Mutable Type vs Mutable Binding

In types definitions, mut T doesn’t exist. Only &mut T

• ✅ &mut T
• ❌ mut T

In variable bindings, mut is used like this:

• let mut foo: T = ...

Note that mut in type and variable bindings serve a distinctly different role:

• Types:
  • &mut T means we borrow T mutably - we are guaranteed exclusive mutable access.
  • Notice that mut T does not make sense in the first place, because we already control mutability via Ownership rules.
• Bindings:
  • let mut means the value can be mutated.
  • Allows lending to functions that accept &mut T arguments

We can rebind T as mutable, but we cannot convert &T to &mut T. (implied by point 1 of Borrowing rules.)