Provides an implicit conversion that adds must methods to String to support the syntax of FlatSpec, WordSpec, fixture.FlatSpec, and fixture.WordSpec.

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

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

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

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

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

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

And finally, it also supportds the registration of subject descriptions with after words in WordSpec and fixture.WordSpec. 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, fixture.FlatSpec, and fixture.WordSpec, 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:

• FlatSpec, WordSpec, fixture.FlatSpec, and fixture.WordSpec mix in MustVerb directly, and
• MustMatchers extends MustVerb, overriding the convertToStringMustWrapper implicit conversion function.

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

Also, because the class of the result of the overriding convertToStringMustWrapper implicit conversion method provided in MustMatchers extends this trait's StringMustWrapperForVerb class, the four uses of must provided here are still available. These four must are in fact available to any class that mixes in MustMatchers, but each takes an implicit parameter that is provided only in FlatSpec and fixture.FlatSpec, or WordSpec and fixture.WordSpec.