More flexible initialization for vectors

[kvanbere]

In one of my projects which I'd like to port to rust in the long term, there is some C99 compliant array initializer like so;

  static const void *dispatch[255] = {
    [opcode_addk] = &&opc_addk,
    [opcode_addr] = &&opc_addr,
    [opcode_addvk] = &&opc_addvk,
    [opcode_addvr] = &&opc_addvr,
    [opcode_inc] = &&opc_inc,
    [opcode_subk] = &&opc_subk,
    [opcode_subr] = &&opc_subr,
    [opcode_subvk] = &&opc_subvk,
    [opcode_subvr] = &&opc_subvr,
    [opcode_dec] = &&opc_dec,
    &&opc_unident,
  };

Here opc_unident fills the parts of the array not specified, and each [index] = val pair puts val at the respective index. This cuts down the boilerplate for some tasks a LOT (In rust, I would have to write out all 255 entries for it to be quickly "maintainable").

I propose some syntax is added for complex initialization of vectors in a similar way.

[lifthrasiir]

#11936 is a sub-issue of this issue. (It does not mention about how to fill the unspecified elements.)

[alexcrichton]

Closing as a dupe of #11936. Filling in unspecified elements is required for soundness regardless of syntax/implementation.