Constant PRUNE_EXIT_CODE 

pub const PRUNE_EXIT_CODE: i32 = 42;

The exit code a child uses to say “prune this trial”, by default.

A child that exits with this code is recorded Pruned rather than failed. Override it with Subprocess::prune_exit_code if 42 collides with a meaningful code of the program being tuned. Only the low 8 bits of an exit code survive on Unix, so a prune code must lie in 1..=255 and differ from 0.