How is a String implemented in Rust? How is it different from a string slice &str?
A String, like a Vec is a smart pointer to data on the heap. Like a Vec, it owns the data on the heap and contains additional metadata such as the length and capacity. In addition, it enforces that all the data it holds are valid UTF-8 characters. A string slice, on the other hand, is a fat pointer reference. It points to and merely borrows data on the heap that the String type owns.
This section from the online Rust book has a good explanation and helpful diagram: https://doc.rust-lang.org/book/ch04-03-slices.html#string-slices
let s = String::from("hello world");
let hello = &s[0..5];
let world = &s[6..11];