bug: iterator modification check has false positives and false negatives

[tserg]

Version Information

• vyper Version (output of vyper --version): ed0a654
• OS: linux
• Python Version (output of python --version): 3.10.8

What's your issue about?

The _check_iterator_modification check in vyper/semantics/analysis/local.py has false positives and false negatives.

The first contract is a false negative, and the second contract is a false positive.

struct A:
    iter: DynArray[uint256, 5]

a: A

@external
def foo():
    self.a.iter = [1, 2, 3]
    for i in self.a.iter:
        self.a = A({iter: [1, 2, 3, 4]})

a: DynArray[uint256, 5]
b: uint256[10]

@external
def foo():
    self.a = [1, 2, 3]
    for i in self.a:
        self.b[self.a[1]] = i

Error compiling: tmp/iterator_false_positive.vy
vyper.exceptions.ImmutableViolation: Cannot modify array during iteration
  contract "tmp/iterator_false_positive.vy:8", function "foo", line 8:15 
       7     for i in self.a:
  ---> 8         self.b[self.a[1]] = i
  ----------------------^
       9

h/t @trocher @ptrcarta

How can it be fixed?

Fill this in if you know how to fix it.