ch19-06 added curly braces to macro output

[freund17]

I'm just learning rust (reading this book) so I might be wrong here...

But shouldn't the generated macro-output include the extra curly braces?

[eddyp]

If you add the curly braces then a scope would be created and the newly created vector would be dropped at the end of that scope.

Since the vector exists after the macro expansion, the proposed equivalent code can't use curly braces (unless the owned result is returned from the macro).

This will become more clear when ownership is discussed.

If after my explanation you understand, then please also close the issue.

[freund17]

If you add the curly braces then a scope would be created and the newly created vector would be dropped at the end of that scope.

Yes, but the vector is also returned by that scope...

Let me illustrate my point with an example:

let test = vec![1, 2, 3];

If the macro got unwrapped to what the book says this would become:

let test = let mut temp_vec = Vec::new();
temp_vec.push(1);
temp_vec.push(2);
temp_vec.push(3);
temp_vec

Whereas if it would become unwrapped the way I expect it to it would become:

let test = {
    let mut temp_vec = Vec::new();
    temp_vec.push(1);
    temp_vec.push(2);
    temp_vec.push(3);
    temp_vec
}

which makes more sense to me.
But also, I may be missunderstanding how macros work... But they seemed to be more or less literal code generators. 🤷‍♂

This also would explain the extra {} in

#[macro_export]
macro_rules! vec {
    ( $( $x:expr ),* ) => {
        { // <-- these ones
            let mut temp_vec = Vec::new();
            $(
                temp_vec.push($x);
            )*
            temp_vec
        } // <-- these ones
    };
}

[eddyp]

As I said, "unless the owned result is returned"...

I agree with the first part.

Not sure what you mean in the second (extra curlies). I think they are connected to the way macros work. If it were plain Rust code, removing them would have no effect since the language is expression based.

[steveklabnik]

It's sorta tough because the actual macro calls a method these days:

https://doc.rust-lang.org/stable/src/alloc/macros.rs.html#38-46

I do think the extra curlies make sense, but also, may be a bit distracting for others. I'm really torn. @carols10cents , any thoughts?

[carols10cents]

I think the curly brackets make sense and won't be too confusing. Thanks!