[clang] The error message for `-gen-reproducer=error` should not request a bug report

[mysterymath]

We have observed the following sequence output when an error occurs with -gen-reproducer=error:

clang++: error: failing because '-gen-reproducer' is used
Fuchsia clang version 22.0.0git (https://llvm.googlesource.com/llvm-project 46e9d6325a825b516826d0c56b6231abfaac16ab)
Target: aarch64-unknown-fuchsia
Thread model: posix
InstalledDir: ../../prebuilt/third_party/clang/custom/bin
Build config: +assertions
clang++: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang++: note: diagnostic msg: clang-crashreports/vnode-cdfcdd.cpp
clang++: note: diagnostic msg: clang-crashreports/vnode-cdfcdd.sh
clang++: note: diagnostic msg:

********************

The mention of a bug report is confusing; it implies that there is a bug in the compiler, and there isn't in this case. This caused a downstream bug to be misfiled by an oncall-er who thought they were looking at a clang crash. Ideally, none of the produced text would imply that there's anything wrong with clang in the case that clang did not itself actually crash.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Daniel Thornburgh (mysterymath)

We have observed the following sequence output when an error occurs with `-gen-reproducer=error`:

clang++: error: failing because '-gen-reproducer' is used
Fuchsia clang version 22.0.0git (https://llvm.googlesource.com/llvm-project 46e9d6325a825b516826d0c56b6231abfaac16ab)
Target: aarch64-unknown-fuchsia
Thread model: posix
InstalledDir: ../../prebuilt/third_party/clang/custom/bin
Build config: +assertions
clang++: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang++: note: diagnostic msg: clang-crashreports/vnode-cdfcdd.cpp
clang++: note: diagnostic msg: clang-crashreports/vnode-cdfcdd.sh
clang++: note: diagnostic msg:

********************

The mention of a bug report is confusing; it implies that there is a bug in the compiler, and there isn't in this case. This caused a downstream bug to be misfiled by an oncall-er who thought they were looking at a clang crash. Ideally, none of the produced text would imply that there's anything wrong with clang in the case that clang did not itself actually crash.

[mysterymath]

There is already a downstream Fuchsia issue for this: http://fxbug.dev/338318264

From the issue:

The messages clang emits when it makes a reproducer are scary-looking and potentially confusing, and pretty much suggest that people should file every error as a compiler bug. That's bad for people who are misled by the messages, and it's bad for the people who have to triage spurious bug reports.

A simple change would be some new switch that allows for custom text for that logging in some fashion. Ideally there would be a way both to use custom text and to have that text embed the reproducer path in some fashion. But it's probably sufficient if using the custom-text option just makes it emit something like:

clang: <custom text we provide>
clang: reproducer file at clang-crashreports/abc123xyz.sh

Then we can provide custom text in the build that is just a short message pointing to a fuchsia.dev URL where we have a docs page fully explaining what it all means and what to do (and not to do) about it.

[shafik]

CC @AaronBallman

[AaronBallman]

I think it's reasonable to make improvements here, but we do document this behavior explicitly: https://clang.llvm.org/docs/UsersManual.html#cmdoption-gen-reproducer

Clang is also capable of generating preprocessed source file(s) and associated run script(s) even without a crash. This is especially useful when trying to generate a reproducer for warnings or errors while using modules.

-gen-reproducer

Generates preprocessed source files, a reproducer script and if relevant, a cache containing: built module pcm’s and all headers needed to rebuild the same modules.

So this option is kind of intentionally similar to a crash dialog like any other. So we may want to also update the documentation if we think this is confusing.