Implement a ReturnVisitor

[GnomedDev]

Closes #13552

changelog: none

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[GnomedDev]

This is now generic enough for the never returns lint, so ready for review!

[dswij]

LGTM, thanks! can we squash some commits?

[GnomedDev]

Done @dswij!

[bors]

☔ The latest upstream changes (presumably #13567) made this pull request unmergeable. Please resolve the merge conflicts.

[Jarcho]

Ping @dswij from triage. Looks like you were ready to merge this.

[Jarcho]

r? @Jarcho

Taking over since @dswij seems to be busy.

This visitor seems to conflate two things which shouldn't be mixed together. A combination of places the code returns from and the values it returns.

For unnecessary_literal_bound, only the returned values are needed. This doesn't walk the any explicit return expressions so this isn't really sufficient as is.

For #13565 this is overly complicated. There you only need to check, for all explicit returns and the implicit return, if the value is of type ! or it's a block where the final expression meets the same conditions.

[rustbot]

☔ The latest upstream changes (possibly d28d234) made this pull request unmergeable. Please resolve the merge conflicts.

[Jarcho]

Ping @GnomedDev from triage. Do you plan to continue working on this?

[GnomedDev]

No, sorry, I don't have the spare time at the moment to help out pushing this over the finish line and it seems like you are saying it's not the right implementation anyway.