"kill failed" isn't actually the message which is printed. What you get is thread '<main>' panicked at 'kill failed: Sys(ESRCH)', ../src/libcore/result.rs:

"kill failed" isn't actually the message which is printed. What you get is thread '<main>' panicked at 'kill failed: Sys(ESRCH)', ../src/libcore/result.rs:746 So you know that it failed because of ESRCH (no such process).