ENV.key

static VALUE
env_key(VALUE dmy, VALUE value)
{
    StringValue(value);
    VALUE str = Qnil;

    rb_encoding *enc = env_encoding();
    ENV_LOCKING() {
        char **env = GET_ENVIRON(environ);
        while (*env) {
            char *s = strchr(*env, '=');
            if (s++) {
                rb_len_t len = strlen(s);
                if (RSTRING_LEN(value) == len && strncmp(s, RSTRING_PTR(value), len) == 0) {
                    str = env_str_new(*env, s-*env-1, enc);
                    break;
                }
            }
            env++;
        }
        FREE_ENVIRON(environ);
    }

    return str;
}