Function get_parent_env 

pub unsafe fn get_parent_env(x: SEXP) -> SEXP

Returns the enclosing environment of env, which will usually be of type ENVSXP, except for the special environment R_EmptyEnv, which terminates the environment chain; its enclosing environment is R_NilValue.

Safety

This function dereferences a raw SEXP pointer. The caller must ensure that x is a valid SEXP pointer to an environment.