removal of closures and functions that contain assert macros cause false positives

[0xalpharush]

Necessist will warn that a test is passing with code remove for the following examples:

• A closure that contains an assertion

fn test1() {
std::thread::.spawn(move || {
                       ...
                        assert!(
                          ...
                        );
}

• A function call that contains an assertion

fn test2() {
   let x = ...
   let y = ...
    my_assertion_helper(x, y);
}
fn my_assertion_helper(x, y) { assert!(...); }

Both of these mutations do not indicate that a test statement is unnecessary, and I think they should be ignored since assert is ignored.

[smoelius]

For both of these cases, I think the right thing to do is walk the function/closure bodies.

Closures are a little tricky, though. For the case of spawning a thread (e.g., the closure body is always executed), I think walking the closure body is probably the right policy.

But if the closure is executed conditionally, the situation becomes more complicated. See, e.g., the example in the README: https://github.com/trailofbits/necessist#example

FWIW, I outlined a strategy for choosing what to remove/ignore in this PR: #474

I think what I wrote here is largely consistent with that PR, though closures require more thought.

[smoelius]

As of version 0.7.0, Necessist walks functions declared within the same files as the tests that call them. So the second bullet should be mitigated in most circumstances.