New rule: disallow to cast to nullable type

[t-kameyama]

Fixes #3479

[codecov]

Codecov Report

Merging #3497 (53c9d49) into master (fce4177) will decrease coverage by 2.15%.
The diff coverage is 86.66%.

@@             Coverage Diff              @@
##             master    #3497      +/-   ##
============================================
- Coverage     79.73%   77.57%   -2.16%     
+ Complexity     2869     2808      -61     
============================================
  Files           461      462       +1     
  Lines          8668     8683      +15     
  Branches       1676     1679       +3     
============================================
- Hits           6911     6736     -175     
- Misses          845     1040     +195     
+ Partials        912      907       -5     

Impacted Files	Coverage Δ	Complexity Δ
...arturbosch/detekt/rules/bugs/CastToNullableType.kt	84.61% <84.61%> (ø)	4.00 <4.00> (?)
...turbosch/detekt/rules/bugs/PotentialBugProvider.kt	100.00% <100.00%> (ø)	3.00 <0.00> (ø)
...o/gitlab/arturbosch/detekt/internal/SharedTasks.kt	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (ø%)
.../arturbosch/detekt/internal/DetektMultiplatform.kt	0.00% <0.00%> (-90.91%)	0.00% <0.00%> (-3.00%)
...b/arturbosch/detekt/extensions/DetektReportType.kt	0.00% <0.00%> (-83.34%)	0.00% <0.00%> (-2.00%)
...kotlin/io/gitlab/arturbosch/detekt/DetektPlugin.kt	0.00% <0.00%> (-82.23%)	0.00% <0.00%> (-10.00%)
...o/gitlab/arturbosch/detekt/internal/DetektPlain.kt	0.00% <0.00%> (-76.48%)	0.00% <0.00%> (-5.00%)
...itlab/arturbosch/detekt/extensions/DetektReport.kt	0.00% <0.00%> (-75.00%)	0.00% <0.00%> (-3.00%)
...tlab/arturbosch/detekt/DetektCreateBaselineTask.kt	0.00% <0.00%> (-43.48%)	0.00% <0.00%> (-16.00%)
...ab/arturbosch/detekt/extensions/DetektExtension.kt	0.00% <0.00%> (-42.86%)	0.00% <0.00%> (-2.00%)
... and 6 more

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[BraisGabin]

Sorry, but I still don't see the value of this rule. And why is OK if the initial type is not nullable. I'd like to know what the other maintainers think about it.

[cortinico]

I think the rule is valuable. To be honest I never really wrote a cast to a nullable type (as String?) and I've always used the as? variant, but I understand that it could be useful to detect them.