T.dest is a unique os.Path (e.g. out/classFiles.dest/ or out/run.dest/) that is assigned to every Target or Command. It is cleared before your task runs, and you can use it as a scratch space for temporary files or a place to put returned artifacts. This is guaranteed to be unique for every Target or Command, so you can be sure that you will not collide or interfere with anyone else writing to those same paths.

T.env is the environment variable map passed to the Mill command when it is run; typically used inside a Task.Input to ensure any changes in the env vars are properly detected.

Note that you should not use sys.env, as Mill's long-lived server process means that sys.env variables may not be up to date.

T.log is the default logger provided for every task. While your task is running, System.out and System.in are also redirected to this logger. The logs for a task are streamed to standard out/error as you would expect, but each task's specific output is also streamed to a log file on disk, e.g. out/run.log or out/classFiles.log for you to inspect later.

Messages logged with log.debug appear by default only in the log files. You can use the --debug option when running mill to show them on the console too.

This is the os.Path pointing to the project root directory.

This is the preferred access to the project directory, and should always be prefered over os.pwd* (which might also point to the project directory in classic cli scenarios, but might not in other use cases like BSP or LSP server usage).