Using format_args!() to build a fmt::Arguments fails to compile

[agrover]

This code:

fn main() {
    let test = &"hello".to_owned();
    let args = format_args!("{}", test);
}

returns this error:

6 |     let args = format_args!("{}", test);
  |                             ^^^^       - temporary value dropped here while still borrowed
  |                             |
  |                             temporary value does not live long enough
7 | }
  | - temporary value needs to live until here
  |
  = note: consider using a `let` binding to increase its lifetime

This makes no sense to me. Doesn't "{}" have a static lifetime?

btw this is related to rust-lang/log#282, I'm trying to recreate a fmt::Arguments to build a log::Record. This with 1.27.0.

Thanks!

[sfackler]

The error message pointing to the "{}" is a bit misleading - the temporary value is actually in the code generated by format_args, and the span happens to point to the format string.

Your example expands to this:

fn main() {
    let test = &"hello".to_owned();
    let args = ::std::fmt::Arguments::new_v1_formatted(
        &[""],
        &match (&test,) {
            (arg0,) => [::std::fmt::ArgumentV1::new(arg0, ::std::fmt::Display::fmt)],
        },
        &[::std::fmt::rt::v1::Argument {
            position: ::std::fmt::rt::v1::Position::At(0usize),
            format: ::std::fmt::rt::v1::FormatSpec {
                fill: ' ',
                align: ::std::fmt::rt::v1::Alignment::Unknown,
                flags: 0u32,
                precision: ::std::fmt::rt::v1::Count::Implied,
                width: ::std::fmt::rt::v1::Count::Implied,
            },
        }],
    );
}

Building that directly points to the actual code that's the problem:

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:7:10
   |
7  |           &match (&test,) {
   |  __________^
8  | |             (arg0,) => [::std::fmt::ArgumentV1::new(arg0, ::std::fmt::Display::fmt)],
9  | |         },
   | |_________^ temporary value does not live long enough
...
20 |       );
   |        - temporary value dropped here while still borrowed
21 |   }
   |   - temporary value needs to live until here
   |
   = note: consider using a `let` binding to increase its lifetime

[agrover]

Thanks for your help! Closing.