Static zsh_subshell 

pub static zsh_subshell: AtomicI32

Port of int zsh_subshell; from Src/params.c:108. Visible $ZSH_SUBSHELL parameter — incremented by entersubsh() each time the shell forks into a subshell (real or fake-exec). Distinct from subsh which records whether we ARE a subshell; zsh_subshell is the visible depth count.