Append new item to $PATH only if it doesn't exist already

zshrc

# $PATH and its friends.
# If a program is invoked from this terminal, then all environment variables
# (including $PATH) are visible to that program.
# Use "append_if_new PATH foo/bar" to append path "foo/bar" to $PATH so we avoid
# duplication -- if executables with the same name exist in multiple paths
# enumerated in $PATH (e.g. caused by "source ~/.zshrc" multiple times), then
# the first find wins anyway.
append_if_new() {
  local variable_name="$1"
  local new_item="$2"

  # Get the current value of the environment variable. This "(P)" takes the
  # value string this variable name refers to, not the name string itself.
  # The equivalence of "(P)" in Bash scripts is "!".
  local variable_value="${(P)variable_name}"

  # Add the new item only if it doesn't already exist
  if [[ ":$variable_value:" != *":$new_item:"* ]]; then
    if [[ -z "$variable_value" ]]; then
      export "$variable_name"="$new_item"
    else
      export "$variable_name"="$variable_value:$new_item"
    fi
  fi
}

append_if_new PATH "$HOME/.cargo/bin"  # Rust