Deprecate scala.collection.mutable.MultiMap

[joshlemer]

In the new Scala-Collection-Contrib module, there exists all three of:

• scala.collection.MultiDict
• scala.collection.immutable.MultiDict
• scala.collection.mutable.MultiDict

They probably even would have been called MultiMap, except that that would cause a name collision with the existing mutable.MultiMap in the standard library.

And so, there is no longer a need for the singular mutable.MultiMap implementation in the core library.

I'd actually prefer that these two data structures (MultiMap/MultiDict and MultiSet/Bag) would be included in core, since they are extremely useful to every day programming. Maybe that can be considered for a future release, but in the meantime we should probably avoid duplicating MultiMap here and there.

Fixes scala/bug#11504

For additional context: scala/scala-collection-contrib#17

[julienrf]

I'd actually prefer that these two data structures (MultiMap/MultiDict and MultiSet/Bag) would be included in core, since they are extremely useful to every day programming. Maybe that can be considered for a future release, but in the meantime we should probably avoid duplicating MultiMap here and there.

I don’t see it as an argument to put it to the stdlib. It’s actually useful to have them in a separate module that we can update independently of the standard library.
I also want to add that these collections haven’t been thoroughly tested.

[julienrf]

Before merging this, do we need stronger guarantees that collections in scala-collection-contrib are usable replacements for this, and more importantly, that that module is going to be published, eventually?

[lrytz]

I'm fine merging this without any further guarantees, as the trait provides very little functionality. Also, I don't see anything that would prevent scala-collection-contrib from being published.

A lighter alternative than a full MultiMap collection would be to turn the methods in the trait being deprecated here into extension methods.

[lrytz]

(pushed a check file update)

[julienrf]

If this PR is merged, consider updating the documentation here: https://github.com/scala/docs.scala-lang/blob/master/_overviews/core/collections-migration-213.md#breaking-changes-with-old-syntax-still-supported

[szeiger]

Looks reasonable for 2.13.1.