Subject:

fun with comments

Date: Mon, 14 Aug 2023
Message-Id: https://www.5snb.club/posts/2023/fun-with-comments/
Tags: #hack(6)

Programming languages typically include one or two ways to comment out code so that the compiler doesn’t read it. One being line comments, such as

foo(); // comment here

which will comment out any code up until the end of the line, and the other being block comments, such as

foo(42 /* a 
    good 
    number */ )

which will comment out code in a block.

Block comments aren’t restricted to a single line, and can sometimes be nested (so that /* /* */ */ behaves correctly).

But what happens when you mix the two?

Well, if you comment out a block comment directive with an active line comment, then the block comment does not apply. This can be seen most simply with

fn main() {
    // /*
    */
}

error: expected expression, found `/`
 --> src/main.rs:3:6
  |
3 |     */
  |      ^ expected expression

Here, the block comment didn’t actually start. Which makes sense, it’s in commented out code.

But active line comment there is important. If a block comment did start, then the line comment does not apply, only block comments do.

fn main() {
    /*
    This is commented out
    // */
    but_this_is_not();
}

The // does not matter, because it’s commented out.

Therefore, if you want to end a block comment if it exists, otherwise do nothing, then you can do // */. That line comment will be active if you’re not currently in a block comment (in which case the */ is just part of the comment), or it will not be active, but it will be commented out, in which case the */ is active and removes a nesting level.

This means that if you have a large code span that you want to quickly comment out and uncomment, you can put a /* at the top, a // */ at the bottom, and then replace the /* with a //* with a single slash.

fn main() {
    //*
    this_is_no_longer_commented_out();
    // */
    and_this_is_not();
}

I have used that in the past when I don’t have IDE shortcuts. Useful? No, not really. But a fun hack.

Finally, if you want to switch between 2 different functions with a single keystroke, you can put a function on the same line as the // */ line, and make use of the fact that the line comment is conditionally active.

fn main() {
    //*
    a();
    // */ b();
}

fn main() {
    /*
    a();
    // */ b();
}

a() can be multi-line, but I don’t think there’s a way to let b() be multi-line.