Rust: Placeholder queries for unused variable, unused value

[geoffw0]

Placeholder queries for unused variable + unused value in Rust.

They are placeholder because we don't have everything we need to write the queries yet, but I want to get the tests and docs reviewed (and merged) ahead of time so that we are ready to move quickly. The implementations will follow when we have a working SSA library.

As for why there are two queries, it turns out that CPP has three "unused variable" type queries:

• cpp/unused-local-variable, which looks for local variables that are never used.
• cpp/unused-static-variable, which looks for static variables that are never used.
• cpp/unused-variable, which looks for assignments to variables that are never accessed.

The unused variable query corresponds with the first two, unused value (probably the more interesting from the perspective of demonstrating accurate AST, CFG and SSA information) corresponds with the last one.

[github-actions]

QHelp previews:

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

Unused value

This rule finds values that are assigned to variables but never used. Unused values should be removed to increase readability and avoid confusion.

Recommendation

Remove any unused values. Also remove any variables that only hold unused values.

Example

In the following example, there is a variable average that is initialized to 0, but that value is never used:

fn get_average(values:&[i32]) -> f64 {
	let mut sum = 0;
	let mut average = 0.0; // BAD: unused value

	for v in values {
		sum += v;
	}

	average = sum as f64 / values.len() as f64;
	return average;
}

The problem can be fixed by removing the unused value:

fn get_average(values:&[i32]) -> f64 {
	let mut sum = 0;
	let average;

	for v in values {
		sum += v;
	}

	average = sum as f64 / values.len() as f64;
	return average;
}

References

• GeeksforGeeks: How to avoid unused Variable warning in Rust?

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

Unused variable

This rule finds variables that are never accessed. Unused variables should be removed to increase readability and avoid confusion.

Recommendation

Remove any unused variables.

Example

In the following example, there is an unused variable average that is never used:

fn get_sum(values:&[i32]) -> i32 {
	let mut sum = 0;
	let mut average; // BAD: unused variable

	for v in values {
		sum += v;
	}

	return sum;
}

The problem can be fixed simply by removing the variable:

fn get_sum(values:&[i32]) -> i32 {
	let mut sum = 0;

	for v in values {
		sum += v;
	}

	return sum;
}

References

• GeeksforGeeks: How to avoid unused Variable warning in Rust?

[subatoi]

Thanks Geoff—LGTM 👍