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Copy clone semantics #1521

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Copy clone semantics #1521

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A poorly defined clone shan't result in UB
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- Feature Name: N/A
- Start Date: (fill me in with today's date, YYYY-MM-DD)
- RFC PR: (leave this empty)
- Rust Issue: (leave this empty)

# Summary
[summary]: #summary

With specialization on the way, we need to talk about the semantics of
`<T as Clone>::clone() where T: Copy`.

It's generally been an unspoken rule of Rust that a `clone` of a `Copy` type is
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You may want to cite rust-lang/rust#23790, which initially proposed making Clone a supertrait of Copy, with the description:

"Copy is a refinement of Clone where memcpy suffices"

The assumption that Copy and Clone would be compatible was also implicitly used in accepting RFC 839, as a premise thereof was the iea that it would be compatible to relax a T: Copy bound to a T: Clone bound later on.

In other words, it's been more than an unspoken rule that clone and copy must be compatible, even if the rule seems not to have made its way onto the documentation of the Clone trait itself.

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(The example of specializing #[derive(Copy, Clone)] that was brought up elsewhere in the thread also seems relevant.)

equivalent to a `memcpy` of that type; however, that fact is not documented
anywhere. This fact should be in the documentation for the `Clone` trait, just
like the fact that `T: Eq` should implement `a == b == c == a` rules.

# Motivation
[motivation]: #motivation

Currently, `Vec::clone()` is implemented by creating a new `Vec`, and then
cloning all of the elements from one into the other. This is slow in debug mode,
and may not always be optimized (although it often will be). Specialization
would allow us to simply `memcpy` the values from the old `Vec` to the new
`Vec` in the case of `T: Copy`. However, if we don't specify this, we will not
be able to, and we will be stuck looping over every value.
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We will be stuck relying on the optimizer to figure it out. Which it can do and insert memcpy, typically for Copy values of 8 bytes or smaller per element, it looks like.

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@bluss exactly.


# Detailed design
[design]: #detailed-design

Specify that `<T as Clone>::clone(t)` shall be equivalent to `ptr::read(t)`
where `T: Copy, t: &T`. An implementation that does not uphold this *shall not*
result in undefined behavior; `Clone` is not an `unsafe trait`.

Also add something like the following sentence to the documentation for the
`Clone` trait:

"If `T: Copy`, `x: T`, and `y: &T`, then `let x = y.clone();` is equivalent to
`let x = *y;`. Manual implementations must be careful to uphold this."

# Drawbacks
[drawbacks]: #drawbacks

This is a breaking change, technically, although it breaks code that was
malformed in the first place.

# Alternatives
[alternatives]: #alternatives

The alternative is that, for each type and function we would like to specialize
in this way, we document this separately. This is how we started off with
`clone_from_slice`.

# Unresolved questions
[unresolved]: #unresolved-questions

What the exact wording should be.
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Alternative wording: "When Copy type has user-defined implementation of Clone it's unspecified whether clone or memcpy is called when cloning a value of this type." or (shamelessly stolen from somewhere) "<...if Copy...> An implementation is allowed to omit the call to clone even if is has side effects. Such elision of calls to clone is called clone elision."

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@petrochenkov it's not that it's unspecified whether clone or ptr::read is called when you actually call .clone(). It's just that libraries (including the standard library) are able to assume that Clone::clone == ptr::read for T: Copy; and, if that requirement is not met, libraries are allowed to exhibit unexpected (but defined) behavior. (Just like if an Ord type doesn't actually have an absolute ordering, or if an Eq type doesn't actually have transitive equality).

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it's not that it's unspecified whether clone or ptr::read is called when you actually call .clone()

Why not? If we go this route anyway, let's give the compiler/libraries all the freedom.
I'd even write "user-defined implementations of Clone for Copy types are ignored", but I'm not sure about generics:

fn f<T: Clone>(arg: T) -> T { arg.clone() } // It's not known if `T` is `Copy` or not and I'm not sure it's convenient to turn `clone`s into `memcpy`s during monomorphisation.

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It's just that libraries (including the standard library) are able to assume that Clone::clone == ptr::read for T: Copy

derive is not a library, so the language already can omit clones as well.

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Also, "clone is always ignored" is better than "clone is sometimes ignored" from pedagogical point of view.

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hmm...

At first I thought this was a bad idea, but then I started thinking. Not "always ignored", but if your clone has different semantics from a ptr::read then it should be implementation defined whether it is called.

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I really don't like the "optimizer magic overriding of clone".

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@arielb1 Yeah, I guess. I do think it would be better if it's just kept to libraries and language constructs like #[derive]