From #42326:

Next steps are:

1. Run crater on experiementally moving it to hard error, to evaluate fallout.
2. If fallout is non-trivial, then it needs to be moved to a proper lint based implementation
3. Likewise, if fallout is non-trival, then ensure that it is included in the cargo report future-incompat output, when relevant.
4. If fallout is trivial, then we can just move to hard error without going through the longer route of replacing with a lint and whatnot.

So let's do a try run, which is a prereq for a crater run.
@bors try

Trying commit 33743a5 with merge d85a750...

Try build successful - checks-actions
Build commit: d85a750 (d85a7505e49363642b460e105973798063a683b4)

@craterbot run mode=check-only

Experiment pr-103914 created and queued.
Automatically detected try build d85a750
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Experiment pr-103914 is now running

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Experiment pr-103914 is completed!
34 regressed and 9 fixed (247167 total)
Open the full report.

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I don't know how to read crater reports. But from clicking around randomly I found at least one clear regression:

[INFO] [stderr]     Checking simple-replace-templete-engine v0.1.0 (/opt/rustwide/workdir)
[INFO] [stdout] error: underscore literal suffix is not allowed
[INFO] [stdout]   --> src/lib.rs:83:61
[INFO] [stdout]    |
[INFO] [stdout] 83 |         let templete_str = "hello _t_name_t_ _t_sec_name_t_"_.to_owned();
[INFO] [stdout]    |                                                    

But I can't find that crate on crates.io or anywhere else.

If you click on the name you'll get sent to the Github repository of the crate. Zero stars and the last commit from 3 years. The author is still active on Github, so I'll just send in a PR.

From above:

1. If fallout is trivial, then we can just move to hard error without going through the longer route of replacing with a lint and whatnot.

AFAICT we have a single failure in the crater run, for a crate that hasn't been touched in 3 years and has few if any users. @Nilstrieb has kindly filed a PR with the trivial fix anyway.

So I think we're good to land this.

r? @petrochenkov

r=me after cleaning up the test.
@rustbot author

@bors r=petrochenkov rollup

Commit dba6fc3 has been approved by petrochenkov

It is now in the queue for this repository.