Add "show tree <task>"

While the SBT inspect tree command is useful, unfortunately too many tasks have dynamic dependencies. One’s ability to visualize the full dependency graph of a given task falls down quickly in these situations.

If SBT recorded the data flow as it was executing the tasks, it could present something like inspect tree does but 1) more accurately and 2) it could even show the values of each task if appropriate.

sbt 1.3.0 added --traces.

$ sbt --traces "clean;compile;"
[success] Total time: 133 s (02:13), completed Apr 22, 2020 3:18:00 PM
wrote /Users/eed3si9n/work/frontend/target/traces/build.trace

This file can then loaded into Chrome chrome://tracing.