Book: Documentation about the dbg! macro

[Centril]

The dbg! macro was just stabilized and will be in Rust 1.32 (see https://github.com/rust-lang/rust/issues/54306). Since one important reason for introducing the macro in the first place was to help beginners it might make sense to use it in the book.

[steveklabnik]

A lot of our printlns would be dbgs, I think. @carols10cents should we do a massive switch-over?

[ngoldbaum]

As a new rust user I really like dbg. I think it would also make the introductory sections of the book clearer because you don't need to gloss over the string formatting rules or explain what "{}" means.

[carols10cents]

Sure, once we switch to 1.32 i'd be up for it. We're currently on 1.31.

[rusty-snake]

since there is code that need 1.32 or newer (#2034) I think the title page should be updated and dbg! can come.

[carols10cents]

Just realized that because dbg! takes ownership, instead of having to explain {} or {:?}, we'll have to explain & in order to use this early on in the book. 🤷‍♀ Not sure that's an improvement.

[dekellum]

It can be used on copy types without that. Or save dbg! for just after borrowing is addressed?

[rusty-snake]

@carols10cents dbg! return its parameter, so you can use it when assigning a variable or you can shadowing it.

fn main() {
    let s = dbg!("abc".to_owned());
    let s = dbg!(s);
    drop(s);
}

Or even

fn main() {
    if "42" == dbg!(answer.get()) {
        println!("The answer is 42");
    }
}

Copy types always work (as @dekellum sayed)

[carols10cents]

@rusty-snake yes, I read the 2 RFCs and the tracking issue before commenting here. I understand the justification behind the dbg! macro taking ownership. I don't think that affects the issue I raised here over whether it would be better to switch the book from using println! to dbg!.

[dekellum]

Isn't there intrinsic value to the reader and early learner, to see both println! and dbg! at some point in the book, say before finding it in code in the wild?

[carols10cents]

@dekellum the issue is that closing this issue is not just a quick find-and-replace println! for dbg! in the book, and that the implementation of dbg! complicates the explanation.

[dekellum]

Fair enough, I still think it has intrinsic value, as early learners will soon encounter both in the wild, and print debugging is rather fundamental.

[nealmcb]

I've been going thru the book, as a newbie, and was already thinking that printing debug info wouldn't be as fun as I'd like.

Then I just ran across dbg! which is indeed fabulous.

I thing getting at least a mention of it in the documentation is a great idea.

We don't have to switch everything to use it in a few places and then later start using it more after & has been explained.