Allow return in tailrec position

[mbovel]

Implement lampepfl/dotty-feature-requests#261.

Joint work with @ghostbuster91 in the framework of the issue spree.

[sjrd]

This shouldn't create any issues now.

However, to be complete, you'll have to actually recurse in all trees all the time. See this comment:
https://github.com/lampepfl/dotty/blob/a34977551e9e13090e6b5201f7d91879464f991c/compiler/src/dotty/tools/dotc/transform/TailRec.scala#L29-L31
Now any nested tree could contain a recursive call, which was not the case before.

Also, this will need more tests before it's ready to merge. ;)

[sjrd]

Looks good to me.

There might be concerns about the performance impact, since now checking needs to be done everywhere all the time. So perhaps ask the performance bot?

[som-snytt]

This is a great first step in the rehabilitation of return. sjrd's eyes make me confident that the feature is not just catering to folks accustomed to the keyword. The other return issue is to allow return followed by miscellaneous defs. lampepfl/dotty-feature-requests#256 I will pursue that, inspired by this PR.

The t-shirt should make reference to Nietzsche, "The eternal return". I don't know if PRs have gotten their own custom t-shirt until this point.

[mbovel]

test performance please

[dottybot]

performance test scheduled: 1 job(s) in queue, 0 running.

[dottybot]

Performance test finished successfully:

Visit https://dotty-bench.epfl.ch/14067/ to see the changes.

Benchmarks is based on merging with master (eb75a1a)

[mbovel]

@sjrd could we merge this or do we need more changes or measurements?

[sjrd]

The benchmark results look good to me. It's good to go as far as I'm concerned.