Try to write the panic message with a single write_all call

[Zoxc]

This writes the panic message to a buffer before writing to stderr. This allows it to be printed with a single write_all call, preventing it from being interleaved with other outputs. It also adds newlines before and after the message ensuring that only the panic message will have its own lines.

Before:

thread 'thread 'thread 'thread 'thread '<unnamed>thread 'thread 'thread 'thread '<unnamed><unnamed>thread '<unnamed>' panicked at ' panicked at <unnamed><unnamed><unnamed><unnamed><unnamed>' panicked at <unnamed>' panicked at src\heap.rssrc\heap.rs' 
panicked at ' panicked at ' panicked at ' panicked at ' panicked at src\heap.rs' panicked at src\heap.rs::src\heap.rssrc\heap.rssrc\heap.rssrc\heap.rssrc\heap.rs:src\heap.rs:455455:::::455:455::455455455455455:455:99:::::9:9:
:
999:
999:
assertion failed: size <= (*queue).block_size:
:
assertion failed: size <= (*queue).block_size:
assertion failed: size <= (*queue).block_size:
:
:
assertion failed: size <= (*queue).block_sizeassertion failed: size <= (*queue).block_sizeassertion failed: size <= (*queue).block_size

assertion failed: size <= (*queue).block_size
assertion failed: size <= (*queue).block_sizeassertion failed: size <= (*queue).block_sizeerror: process didn't exit successfully: `target\debug\direct_test.exe` (exit code: 0xc0000409, STATUS_STACK_BUFFER_OVERRUN)

After:

thread '<unnamed>' panicked at src\heap.rs:455:9:
assertion failed: size <= (*queue).block_size


thread '<unnamed>' panicked at src\heap.rs:455:9:
assertion failed: size <= (*queue).block_size


thread '<unnamed>' panicked at src\heap.rs:455:9:
assertion failed: size <= (*queue).block_size

error: process didn't exit successfully: `target\debug\direct_test.exe` (exit code: 0xc0000409, STATUS_STACK_BUFFER_OVERRUN)

r? @the8472

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[RalfJung]

Suggested change

	write_msg(err)
	// The message does not fit into the buffer, write it directly to stderr instead.
	write_msg(err)

[RalfJung]

I think it'd be less confusing to move the let _ into the arms (showing more clearly that it's just ignoring IO errors).

[RalfJung]

I'd suggest to separate the functional change (more newlines) from the implementation change. Makes it easier to discuss and decide on these separately.

A leading newline is not very common in Rust output, so if it stays it definitely needs a comment explaining the purpose.

[the8472]

Panic output isn't stable. There's some work to add thread IDs to the panic message for example.

Yeah the extra newline is unusual, but this is an unsynchronized write to a shared output so we don't have many options to clean up the formatting in cases where a panic is emitted in the middle of something else that's printing.

proc_macro_generated_packed.stderr is a good example of this. It start printing the panic message in the middle of reporting another error.

Do you think it's disruptive enough that it warrants separate discussion?

[rustbot]

Some changes occurred in run-make tests.

cc @jieyouxu

[bors]

☔ The latest upstream changes (presumably #122267) made this pull request unmergeable. Please resolve the merge conflicts.

[the8472]

I think it's better to filter out blank lines here so that the test isn't too sensitive to such changes.
The function name says it's supposed to test for the absence of backtraces, so blank lines shouldn't be relevant.