extern crate walkdir;
use walkdir::WalkDir;
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)
}

fn main() {
    let mut cpp_files = Vec::new();

    for e in WalkDir::new(".").into_iter().filter_map(|e| e.ok()) {
        if e.metadata().unwrap().is_file() {
            let p = e.path().display().to_string().as_str();

            if get_extension_from_filename(p) == Some("cpp") {
                cpp_files.push(p);
            }
        }
    }
}

Error:
error[E0716]: temporary value dropped while borrowed
  --> src\main.rs:17:21
   |
17 |             let p = e.path().display().to_string().as_str();
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^         - temporary value is freed at the end of this statement
   |                     |
   |                     creates a temporary which is freed while still in use
18 |
19 |             if get_extension_from_filename(p) == Some("cpp") {
   |                                            - borrow used here, in later iteration of loop
   |
   = note: consider using a `let` binding to create a longer lived value