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]

Regarding the second bullet, I am jotting down some notes about how it could work. These ideas are by no means set in stone.

• A type LocalFunction is added to this trait:

  necessist/frameworks/src/parsing.rs

  Line 58 in b88071c

  pub trait AbstractTypes {

• A method call_of_local_function is added to this trait:

  necessist/frameworks/src/parsing.rs

  Line 74 in b88071c

  pub trait ParseLow: Sized {

  The function returns an Option<<Self::Types as AbstractTypes>::LocalFunction<'ast>>.
• A method visit_local_function is added to the same trait.

The above are used as follows:

• When the GenericVisitor encounters a call, it asks the backend whether the call is of a local function (i.e., a function defined in the same test file). If so, the GenericVisitor adds the local function to a queue.
• Periodically, the GenericVisitor pops a local function from the queue and asks the backend to visit it.

A crucial point is that logic that is common among the backends is in the GenericVisitor, and is not duplicated among the backends.