RFC: C99-style designated initializers

[edwardw]

On several occasions I found myself in the need of C99-style designated initializer to initialize a static sparse array, like so:

int a[128] = { [0] = 1, [64] = 2 };

It is just too error-prone, tedious and unsightly to type all remaining zeros. Note C99 designated initializer can also be applied to aggregate types but I doubt it would be as useful in rust.

[lifthrasiir]

What should be its syntax? We already have [expr, expr, expr] and [expr, ..expr] which severely limit remaining choices. For example, [[expr] = expr, <...>] or [expr = expr, <...>] is out, and [[expr]: expr, <...>] or [expr: expr, <...>] is possible but might conflict with #10502.

[kvanbere]

Don't forget unspecified elements need a default fill too in some applications (#12680).

[edwardw]

I was thinking something along the line of macro and / or compile time function evaluation (CTFE) may work but nothing concrete yet.

[thestinger]

This would need to go through the formal RFC process now.