Unable to set permissions in RUST - rust

Question

Hi I have just started to program in Rust.
Here is my build.rs
use std::fs::{File, self};
use std::io::Write;
use std::os::unix::prelude::PermissionsExt;
use std::env;
use std::path::{Path, PathBuf};
fn main() {
let out_dir = env::var_os("CARGO_MANIFEST_DIR").unwrap();
let dest_path: PathBuf = Path::new(&out_dir).join("target/release/Settings.toml");
println!("{:?}", dest_path.as_os_str());
let f = File::create(dest_path);
f.as_ref()
.unwrap()
.write_all("largest_number = 7".as_bytes())
.unwrap();
set_permissions(f);
}
fn set_permissions(f: Result<File, std::io::Error>) {
f.as_ref()
.unwrap()
.metadata()
.unwrap()
.permissions()
.set_mode(0x777);
}
The problem I am having is even after executing cargo build the permissions of the Settings.toml file is not 777.
root#xxxx:~/code/rust/addition# ls -larth target/release/
total 5.3M
drwxrwxrwx 2 root root 4.0K Feb 14 19:16 incremental
drwxrwxrwx 2 root root 4.0K Feb 14 19:16 examples
-rwxrwxrwx 1 root root 0 Feb 14 19:16 .cargo-lock
drwxrwxrwx 4 root root 4.0K Feb 14 19:16 ..
-rwxrwxrwx 1 root root 233 Feb 14 22:32 addition.d
drwxrwxrwx 2 root root 20K Feb 16 20:31 deps
-rwxrwxr-x 2 vinay vinay 5.2M Feb 16 20:31 addition
drwxrwxrwx 29 root root 4.0K Feb 16 22:02 build
drwxrwxrwx 90 root root 4.0K Feb 16 22:02 .fingerprint
drwxrwxrwx 7 root root 4.0K Feb 16 22:58 .
-rw-r--r-- 1 root root 18 Feb 16 23:17 Settings.toml
root#xxxx:~/code/rust/addition#

Answer 1

From Permissions::set_readonly() docs (the same apply for set_mode()):
This operation does not modify the files attributes. This only changes the in-memory value of these attributes for this Permissions instance. To modify the files attributes use the set_permissions function which commits these attribute changes to the file.

Answer 2

Well your code does not change the permissions on the file but only the in memory representation of them. To set the permissions of a file you have to call it's set_permissions method:
fn set_permissions(f: Result<File, std::io::Error>) {
let perm = Permissions::from_mode(0o777);
f.as_ref().unwrap().set_permissions(perm).unwrap();
}
Note: I also changed from hexadecimal to octal numbers which is very likely what you really want.