Rust: Handle question mark operator in CFG

[paldepind]

The primary goal of this PR is to handle the ? operator (TryExpr) in the CFG, as that is currently the cause of dead-ends.

The PR does a few other things as well:

• Fixes a bug where abnormal control flow in the scrutinee of a match expression would incorrectly continue to match arms.
• Simplifies the implementation of return to use a standard tree.

With this PR, the code

fn test_question_mark_operator_1(s: &str) -> Option<i32> {
    str.parse::<u32>()? + 4
}

results in the CFG below. The CFG is roughly as if the a? operator is sugar for (if a.is_ok() { a.unwrap() } else { return None }) which I think is a fine way to handle it. But, do note that match edges are not created in the CFG, so a successful ? will not look the same in the CFG as a succesfull match on Some(foo). If we end up needing/wanting that, we should probably do it through desugaring to HIR.

flowchart TD
1["enter test_question_mark_operator_1"]
10["... + ..."]
11["4"]
2["exit test_question_mark_operator_1"]
3["exit test_question_mark_operator_1 (normal)"]
4["s"]
5["Param"]
6["BlockExpr"]
7["PathExpr"]
8["MethodCallExpr"]
9["TryExpr"]

1 --> 4
3 --> 2
4 -- match --> 5
5 --> 7
6 --> 3
7 --> 8
8 --> 9
9 -- return --> 3
9 --> 11
10 --> 6
11 --> 10

Loading

[aibaars]

Looks good to me!