builtins.toString
#builtins
#nix1.18.1
https://nixos.org/manual/nix/stable/language/builtins.html#builtins-toString
おそらく実装はここ
https://github.com/NixOS/nix/blob/2.18.1/src/libexpr/primops.cc#L3652-L3659
builtinsを経由しなくても呼べるようだ。
code: nix repl
toString(/hoge/hoge.nix)
"/hoge/hoge.nix"
toString(1)
"1"