Fix Termination impl panic on closed stderr

[dtolnay]

Repro:

#![feature(backtrace)]

use std::backtrace::Backtrace;
use std::io::{self, Write as _};
use std::panic::{self, PanicInfo};

#[derive(Debug)]
pub struct Error;

fn panic_hook(panic_info: &PanicInfo) {
    let backtrace = Backtrace::force_capture();
    let _ = write!(io::stdout(), "{}\n{}", panic_info, backtrace);
}

fn main() -> Result<(), Error> {
    panic::set_hook(Box::new(panic_hook));
    let stderr = io::stderr();
    let mut stderr = stderr.lock();
    while stderr.write_all(b".\n").is_ok() {}
    Err(Error)
}

Before:

$ target/debug/repro 3>&2 2>&1 1>&3 | head
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panicked at 'failed printing to stderr: Broken pipe (os error 32)', library/std/src/io/stdio.rs:1016:9
   0: testing::panic_hook
             at ./src/main.rs:11:21
   1: core::ops::function::Fn::call
             at /git/rust/library/core/src/ops/function.rs:77:5
   2: std::panicking::rust_panic_with_hook
   3: std::panicking::begin_panic_handler::{{closure}}
   4: std::sys_common::backtrace::__rust_end_short_backtrace
   5: rust_begin_unwind
   6: core::panicking::panic_fmt
   7: std::io::stdio::_eprint
   8: <core::result::Result<!,E> as std::process::Termination>::report
             at /git/rust/library/std/src/process.rs:2164:9
   9: <core::result::Result<(),E> as std::process::Termination>::report
             at /git/rust/library/std/src/process.rs:2148:25
  10: std::rt::lang_start::{{closure}}
             at /git/rust/library/std/src/rt.rs:145:18
  11: std::rt::lang_start_internal
  12: std::rt::lang_start
             at /git/rust/library/std/src/rt.rs:144:17
  13: main
  14: __libc_start_main
             at /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
  15: _start

After:

$ target/debug/repro 3>&2 2>&1 1>&3 | head
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[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

r? @Mark-Simulacrum

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[joshtriplett]

@bors r+

[bors]

📌 Commit 563aa12 has been approved by joshtriplett

[bors]

⌛ Testing commit 563aa12 with merge 56db11c0496f579a182c562fcd5c46afce6f0b1c...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

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