Panic when calling `.format`: "Format: was already formatted once"

[flip1995]

I tried this code:

use itertools::Itertools;
fn main() {
    let mut v = Vec::new();
    v.push("foo");
    assert!(
        v.is_empty(),
        "{}",
        v.iter().format(", "),
    );
}

Playground

And when trying to run it, I got this panic:

Format: was already formatted once

thread 'main' panicked at 'thread 'main' panicked at 'Format: was already formatted once', /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/itertools-0.10.5/src/format.rs:81:21
stack backtrace:
   0:     0x55575722e373 - std::backtrace_rs::backtrace::libunwind::trace::hb9265895128f1f9c
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x55575722e373 - std::backtrace_rs::backtrace::trace_unsynchronized::h028901cb3a0224b1
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x55575722e373 - std::sys_common::backtrace::_print_fmt::hfa5c37f64d314fee
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x55575722e373 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h5b30f1684e8bd3e6
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x55575724982f - core::fmt::write::h997ecd0e22e2af7a
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/fmt/mod.rs:1247:17
   5:     0x55575722c621 - std::io::Write::write_fmt::h291317e85b978e13
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/io/mod.rs:1698:15
   6:     0x55575722e185 - std::sys_common::backtrace::_print::h0133b24def7b99d9
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x55575722e185 - std::sys_common::backtrace::print::h4a42174c7f3bd207
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x55575722f4ff - std::panicking::default_hook::{{closure}}::h0847452fe926d6e5
   9:     0x55575722f1b7 - std::panicking::default_hook::hbcdd7833994c05d0
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:293:9
  10:     0x55575722fa21 - std::panicking::rust_panic_with_hook::hb55c52c8e21298f0
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:700:13
  11:     0x555757215394 - std::panicking::begin_panic::{{closure}}::hb08efbb470f664d6
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:624:9
  12:     0x5557572152bd - std::sys_common::backtrace::__rust_end_short_backtrace::hbf8634da0334b4e2
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:150:18
  13:     0x555757215307 - std::panicking::begin_panic::h61b21b87c2e55e8f
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:623:12
  14:     0x555757214cdb - itertools::format::Format<I>::format::heb0f74abc06f4991
                               at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/itertools-0.10.5/src/format.rs:81:21
  15:     0x5557572149b3 - <itertools::format::Format<I> as core::fmt::Display>::fmt::h064530a6ea8ffcbf
                               at /playground/.cargo/registry/src/index.crates.io-6f17d22bba15001f/itertools-0.10.5/src/format.rs:105:21
  16:     0x55575724982f - core::fmt::write::h997ecd0e22e2af7a
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/fmt/mod.rs:1247:17
  17:     0x55575724982f - core::fmt::write::h997ecd0e22e2af7a
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/fmt/mod.rs:1247:17
  18:     0x55575722c621 - std::io::Write::write_fmt::h291317e85b978e13
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/io/mod.rs:1698:15
  19:     0x55575722f3b3 - std::panicking::default_hook::{{closure}}::h0847452fe926d6e5
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:255:21
  20:     0x55575722f1b7 - std::panicking::default_hook::hbcdd7833994c05d0
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:293:9
  21:     0x55575722fa21 - std::panicking::rust_panic_with_hook::hb55c52c8e21298f0
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:700:13
  22:     0x55575722f913 - std::panicking::begin_panic_handler::{{closure}}::h51ad0ee3ee1b4d02
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:595:13
  23:     0x55575722e7b6 - std::sys_common::backtrace::__rust_end_short_backtrace::hd1006853c72eedaf
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:150:18
  24:     0x55575722f612 - rust_begin_unwind
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:584:5
  25:     0x555757212ed3 - core::panicking::panic_fmt::h0405aceea833a54b
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/panicking.rs:67:14
  26:     0x555757213f7f - core::panicking::panic_display::he369e4705b14dda4
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/panicking.rs:150:5
  27:     0x555757215201 - playground::main::hed8c86f138364518
                               at /playground/src/main.rs:5:5
  28:     0x5557572137db - core::ops::function::FnOnce::call_once::hbebd13fb81ee85e2
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/ops/function.rs:250:5
  29:     0x5557572134ee - std::sys_common::backtrace::__rust_begin_short_backtrace::h8be5514a066de9a8
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/sys_common/backtrace.rs:134:18
  30:     0x555757213561 - std::rt::lang_start::{{closure}}::h283d7d606f728596
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/rt.rs:166:18
  31:     0x55575722a432 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::ha628a292c7e4d1fe
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/core/src/ops/function.rs:284:13
  32:     0x55575722a432 - std::panicking::try::do_call::haef2b0ee78305734
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:490:40
  33:     0x55575722a432 - std::panicking::try::h18b79ccaa1dcd46e
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:454:19
  34:     0x55575722a432 - std::panic::catch_unwind::h44fa38099b98dbce
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panic.rs:140:14
  35:     0x55575722a432 - std::rt::lang_start_internal::{{closure}}::h90d1d020380fa86c
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/rt.rs:148:48
  36:     0x55575722a432 - std::panicking::try::do_call::h96eb402652927015
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:490:40
  37:     0x55575722a432 - std::panicking::try::h9ac5ba472c3806a8
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panicking.rs:454:19
  38:     0x55575722a432 - std::panic::catch_unwind::ha6f72383b6160344
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/panic.rs:140:14
  39:     0x55575722a432 - std::rt::lang_start_internal::hc00ff9425137690f
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/rt.rs:148:20
  40:     0x55575721353a - std::rt::lang_start::hf0c3703aafa2e063
                               at /rustc/b628260df0587ae559253d8640ecb8738d3de613/library/std/src/rt.rs:165:17
  41:     0x55575721523e - main
  42:     0x7f45f2cf9083 - __libc_start_main
  43:     0x55575721340e - _start
  44:                0x0 - <unknown>
thread panicked while panicking. aborting.
timeout: the monitored command dumped core

My workaround is to use join instead.

[flip1995]

Trying this with different compiler versions, it seems like this is a regression in 1.71.0 nightly. I will open a rust-lang/rust issue about this as well.

EDIT: rust-lang/rust#110717

[phimuemue]

Seems to be the same as #667.

[flip1995]

With rust-lang/rust#110721 merged, this will work in the next nightly Rust version again.