Support pointwise addition for arrays and tuples

[matthiasgoergens]

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Resolves https://github.com/JelteF/derive_more/issues/342

Synopsis

We want to support examples like these:

struct StructRecursive {
    a: i32,
    b: [i32; 2],
    c: [[i32; 2]; 3],
    d: (i32, i32),
    e: ((u8, [i32; 3]), i32),
    f: ((u8, i32), (u8, ((i32, u64, ((u8, u8), u16)), u8))),
    g: i32,
}

struct TupleRecursive((i32, u8), [(i32, u8); 10]);

Supporting arrays and tuples inside of enums would also be useful, but that's not in this PR.

Solution

Overhaul some helper functions in add_helpers and make them recursive.

Checklist

• [ ] Documentation is updated (if required)
• [x] Tests are added/updated (if required)
• [ ] CHANGELOG entry is added (if required)

[matthiasgoergens]

Unfortunately, as implemented this breaks for

#[derive(Add)]
pub struct GenericArrayStruct<T> {
    pub a: [T; 2],
}

A workaround is:

#[derive(Add)]
pub struct GenericArrayStruct<T: Copy> {
    pub a: [T; 2],
}

But a more proper fix would be useful.

[matthiasgoergens]

The latest commit on this branch has an alternative that uses Vec and iterators, but does not require Copy. I'm not sure if that's better or worse?

[JelteF]

Moved this to the 1.1.0 milestone as this isn't a breaking change.