Change default panic handler message format.

[m-ou-se]

Note: See below for the snippet for the Rust 1.73 release blog post.

---

This changes the default panic hook's message format from:

thread '{thread}' panicked at '{message}', {location}

to

thread '{thread}' panicked at {location}:
{message}

This puts the message on its own line without surrounding quotes, making it easiser to read. For example:

Before:

thread 'main' panicked at 'env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`', src/main.rs:4:6

After:

thread 'main' panicked at src/main.rs:4:6:
env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`

---

See this PR by @nyurik, which does that for only multi-line messages (specifically because of assert_eq): #111071

This is the change that does that for all panic messages.

[m-ou-se]

@craterbot run mode=build-and-test

[craterbot]

🎉 Experiment pr-112849 is completed!
📊 210 regressed and 118 fixed (303810 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the blacklist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[nyurik]

Thx everyone for getting this one through! Turns out the simplest PRs are the hardest to merge :)

I re-based #111071 on top of the main branch, so now its much simpler. I think we should release both panic message changes in the same release if possible, as they are closely related.

[rust-timer]

Finished benchmarking commit (828bdc2): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	2
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.3%]	4
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	2

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.5%	[-4.5%, -4.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[1.7%, 4.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-5.3%, -1.4%]	4
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: missing data

[lqd]

@rust-lang/wg-compiler-performance: We may need to do something about #114318 breaking the bootstrap timings. They're still missing for more recent PRs, like this one.

[m-ou-se]

Snippet for the release blog post (in Markdown), for both this PR and #111071:

## Cleaner panic messages

The output produced by the default panic handler has been changed
to put the panic message on its own line instead of wrapping it in quotes.
This can make panic messages easier to read, as shown in this example:

<div style="margin:1em"><pre><code>fn main() {
    let file = "ferris.txt";
    panic!("oh no! {file:?} not found!");
}</code></pre>
Output before Rust 1.73:
<pre style="margin-top:0"><code style="background:#000;color:#ccc" class="language-text">thread 'main' panicked at 'oh no! "ferris.txt" not found!', src/main.rs:3:5</code></pre>
Output starting in Rust 1.73:
<pre style="margin-top:0"><code style="background:#000;color:#ccc" class="language-text">thread 'main' panicked at src/main.rs:3:5:
oh no! "ferris.txt" not found!</code></pre></div>

This is especially useful when the message is long, contains nested quotes, or spans multiple lines.

Additionally, the panic messages produced by `assert_eq` and `assert_ne` have
been modified, moving the custom message (the third argument)
and removing some unnecessary punctuation, as shown below:

<div style="margin:1em"><pre><code>fn main() {
    assert_eq!("🦀", "🐟", "ferris is not a fish");
}</code></pre>
Output before Rust 1.73:
<pre style="margin-top:0"><code style="background:#000;color:#ccc" class="language-text">thread 'main' panicked at 'assertion failed: `(left == right)`
 left: `"🦀"`,
right: `"🐟"`: ferris is not a fish', src/main.rs:2:5</code></pre>
Output starting in Rust 1.73:
<pre style="margin-top:0"><code style="background:#000;color:#ccc" class="language-text">thread 'main' panicked at src/main.rs:2:5:
assertion `left == right` failed: ferris is not a fish
 left: "🦀"
right: "🐟"</code></pre></div>

Rendered

[safinaskar]

Despite all this effort, bug #108341 still persists. I. e. I still see ugly \n in assert output if line is too long

[safinaskar]

Also, backtraces are still completely impossible to understand: #99560

[workingjubilee]

Naturally. This PR was not intended to fix either of those problems. If you want to help us improve our panic and backtrace diagnostics, a good first step would be to find all the issues that have to deal with panic and labeling them like so:

@rustbot label: +A-panic

That is something anyone can do, and it will significantly help us sort these issues out by making it easier to try to figure out which issues are related to each other.