implement `Default` for arrays

[vks]

Something like

impl<T: Default> for [T; 2] {                                                       
    fn default() -> [T; 2] {                                                        
        [Default::default(), Default::default()]                                    
    }                                                                               
}

for N=0..33 should be added to src/libcore/default.rs. The question is how to do that with a macro.

[lilyball]

What's special about N=0..33?

[vks]

Due to lack of generics over integers, we cannot implement it for all N. Typically traits on arrays in the standard library are implemented for N=0..33, see for example Eq.

[nham]

I looked at this a bit today and I'm stuck. Here's one way to do it, but it requires an extraneous Copy bound on T (inside libcore/array.rs macro):

        impl<T: Copy + Default> Default for [T; $N] {
            #[inline]
            fn default() -> [T; $N] {
               [Default::default(); $N]
            }                                                                               
        }

Not sure if there's any way to achieve the same effect as array initialization without requiring Copy.

[vks]

You could write [Defaul::default(), Default::default(), ...] instead.

[shepmaster]

I think you have to write it out as [Default::default(), Default::default(), ...]. If the Default implementation actually calls something stateful, like rand, you presumably would want there to be different random values for each item.

[apasel422]

This was implemented in #27825 and can be closed.