Mutable references from immutable variables are allowed and result in copies

[nventuro]

Aim

I should not be allowed to take a &mut from a non-mut variable. Not only does Noir allow it, it apparently creates a copy of the value when doing so, which means that the mutable reference does not refer to the original value, leading to hard-to-track bugs in which expected mutations are not carried out.

Expected Behavior

Given

fn bar(x: &mut Field) {
  *x = 2;
}

then none of the following calls to bar should compile, but they all do

fn foo(x: Field) {
  bar(&mut x);
  
  let y = x;
  bar(&mut y);

  let (j, k) = baz(x);
  bar(&mut k);
}

fn baz(x: Field) -> (Field, Field) {
  (x, x)
}

Bug

These two tests pass, but take_ref_from_non_mut should not even compile.

fn mutate_contents(x: &mut Field) {
    *x = 2;
}

fn take_ref_from_non_mut(x: Field) -> Field {
    mutate_contents(&mut x);
    x
}

fn take_ref_from_mut(mut x: Field) -> Field {
    mutate_contents(&mut x);
    x
}

fn test_ref_from_non_mut() {
    assert_eq(take_ref_from_non_mut(5), 0);
}

fn test_ref_from_mut() {
    assert_eq(take_ref_from_mut(5), 2);
}