Use sbt as Metals build server

Objective

I want to use Metals on VS Code with sbt as the build server.

Steps

To use Metals on VS Code:

1. Install Metals from Extensions tab:
   
2. Open a directory containing a build.sbt file.
3. From the menubar, run View > Command Palette... (Cmd-Shift-P on macOS) "Metals: Switch build server", and select "sbt"
   
4. Once the import process is complete, open a Scala file to see that code completion works:
   

Use the following setting to opt-out some of the subprojects from BSP.

bspEnabled := false

When you make changes to the code and save them (Cmd-S on macOS), Metals will invoke sbt to do the actual building work.

Interactive debugging on VS Code

1. Metals supports interactive debugging by setting break points in the code:
   
2. Interactive debugging can be started by right-clicking on an unit test, and selecting "Debug Test." When the test hits a break point, you can inspect the values of the variables:
   

See Debugging page on VS Code documentation for more details on how to navigate an interactive debugging session.

Logging into sbt session

While Metals uses sbt as the build server, we can also log into the same sbt session using a thin client.

• From Terminal section, type in sbt --client
  

This lets you log into the sbt session Metals has started. In there you can call testOnly and other tasks with the code already compiled.