Memory

• Does not have a Garbage Collector.

Ownership

Borrow Checker

• Explanation .

  • Quite interesting.

• Video demonstrating 'Stack', 'Heap', 'Borrowing', and 'Ownership' .

  • The memory explanation is good, but the rest of the video is not well explained.

Lifetime

Syntax with '

&i32        // a reference
&'a i32     // a reference with an explicit lifetime
&'a mut i32 // a mutable reference with an explicit lifetime

Usage and need

• The return type needs a generic lifetime parameter on it because Rust can’t tell whether the reference being returned refers to x  or y .

  fn longest(x: &str, y: &str) -> &str {
      if x.len() > y.len() {
          x
      } else {
          y
      }
  }
  

• This tells Rust that the string slice returned from the function will live at least as long as lifetime 'a

  fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
      if x.len() > y.len() {
          x
      } else {
          y
      }
  }
  

• Example:

  use std::fmt::Display;
  
  fn longest_with_an_announcement<'a, T>(
      x: &'a str,
      y: &'a str,
      ann: T,
  ) -> &'a str
  where
      T: Display,
  {
      println!("Announcement! {ann}");
      if x.len() > y.len() {
          x
      } else {
          y
      }
  }
  

Requirements

• The return lifetime of the function must be one of the lifetimes of its parameters.

  • Does not work :

  fn longest<'a>(x: &str, y: &str) -> &'a str {
      let result = String::from("really long string");
      result.as_str()
  }
  

  • result  stops existing when the function scope ends, so its lifetime cannot be used.

• Lifetime annotations become part of the function’s contract, similar to types in its signature.

Conclusion

• "The Lifetime is NOT  changed. It only creates relationships between the lifetimes of multiple references."

• The generic lifetime 'a  will get the concrete lifetime that equals the shorter of the lifetimes  of x  and y .

• The returned reference will also be valid for the shorter lifetime  of x  and y .

• "Is the smallest lifetime still valid?"