Nix 2.10 introduced a new function, builtins.traceVerbose, that works like builtins.trace but is no-op until the user explicitly enables it.
For eg., running nix build will not cause traceVerbose to have any effect. But if you run nix build --trace-verbose, then logs from traceVerbose will be printed to the console.
Examples
- haskell-flake uses this to provide conditional logging