New Rule: DoubleMutabilityForCollection

[cortinico]

I'm adding a new rule that detects if you're declaring a var with also a Mutable type like:

var myList = mutableListOf(1,2,3)

Instead you should prefer to:

val myList = mutableListOf(1,2,3)
// OR
var myList = listOf(1,2,3)

Having "double mutability" is considered an anti-pattern as the myList variable can both be reassigned and .clear()-ed.

[codecov]

Codecov Report

Merging #3553 (fa98b54) into master (b5aac52) will increase coverage by 0.03%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##             master    #3553      +/-   ##
============================================
+ Coverage     77.53%   77.56%   +0.03%     
- Complexity     2829     2833       +4     
============================================
  Files           464      465       +1     
  Lines          8769     8795      +26     
  Branches       1713     1716       +3     
============================================
+ Hits           6799     6822      +23     
  Misses         1046     1046              
- Partials        924      927       +3     

Impacted Files	Coverage Δ	Complexity Δ
...detekt/rules/bugs/DoubleMutabilityForCollection.kt	88.00% <88.00%> (ø)	4.00 <4.00> (?)
...n/io/github/detekt/report/sarif/RuleDescriptors.kt	20.00% <100.00%> (ø)	0.00 <0.00> (ø)
...turbosch/detekt/rules/bugs/PotentialBugProvider.kt	100.00% <100.00%> (ø)	3.00 <0.00> (ø)

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[BraisGabin]

👍

Once we have a real support for severity I'd like to add a rule to detect all the usages of var. There are very few cases where it uses make sense.

[chao2zhang]

Let's keep in mind that we need to annotate this rule later with the new @RequiresTypeResolution #3579

[BraisGabin]

Let's keep in mind that we need to annotate this rule later with the new @RequiresTypeResolution #3579

#3579 is merged already so I think that we should do this change and we can merge this. The js problem can be faced at #3613