I personally choose not to use optionalAttrs in that way, instead choosing

  ${if cond then "foo" else null} = value;

xovidium has said that empty merges are free on new nix versions though

how does nix handle these differently when it comes to laziness?

let
  inherit (builtins) isInt genList;
  assertMsg = pred: msg: pred || throw msg;
  v1 =
    x:
    assert isInt x || throw "error message";
    x * 2;
  v2 =
    x:
    assert assertMsg (isInt x) "error message";
    x * 2;
  indices = genList (i: i) 100;
in
map v2 indices

i understand that assertMsg and similar functions (throwIf, warnIf, etc) are bad for performance - i can run stats on both versions and tell you that we go from 7 -> 107 thunks, 200 -> 400 function calls, and 4300 -> 7500 envs.bytes when using v2. what i don't get is how we're able to only have 7 thunks in the || case. how do we not need to create a thunk on every call?