PreferSetEmpty,etc. seem misleading

[gawashburn]

The inspection currently states

 "`Set[T]()` allocates an intermediate object. Consider `Set.empty` which returns a singleton instance without creating a new object."

Arguably the apply() method does not document this, but Set[T]() does not allocate a new object in practice in any version of Scala I have tested. So it might be clearer to say that the documentation does not make this guarantee about future versions.

[gawashburn]

Actually I guess you probably mean that a Seq is created because of the argument elems: A*, which I not what I thought it meant. So perhaps the message could clarified.

[mccartney]

Let's establish what's the truth first. And then I suggest we can adjust the message to reflect that.

I am checking for Scala 2.12.11. If I am reading the code correctly:

• Set() calls Set.newBuilder
• newBuilder for Set has a line like this:
  https://github.com/scala/scala/blob/v2.12.11/src/library/scala/collection/immutable/Set.scala#L67
  which is the intermediate object being created the message is mentioning.

Not sure if you agree with this reasoning.

For 2.13.5 (just checking) I don't think that's the case indeed that an intermediate object is created:

1. Set() calls Set.from() with an empty collection as an argument
2. Set.from delegates to empty:
   https://github.com/scala/scala/blob/v2.13.5/src/library/scala/collection/immutable/Set.scala#L102
3. Set.empty uses a singleton:
   https://github.com/scala/scala/blob/v2.13.5/src/library/scala/collection/immutable/Set.scala#L95
   which does not seem to create an intermediate object.

So without checking other versions my hunch is:

• 2.11 - there's an intermediate object (this is what my memory tells me)
• 2.12 - there's an intermediate object
• 2.13 - there's no intermediate object

And the risk is that it's more sophisticated. Maybe the difference is even in the minor versions.

[gawashburn]

@mccartney I don't disagree the about the creation of the intermediate object in some version. As my second comment on the issue indicates, the problem is that message could be clearer:

Consider 'Set.empty' which returns a singleton instance without creating a new object.

The way that sentence is worded makes me think a new empty Set instance is being created, which is not the case. This is because to me the most direct referent for "new object" is the Set being returned, not some allocations that may take place.