The compiler enforces valid borrowing using its borrow checker. To accomplish this, it keeps track of two things:

• The lifetime of objects, and
• The scope of blocks

The lifetime of an object starts when the object is created and ends when it goes out of scope (i.e. it gets destroyed, because of the RAII discipline).

A lifetime looks like this: 'burrito, which reads as: “lifetime burrito”.

All references actually have a type signature of the form &'a T, where 'a is the lifetime of the referenced object. The compiler takes care of inserting the lifetime part 'a so we can simply type annotate references with &T.

For example:

let integer: int = 5
let ref_to_int: &int = &integer;

• integer has lifetime 'i (it could be any other name, like 'foo)
• ref_to_int has lifetime 'r (references also have lifetimes!)
• ref_to_int type signature actually is &'i int (the compiler inserts the 'i for us)
• The type signature &'i int reads as:
  • &: reference to an
  • int: integer with
  • 'i: lifetime i (i is the lifetime of integer!)

Because the compiler keeps track of the lifetime of referenced objects in the type system, it can avoid several memory bugs.

Haven‘t grok what is a lifetime yet? Don’t dismay! See the next page