1. sbt -jvm-debug 9999 run
2. M-x ensime-db-attach
https://t.co/guaedHGNmK
— shigemk2 (@shigemk2) December 13, 2015
ってなるけど、
Type C-c C-d r to launch the embedded Scala Debugger. ENSIME will prompt you for the class (with 'main' function) that you want to run (tab-completion works here), and then launch the debug VM. Alternatively, use M-x ensime-db-attach to attach to a remote VM that's already running. The first breakpoint your program hits will be highlighted and centered in Emacs.
普通にensime-db-runしてもよい。