Add -Yimports compiler flag

[adampauls]

Fixes lampepfl/dotty-feature-requests#138. I was always unclear on the interaction between -Yimports and -Yno-imports -- are they additive or redundant -- so I made the name of the flag -Yadditional-imports so that its semantics are very clear.

EDIT: Okay, I kept the original (confusing IMHO) behavior of -Yimports from Scala 2.

I think this might be possibly controversial because it is a flag that technically affects typechecking, but I still think it would be quite handy to have.

[dwijnand]

Also, we should stick to how Scala 2 calls it, ie. -Yimports

[adampauls]

I changed the name because it's really not clear how -Yno-imports and -Yimports foo interact. As in, I don't even know what the spec is, I think I'd have to look at the code to understand. It's possible that the old -Yimports was a set and not an add, in which case I think it's right to change the name. Put another way, if I had

class Foo(myImports: List[String]) {
  def noImports(): Foo = ???
  def imports(packages: List[String]): Foo = ???
}

What would you expect imports to do if you couldn't see the implementation? I don't think it would be to add them.

[adampauls]

Hmm, if I'm reading the Scala 2 code right, -Yimports is a set and nopredef and -Yno-imports don't actually do anything? This is all I could find https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/typechecker/Contexts.scala#L159.

[adampauls]

Apologies, it appears that the other import flags mutate the import settings https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala#L195. So imports is a set and I think that's unfortunate because appending imports to the defaults becomes painful. However, if that's the preferred behavior, I'll change the behavior and name here.

[dwijnand]

Yeah, we should match the behaviour, whatever it is. Even if we don't like it any more, changing it would create more confusion so should only be done for a very good reason.

[adampauls]

I respectfully disagree -- it's not confusing when an old flag goes away and a new one with different behavior appears. But as it so happens, I'm mostly interested in the version where you would pair it with -Yno-imports anyways, so I suppose I shouldn't complain too much.

[adampauls]

Okay, I made the changes. I left the move over to comment-based CLI args as a separate commit.

[som-snytt]

FWIW, scala 2 -Yimports defines the (ordered) root contexts as described in the (revised) spec https://scala-lang.org/files/archive/spec/2.13/02-identifiers-names-and-scopes.html

Ideally it would be -Xroot-contexts. There is history behind the existing name and behavior, and not everyone likes the behavior and would prefer a different feature for "preamble imports" or "preamble code".

There is precedent for renaming a compiler option but keeping the old name as an alias.

[adampauls]

I'm happy with whatever will get approved!

[adampauls]

Friendly bump on this PR?

[dwijnand]

Coincidentally ended up spending a long detour on imports in the REPL (#16389), which sets me up to be much better informed on imports. I'm reviewing this next. Sorry for the long delay 😞

[som-snytt]

This definitely does not qualify as a long delay in this repository.