fn default_config_location() -> Result<PathBuf, CredentialsError> {
        let env = non_empty_env_var(AWS_CONFIG_FILE);
        match env {
            Some(path) => Ok(PathBuf::from(path)),
            None => ProfileProvider::hardcoded_config_location(),
        }
    }
Default config file location:
1: if set and not empty, use the value from environment variable ```AWS_CONFIG_FILE```
2. otherwise return `~/.aws/config` (Linux/Mac) resp. `%USERPROFILE%\.aws\config` (Windows)
rusoto/credential/src/profile.rs