Merge pull request #2133 from arielb1/all-the-clones

RFC - compiler-generated Clone impls for Arrays and Tuples

text/2133-all-the-clones.md

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+- Feature Name: all_the_clones
+- Start Date: 2017-08-28
+- RFC PR: https://github.com/rust-lang/rfcs/pull/2133
+- Rust Issue: https://github.com/rust-lang/rust/issues/44496
+
+# Summary
+[summary]: #summary
+
+Add compiler-generated `Clone` implementations for tuples and arrays with `Clone` elements of all lengths.
+
+# Motivation
+[motivation]: #motivation
+
+Currently, the `Clone` trait for arrays and tuples is implemented using a [macro] in libcore, for tuples of size 11 or less and for `Copy` arrays of size 32 or less. This breaks the uniformity of the language and annoys users.
+
+Also, the compiler already implements `Copy` for all arrays and tuples with all elements `Copy`, which forces the compiler to provide an implementation for `Copy`'s supertrait `Clone`. There is no reason the compiler couldn't provide `Clone` impls for all arrays and tuples.
+
+[macro]: https://github.com/rust-lang/rust/blob/f3d6973f41a7d1fb83029c9c0ceaf0f5d4fd7208/src/libcore/tuple.rs#L25
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Arrays and tuples of `Clone` arrays are `Clone` themselves. Cloning them clones all of their elements.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Make `clone` a lang-item, add the following trait rules to the compiler:
+
+```
+n number
+T type
+T: Clone
+----------
+[T; n]: Clone
+
+T1,...,Tn types
+T1: Clone, ..., Tn: Clone
+----------
+(T1, ..., Tn): Clone
+```
+
+And add the obvious implementations of `Clone::clone` and `Clone::clone_from` as MIR shim implementations, in the same manner as `drop_in_place`. The implementations could also do a shallow copy if the type ends up being `Copy`.
+
+Remove the macro implementations in libcore. We still have macro implementations for other "derived" traits, such as `PartialEq`, `Hash`, etc.
+
+Note that independently of this RFC, we're adding builtin `Clone` impls for all "scalar" types, most importantly fn pointer and fn item types (where manual impls are impossible in the foreseeable future because of higher-ranked types, e.g. `for<'a> fn(SomeLocalStruct<'a>)`), which are already `Copy`:
+```
+T fn pointer type
+----------
+T: Clone
+
+T fn item type
+----------
+T: Clone
+
+And just for completeness (these are perfectly done by an impl in Rust 1.19):
+
+T int type | T uint type | T float type
+----------
+T: Clone
+
+T type
+----------
+*const T: Clone
+*mut T: Clone
+
+T type
+'a lifetime
+----------
+&'a T: Clone
+
+----------
+bool: Clone
+char: Clone
+!: Clone
+```
+
+This was considered a bug-fix (these types are all `Copy`, so it's easy to witness that they are `Clone`).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The MIR shims add complexity to the compiler. Along with the `derive(Clone)` implementation in `libsyntax`, we have 2 separate sets of implementations of `Clone`. 
+
+Having `Copy` and `Clone` impls for all arrays and tuples, but not `PartialEq` etc. impls, could be confusing to users.
+
+# Rationale and Alternatives
+[alternatives]: #alternatives
+
+Even with all proposed expansions to Rust's type-system, for consistency, the compiler needs to have at least *some* built-in `Clone` implementations: the type `for<'a> fn(Foo<'a>)` is `Copy` for all user-defined types `Foo`, but there is no way to implement `Clone`, which is a supertrait of `Copy`, for it (an `impl<T> Clone for fn(T)` won't match against the higher-ranked type).
+
+The MIR shims for `Clone` of arrays and tuples are actually pretty simple and don't add much complexity after we have `drop_in_place` and shims for `Copy` types.
+
+## The array situation
+
+In Rust 1.19, arrays are `Clone` only if they are `Copy`. This code does not compile:
+```Rust
+fn main() {
+    let x = [Box::new(0)].clone(); //~ ERROR
+    println!("{:?}", x[0]);
+}
+```
+
+~~The reason (I think) is that there is no good way to write a variable-length array expression in macros. This wouldn't be fixed by the first iteration of const generics.~~ Actually, this can be done using a for-loop (`ArrayVec` is used here instead of a manual panic guard for simplicity, but it can be easily implemented given const generics).
+```Rust
+impl<const n: usize; T: Clone> Clone for [T; n] {
+    fn clone(&self) -> Self {
+        unsafe {
+            let result : ArrayVec<Self> = ArrayVec::new();
+            for elem in (self as &[T]) {
+                result.push(elem.clone());
+            }
+            result.into_inner().unwrap()
+        }
+    }
+}
+```
+
+OTOH, this means that making non-`Copy` arrays `Clone` is less of a bugfix and more of a new feature. It's however a nice feature - `[Box<u32>; 1]` not being `Clone` is an annoying and seemingly-pointless edge case.
+
+## Implement `Clone` only for `Copy` types
+
+As of Rust 1.19, the compiler *does not* have the `Clone` implementations, which causes ICEs such as [rust-lang/rust#25733] because `Clone` is a supertrait of `Copy`.
+
+One alternative, which would solve ICEs while being conservative, would be to have compiler implementations for `Clone` only for *`Copy`* tuples of size 12+ and arrays, and maintain the `libcore` macros for `Clone` of tuples (in Rust 1.19, arrays are only `Clone` if they are `Copy`).
+
+This would make the shims *trivial* (a `Clone` implementation for a `Copy` type is just a memcpy), and would not implement any features that are not needed.
+
+When we get variadic generics, we could make all tuples with `Clone` elements `Clone`. When we get const generics, we could make all arrays with `Clone` elements `Clone`.
+
+## Use a MIR implementation of `Clone` for all derived impls
+
+The implementation on the other end of the conservative-radical end would be to use the MIR shims for *all* `#[derive(Clone)]` implementations. This would increase uniformity by getting rid of the separate `libsyntax` derived implementation. However:
+
+1. We'll still need the `#[derive_Clone]` hook in libsyntax, which would presumably result in an attribute that trait selection can see. That's not a significant concern.
+
+2. The more annoying issue is that, as a workaround to trait matching being inductive, derived implementations are imperfect - see [rust-lang/rust#26925]. This means that we either have to solve that issue for `Clone` (which is dedicatedly non-trivial) or have some sort of type-checking for the generated MIR shims, both annoying options.
+
+3. A MIR shim implementation would also have to deal with edge cases such as `#[repr(packed)]`, which normal type-checking would handle for ordinary `derive`. I think drop glue already encounters all of these edge cases so we have to deal with them anyway.
+
+## `Copy` and `Clone` for closures
+
+We could also add implementations of `Copy` and `Clone` to closures. That is [RFC #2132] and should be discussed there.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+See Alternatives.
+
+[RFC #2132]: https://github.com/rust-lang/rfcs/pull/2132
+[rust-lang/rust#25733]: https://github.com/rust-lang/rust/issues/25733
+[rust-lang/rust#26925]: https://github.com/rust-lang/rust/issues/26925