Format const array as block if remaining width is less than half

[sanxiyn]

Motivated by poor formatting in Manishearth/rust-clippy#540.

[nrc]

Could you use a new enum instead of a bool here please? It makes the caller code easier to read.

[nrc]

Looks good and I think this is a definite improvement in the formatted output. There's a bunch of nits to fix, but they're all pretty minor. r+ with them addressed

[marcusklaas]

I don't know if we should hold off merging this, but in my mind we could be a lot more general in our approach. Instead of adding heuristics for arrays, we could implement a more general heuristic for assignments where we break early if this ends up taking fewer lines.

For example:

looooooooooooooooooooooooooooooooong_name = funk(a,
                                                 b,
                                                 c);

would become

looooooooooooooooooooooooooooooooong_name =
    funk(a, b, c);

[nrc]

That is probably a good idea, we'd keep arrays formatted consistently then and handle non-array cases. I wonder if there are any cases where this PR helps, but which are not assignments?

[nrc]

ping @sanxiyn What do you think about @marcusklaas 's suggestion? Do you think this PR would help even if we were to implement that?

[sanxiyn]

@marcusklaas's suggestion sounds good to me. I will try to implement it. There seems to be an issue(#497) for this already?

I think this PR is orthogonal, but I also think it is better implemented after line breaking at assignment is done.

[marcusklaas]

Superseded by #893.