Whyyyy

{
  system ? builtins.currentSystem,
  pkgs ? import <nixpkgs> {
    inherit system;
    crossSystem = "wasm32-unknown-none";
    overlays = [
      (final: prev: {
        lib = prev.lib // {
          platforms = prev.lib.platforms // {
            wasm32-unknown-none = [ "wasm32-unknown-none" ];
            all = prev.lib.platforms.all ++ final.lib.platforms.wasm32-unknown-none;
          };
        };
        rustc = prev.rustc // {
          targetPlatforms = prev.rustc.targetPlatforms ++ final.lib.platforms.wasm32-unknown-none;
        };
      })
    ];
  },
}:
pkgs.callPackage ./package.nix { }

$ nix-instantiate --eval -E '(import ./. { }).meta.platforms'
[ ]

Oh you also need to redefine rustPlatform, I see:

rustPlatform = prev.makeRustPlatform {
  inherit (final) rustc cargo;
};

Wait what:

error: Package <...> is not available on the requested hostPlatform:
         hostPlatform.config = "wasm32-unknown-none"
         package.meta.platforms = [
           "wasm32-unknown-none"
         ]