error[E0384]: cannot assign twice to immutable variable `b`
  --> $DIR/issue-45199.rs:6:5
   |
LL |     let b: Box<isize>;
   |         - help: make this binding mutable: `mut b`
...
LL |     b = Box::new(1);
   |     - first assignment to `b`
LL |     b = Box::new(2);
   |     ^ cannot assign twice to immutable variable

error[E0384]: cannot assign twice to immutable variable `b`
  --> $DIR/issue-45199.rs:14:5
   |
LL |     let b = Box::new(1);
   |         -
   |         |
   |         first assignment to `b`
   |         help: make this binding mutable: `mut b`
...
LL |     b = Box::new(2);
   |     ^ cannot assign twice to immutable variable

error[E0384]: cannot assign to immutable argument `b`
  --> $DIR/issue-45199.rs:20:5
   |
LL | fn test_args(b: Box<i32>) {
   |              - help: make this binding mutable: `mut b`
LL |
LL |     b = Box::new(2);
   |     ^ cannot assign to immutable argument

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0384`.