Add linting of unused context bounds (via -Wunused:synthetics or -Wunused:params)

[som-snytt]

Warn under -Wunused:synthetics which is included by -Wunused:params.

Warnings for unused value parameters can be selectively enabled. -Xlint enables -Wunused:implicits to warn about unused implicit parameters. Warnings about ordinary parameters can be enabled with -Wunused:explicits; that is deemed noisy for ordinary use, so it is not enabled by -Xlint. Now, "unused context bounds", which are synthetic parameters, can be detected by enabling -Wunused:synthetics. All three warnings are enabled by -Wunused:params.

  def f[A](implicit ctx: Context[A]) = ???  // -Wunused:implicits
  def g[A: Context] = ???                   // -Wunused:synthetics

Fixes scala/bug#10349

[SethTisue]

@som-snytt this appears just about ready to merge, but because it adds a new flag, would you mind supplying a nice PR description that explains this from the user's point of view (and without requiring them to follow links), for linking from the release notes? in particular, I'm wondering what the recommended way of suppressing the warning is (and should there be a test that tests that?), since @unused won't work here since there's nowhere to put it.

[dwijnand]

in particular, I'm wondering what the recommended way of suppressing the warning is (and should there be a test that tests that?), since @unused won't work here since there's nowhere to put it.

I merged it already, but I think this would be handy to have written down. 🤗

[SethTisue]

I'm always a little hesitant about letting flags proliferate, but this is an area where it's probably harmless and will benefit at least some users.

I just had a look at sbt-tpolecat and it does enable -Wunused:params, so that means this change will automatically get some uptake/testing once 2.13.5 is released. (And sbt-tpolecat users aren't too put off by occasional false positives.)

[SethTisue]

OH: “I just eliminated six unused constraints”, not sure who it was, but it sounded like they were wearing ski goggles, relative of yours @tpolecat?

oh heh nm, I see you thanked him on Gitter 👍