filter mutations to defensive unmodifiable collection returns

[hcoles]

It's common practice in some teams to wrap mutable collections with unmodifiable wrappers before returning them from a class to prevent accidental modification of state.

class Foo {
   private final List<String> field;

   ...

   List<String> doStuff() {
     return Collections.unmodifiableList(field);
   }
}

Although the presence of the wrapper can be easily detected by a test that attempts to modify the state of the list, some teams do not wish to specify this behaviour.

This change provides the option to filter out mutations implementing this defensive coding pattern.

The filter is disabled by default, but can be enabled with the +funmodifiablecollection feature string.

[romani]

@hcoles , is this a way to activate this filter https://pitest.org/quickstart/maven/#features ?

[hcoles]

Yes, the filtering is controlled by a feature named funmodifiablecollection. It can be activated by adding +funmodifiablecollection in that section of the maven config.