rustdoc: detect docs that use re-export concat

[notriddle]

DO NOT MERGE WITHOUT SETTING THE DEFAULT BACK TO WARN

This is a prereq for implementing LaTeX math (tracking issue: #162365). Because I want to be able to enable it on a per-docblock basis, re-exports need to be parsed with a different set of markdown parsing extensions than the items they link to, which means we can't just concatenate the strings and parse like we do now.

This was pointed out, in a similar case, by rust-lang/rfcs#3958 (comment)

IMO, markdown that relies on rustdoc’s current behavior is hard to understand, because string concatenation on separate doc comments is stupid.

This lint is mostly added here to run a Crater, so that we can figure out if there are significant numbers of crates are relying on the current behavior.

• If I find that the number of docs fixed by parsing re-exports separately outnumbers docs that are broken, I think we should just make the change and open PRs against the few crates that were broken.
• If it turns out that docs are purposefully using this, then I want to ship this compatibility lint for about a year after making the change, just like when people’s docs were being rendered differently because of the pulldown-cmark upgrade.

[notriddle]

@bors try

@craterbot run mode=rustdoc

[craterbot]

🚨 Error: missing start toolchain

🆘 If you have any trouble with Crater please ask in t-infra on Zulip
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[rust-bors]

☀️ Try build successful (CI)
Build commit: da3f5c3 (da3f5c31e5535b3dc210067de660a45784ae1e9c)
Base parent: 824336a (824336ad4127ce295849937a24c08a4aeff6ada7)

[notriddle]

@craterbot run mode=rustdoc

[craterbot]

👌 Experiment pr-162169 created and queued.
🤖 Automatically detected try build da3f5c3
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-162169 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-162169 is completed!
📊 1053 regressed and 1 fixed (1103137 total)
📊 5067 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

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

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-162169/retry-regressed-list.txt ↩