Rust: What is the difference between &T and &mut T?

&T — immutable reference. &mut T — mutable reference.

&T:

• Multiple allowed.
• Read-only.
• Safe.

&mut T:

• Only one allowed.
• Read/write.
• Exclusive.

1fn read(data: &[i32]) {
2    println!("{:?}", data);
3}
4
5fn write(data: &mut Vec<i32>) {
6    data.push(1);
7}

Borrow checker enforces:

• No &mut T while &T exists.
• No &mut T while another &mut T exists.

Key: &T for reading, &mut T for writing.