Extracting a file extension from a given path in Rust idiomatically

In idiomatic Rust the return type of a function that can fail should be an Option or a Result. In general, functions should also accept slices instead of Strings and only create a new String where necessary. This reduces excessive copying and heap allocations.

You can use the provided extension() method and then convert the resulting OsStr to a &str:

use std::path::Path;
use std::ffi::OsStr;

fn get_extension_from_filename(filename: &str) -> Option<&str> {
    Path::new(filename)
        .extension()
        .and_then(OsStr::to_str)
}

assert_eq!(get_extension_from_filename("abc.gz"), Some("gz"));

Using and_then is convenient here because it means you don't have to unwrap the Option<&OsStr> returned by extension() and deal with the possibility of it being None before calling to_str. I also could have used a lambda |s| s.to_str() instead of OsStr::to_str - it might be a matter of preference or opinion as to which is more idiomatic.

Notice that both the argument &str and the return value are references to the original string slice created for the assertion. The returned slice cannot outlive the original slice that it is referencing, so you may need to create an owned String from this result if you need it to last longer.

What's more idiomatic than using Rust's builtin method for it?

Path::new(&filename).extension()