mkdir i1 i2; echo a > i1/É; echo a > i2/é

$ LC_ALL=C ./target/release/diffutils diff --ignore-file-name-case i1/É i2
# Output: (none)
# Exit code: 0
$ LC_ALL=C ./target/release/diffutils diff -r --ignore-file-name-case i1 i2
# Output: (none)
# Exit code: 0

$ LC_ALL=C diff --ignore-file-name-case i1/É i2
diff: 'i2/'$'\303\211': No such file or directory
# Exit code: 2
$ LC_ALL=C diff -r --ignore-file-name-case i1 i2
# two "Only in" lines, one per directory
# Exit code: 1

pub fn fold_file_name_case(name: &OsStr) -> Vec<u8> {
    match name.to_str() {
        Some(name) => name.to_lowercase().into_bytes(),
        None => name.as_encoded_bytes().to_ascii_lowercase(),
    }
}

#[test]
#[cfg(unix)]
fn ignore_file_name_case_is_ascii_only_in_c_locale() -> Result<(), Box<dyn std::error::Error>> {
    let tmp_dir = tempdir()?;
    let (d1, d2) = (tmp_dir.path().join("i1"), tmp_dir.path().join("i2"));
    std::fs::create_dir(&d1)?;
    std::fs::create_dir(&d2)?;
    std::fs::write(d1.join("É"), "a\n")?;
    std::fs::write(d2.join("é"), "a\n")?;
    let mut cmd = cargo_bin_cmd!("diffutils");
    cmd.env("LC_ALL", "C")
        .arg("diff")
        .arg("-r")
        .arg("--ignore-file-name-case")
        .arg(&d1)
        .arg(&d2);
    cmd.assert().code(predicate::eq(1));
    Ok(())
}