Align leading infix operator with Scala 3 improvements

[som-snytt]

Align "leading infix operator" with Scala 3. The feature is available under -Xsource:3, and a migration warning is available under -Xmigration:2.13.

Improvements include allowing an infix operator on a line by itself, and a better heuristic for whether to take a back-quoted identifier as an infix operator.

Fixes scala/bug#12071

[SethTisue]

Expected a toplevel definition (or pure expr warning, here)

This doesn't seem like a final error message — was this a reminder to yourself to do some more work on the code, or is it just something we need to find a better wording for?

[som-snytt]

@SethTisue that was a Scala 3 test. I tried to keep the code diff close to Dotty, as these are backports.

[SethTisue]

Fixes scala/bug#12071

This is related to that bug, but it doesn't fix that bug. In that bug, we agreed that the main thing we want fixed is:

scala> def `test-1`: Int = 23; def `test-2`: Int = 42
def test$minus1: Int
def test$minus2: Int

scala> def y = 1 +
     |   `test-2` + `test-1`
         `test-2` + `test-1`
         ^
On line 2: warning: Line starts with an operator that in future
       will be taken as an infix expression continued from the previous line.
       To force the previous interpretation as a separate statement,
       add an explicit `;`, add an empty line, or remove spaces after the operator.

which is just plain wrong.

I guess that's what you meant by:

This does not yet address the "false positive"

You also wrote:

the fix for which is to change the message to "Please enable -Xsource:3".

but that doesn't make sense to me. No warning should be issued, regardless of whether -Xsource:3 is enabled. (Right?)

(Perhaps this PR is mergeable regardless, that's a separate question...)

[som-snytt]

Sorry I missed Seth's question. Yes, that's basically what I mean.

About to push rebase and Dale's idea to just skip the warning, modulo -Xmigration. I see the semantics of that setting is: I'm migrating from "2.13.2", so warn me if something changes. Thereafter, if I bump the flag, I won't see the message again, even if I accidentally write code to the wrong (old) syntax. I guess that is OK because we also check that -Xsource:3, so everything is assumed to be intentional.

[dwijnand]

Difficult to follow it all, but the test cases and test case diffs look good. Thank you Som for spending the time to re-align our impl with Scala 3's!

[dwijnand]

Oh, @som-snytt could you tweak the PR description a little with the changes? Seth and I will also do a pass, but it's probably easier for you to describe them.