Rust: Unreachable code query

[geoffw0]

Unreachable code query for Rust.

@paldepind please would you review, particularly whether the logic in firstUnreachable and skipNode is reasonable and written as cleanly as possible right now; and whether there are obvious explanations for the false positive results in the test?

[github-actions]

QHelp previews:

rust/ql/src/queries/unusedentities/UnreachableCode.qhelp

Unreachable code

This rule finds code that is never reached. Unused code should be removed to increase readability and avoid confusion.

Recommendation

Remove any unreachable code.

Example

In the following example, the final return statement can never be reached:

fn fib(input: u32) -> u32 {
	if (input == 0) {
		return 0;
	} else if (input == 1) {
		return 1;
	} else {
		return fib(input - 1) + fib(input - 2);
	}

	return input; // BAD: this code is never reached
}

The problem can be fixed simply by removing the unreachable code:

fn fib(input: u32) -> u32 {
	if (input == 0) {
		return 0;
	} else if (input == 1) {
		return 1;
	} else {
		return fib(input - 1) + fib(input - 2);
	}
}

References

• Wikipedia: Unreachable code