Add named ellipsis

[Synthetica9]

This PR adds a new language feature: named ellipsis.

nix-repl> ({ x, ... @ y } : y)  { x = 1; a = 1; }
{ a = 1; }

Often, we see the following pattern:

{ foo, bar, ... } @ args:
  let
    strippedArgs = builtins.removeAttrs args [ "foo" "bar" ];
  in
    ...

This is error-prone and repeats all the formal arguments. With this PR, this can be replaced with:

{ foo, bar, ... @ strippedArgs }:
  ...

TODO

• Write an RFC to get this properly discussed in the community
• Hide this behind a flag
• Write tests
• Write documentation

[domenkozar]

NixOS/rfcs#58

[stale]

I marked this as stale due to inactivity. → More info