[BUG] Mutating list while holding immutable reference and mutating immutable references compile successfully.

[Deftioon]

Bug description

Mutating a list while holding an immutable reference to an item in a list does not error, and mutating this immutable reference also does not error.

Steps to reproduce

struct MyList:
    var list: List[Int]
    
    fn __init__(out self):
        self.list = List[Int]()
    
    fn __getitem__(self, index: Int) -> ref[self.list] Int:
        return self.list[index]
    
    fn append(mut self, value: Int) -> None:
        self.list.append(value)
    
    fn do_stuff(mut self, index: Int) -> None:
        self.list[index] = 1

fn main() raises -> None:
    var mylist = MyList()
    mylist.append(0)
    var q1 = mylist[0]
    print(q1)
    mylist.do_stuff(0)
    print(q1)
    print(mylist[0])

This code snippet creates an immutable reference q1 and then mutates the list, and compiles successfully. Outputs:

0
0
1

Running this function:

fn main() raises -> None:
    var mylist = MyList()
    mylist.append(0)
    var q1 = mylist[0]
    print(q1)
    q1 = 1
    print(q1)
    print(mylist[0])

Also compiles successfully. Outputs:

0
1
0

System information

- What OS did you do install Mojo on ?
- Provide version information for Mojo by pasting the output of `mojo -v`
- Provide Magic CLI version by pasting the output of `magic -V` or `magic --version` 
- Optionally, provide more information with `magic info`.

OS: `Ubuntu 24.04.1 LTS`
Mojo Version: magic 0.4.0 - (based on pixi 0.33.0)
Magic CLI version: `magic 0.4.0 - (based on pixi 0.33.0)`
A bit of `magic info` output:

      Pixi version: 0.33.0
          Platform: linux-64
  Virtual packages: __unix=0=0
                  : __linux=6.8.0=0
                  : __glibc=2.39=0
                  : __cuda=12.4=0
                  : __archspec=1=icelake
         Cache dir: /home/deftioon/.cache/rattler/cache
      Auth storage: /home/deftioon/.rattler/credentials.json
  Config locations: No config files found