now not Send, and getting errors very far away in the codebase because of it.[0]

If I have time in a bit I’ll try and reproduce it, but I think Send conformance may be an exception to your statement, particularly around async code. (It also may have been a compiler bug.)

[0] It had something to do with carrying something across an await boundary, and if I got rid of the async call it went back to being Send again. I didn’t change the signature, it was an async method in both cases.

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2. kd5bjo ◴[08 Jul 25 11:50 UTC] No.44499138[source]▶
`Send`, `Sync`, and `Unpin` are special because they're so-called 'auto traits': The compiler automatically implements them for all compound types whose fields also implement those traits. That turns out to be a double-edged sword: The automatic implementation makes them pervasive in a way that `Clone` or `Debug` could never be, but it also means that changes which might be otherwise private can have unintended far-reaching effects.

In your case, what happens is that async code conceptually generates an `enum` with one variant per await point which contains the locals held across that point, and it's this enum that actually gets returned from the async method/block and implements the `Future` trait.