For example, this trait enables syntax such as the following test registration in FlatSpec and FixtureFlatSpec:

 "A Stack (when empty)" must "be empty" in { ... }
                        ^
 

It also enables syntax such as the following shared test registration in FlatSpec and FixtureFlatSpec:

 "A Stack (with one item)" must behave like nonEmptyStack(stackWithOneItem, lastValuePushed)
                           ^
 

In addition, it supports the registration of subject descriptions in WordSpec and FixtureWordSpec, such as:

 "A Stack (when empty)" must { ...
                        ^
 

And finally, it also supportds the registration of subject descriptions with after words in WordSpec and FixtureWordSpec. For example:

    def provide = afterWord("provide")

   "The ScalaTest Matchers DSL" must provide {
                                ^
 

The reason this implicit conversion is provided in a separate trait, instead of being provided directly in FlatSpec, WordSpec, FixtureFlatSpec, and FixtureWordSpec, is because an implicit conversion provided directly would conflict with the implicit conversion that provides must methods on String in the MustMatchers trait. By contrast, there is no conflict with the separate MustVerb trait approach, because:

1. FlatSpec, WordSpec, FixtureFlatSpec, and FixtureWordSpec mix in MustVerb directly, and
2. MustMatchers extends MustVerb, overriding the convertToStringMustWrapper implicit conversion function.

So whether or not a FlatSpec, WordSpec, FixtureFlatSpec, or FixtureWordSpec mixes in MustMatchers, there will only be one implicit conversion in scope that adds must methods to Strings.