Function settings_path 

pub fn settings_path(
    platform: Platform,
    variable: impl Fn(&str) -> Option<String>,
    writable: impl Fn(&Path) -> bool,
    temporary: impl FnOnce() -> PathBuf,
) -> PathBuf

Where the settings file is on platform, as music21’s getSettingsPath finds it.

variable reads the environment, writable says whether a directory can be written to, and temporary is the directory to fall back on where the home directory cannot be: music21’s getDefaultRootTempDir.

use std::path::{Path, PathBuf};
use music21::environment::{settings_path, Platform};

let home = |name: &str| (name == "HOME").then(|| "/home/me".to_owned());
let found = settings_path(Platform::Nix, home, |_: &Path| true, || PathBuf::from("/tmp/music21"));
assert_eq!(found, PathBuf::from("/home/me/.music21rc"));
let found = settings_path(Platform::Nix, |_: &str| None, |_: &Path| true, || PathBuf::from("/tmp/music21"));
assert_eq!(found, PathBuf::from("/tmp/music21/.music21rc"));