Adds version of mkString_ with no prefix/suffix, matching the std lib.

[matthughes]

Thank you for contributing to Cats!

This is a kind reminder to run sbt prePR and commit the changed files, if any, before submitting.

[matthughes]

Fixes #2663 but as it stands, this is going to fail binary compatibility rules. Not sure how best to get around? I could change the name but I got no suggestions.

[kailuowang]

I am not sure if this is going to break binary compatibility. Right now the build breaks due to format issue, a sbt prePR will format the code and also check if it breaks binary compatibility.

[matthughes]

So this does fail with binary incompatibility as you can see here: https://travis-ci.org/typelevel/cats/jobs/484371830.

I don't quite grok the FoldableOps/0/1 pattern but it looks like it has something to do with getting around bin incompatibility. If I this new method to FoldableOps0 instead of FoldableOps, MiMa is happy. But now I'm faced with how to implement mkString_. Previously, I was just doing a simple delegation to the prefix/d/suffix version, but that's no longer in scope. I could:

1. copy the relevant implementation over into the other trait.
2. call new FoldableOps(f).mkString_("", delim, "")

I'm find with either option but looking for input on how you'd like it.

[kailuowang]

@matthughes thanks!
The short answer is: yes, please move it FoldableOps0 and I'd perfer call new FoldableOps(f).mkString_("", delim, "")

The longer answer:
ah, it looks like adding an override to a method also breaks BC. Good to learn something new.
FoldableOps0 and FoldableOps1 were created recently to get around binary incompatibility. Mima is happy when you move the method to FoldableOps0 because that one is not officially released yet, it's released on 1.6.0-RC1, and mima doesn't check BC against RC releases. If we can make this one in 1.6.0 we should be good. Just noticed that your PR is against 1.6.x branch rather than master, so we are good on that as well.

[mpilquist]

Note: adding an overload to a regular class doesn't break binary compat but doing so to a value class does. The existence of an overload changes the scheme used for generation of the synthetic method names in the companion class.

Consider these two classes, representing 2 different versions of the same class:

class Foo1(val x: Int) extends AnyVal {
  def foo(y: Int): Int = x + y
}

class Foo2(val x: Int) extends AnyVal {
  def foo(y: Int): Int = x + y
  def foo(y: Int, z: Int): Int = x + y + z
}

Foo1.class and Foo2.class are binary compatible (modulo the class name difference of Foo1 vs Foo2). However, Foo1$.class and Foo2$.class are not.

Foo1$.class has this method:

  public final int foo$extension(int, int);
    descriptor: (II)I
    flags: ACC_PUBLIC, ACC_FINAL

Foo2$.class has these methods:

  public final int foo$extension0(int, int);
    descriptor: (II)I
    flags: ACC_PUBLIC, ACC_FINAL
<snip>

  public final int foo$extension1(int, int, int);
    descriptor: (III)I
    flags: ACC_PUBLIC, ACC_FINAL

[kailuowang]

@mpilquist ah, that makes sense, thanks for the detail! In hindsight, maybe the first one should've been made foo$extension0 rather than foo$extension. I'll see if I can find an existing scalac issue for this one.

[kailuowang]

@matthughes I updated the CatsSuite which is missing the newly added syntax trait and caused the broken build. Hopefully this one will be ready to merge soon.

[codecov-io]

Codecov Report

Merging #2709 into 1.6.x will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            1.6.x    #2709      +/-   ##
==========================================
+ Coverage   95.12%   95.12%   +<.01%     
==========================================
  Files         365      365              
  Lines        6795     6796       +1     
  Branches      314      294      -20     
==========================================
+ Hits         6464     6465       +1     
  Misses        331      331

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/CatsSuite.scala	70% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/foldable.scala	100% <100%> (ø)	⬆️

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