Expand tilde in Rust Path idiomatically

The most idiomatic way would be to just use an existing crate, in this case shellexpand (github, crates.io) seems to do what you want:

extern crate shellexpand; // 1.0.0

#[test]
fn test_shellexpand() {
    let home = std::env::var("HOME").unwrap();
    assert_eq!(shellexpand::tilde("~/foo"), format!("{}/foo", home));
}

Alternatively, you could try it with dirs (crates.io). Here is a sketch:

extern crate dirs; // 1.0.4

use std::path::{Path, PathBuf};

fn expand_tilde<P: AsRef<Path>>(path_user_input: P) -> Option<PathBuf> {
    let p = path_user_input.as_ref();
    if !p.starts_with("~") {
        return Some(p.to_path_buf());
    }
    if p == Path::new("~") {
        return dirs::home_dir();
    }
    dirs::home_dir().map(|mut h| {
        if h == Path::new("https://stackoverflow.com/") {
            // Corner case: `h` root directory;
            // don't prepend extra `/`, just drop the tilde.
            p.strip_prefix("~").unwrap().to_path_buf()
        } else {
            h.push(p.strip_prefix("~/").unwrap());
            h
        }
    })
}

Usage examples:

#[test]
fn test_expand_tilde() {
    // Should work on your linux box during tests, would fail in stranger
    // environments!
    let home = std::env::var("HOME").unwrap();
    let projects = PathBuf::from(format!("{}/Projects", home));
    assert_eq!(expand_tilde("~/Projects"), Some(projects));
    assert_eq!(expand_tilde("/foo/bar"), Some("/foo/bar".into()));
    assert_eq!(
        expand_tilde("~alice/projects"),
        Some("~alice/projects".into())
    );
}

Some remarks:

• The P: AsRef<Path> input type imitates what the standard
  library does. This is why the method accepts all Path-like
  inputs, like &str, &OsStr, and &Path.
• Path::new doesn’t allocate anything, it points to
  exactly the same bytes as the &str.
• strip_prefix("~/").unwrap() should never fail here,
  because we checked that the path starts with ~ and
  is not just ~. The only way how this can be is that
  the path starts with ~/ (because of how starts_with
  is defined).