Why can't I return `format_args` from a closure?

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1

(Playground link)

use itertools::Itertools;

fn main() {
    let v = vec![1,2,3,4,5];
    // why this line doesn't work?
    v.iter().map(|it| format_args!("x{}", it)).intersperse(format_args!(", ")).for_each(|args| println!("{}", args));
    
    // whereas this line work
    println!("{}", v.iter().format_with(", ", |it, f| f(&format_args!("x{}", it))));
}

The error message reads "returns a value referencing data owned by the current function", it is not owned by the current function, it is owned by the vector. What's wrong with the code?

2

The macro evaluates each expression argument into macro-generated locals, then passes references to those in the Arguments structure. It seems simple in your case that it could just use your given reference, but that doesn't generalize.

3

Oh, I see. In this case, is there any way to tell the macro to use the expression as a reference directly?

4

You can create internal type which implements std::fmt::Display, and return it from a function using impl std::fmt::Display + 'a.
With this way, You can return to-be-formatted objects without rendering them too earlily.
However, this requires lots of boilerplates and would not be useful for some cases.

5

That part can be factored out by being generic over a formatting closure, and a macro that converts format!-like syntax into such a closure:

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