swift-library-variant-task-graph.png...

  compileTestVariantCpp -> linkTestVariant -> installTestVariant -> runTestVariant [dir=back, style=dashed]
  compileTestVariantCpp[label=<compile<i>Variant</i>Cpp>]
  linkTestVariant[label=<link<i>Variant</i>>]
  installTestVariant[label=<installTest<i>Variant</i>>]
  runTestVariant[label=<runTest<i>Variant</i>>]

cpp-application-variant-task-graph.png...

swift-application-variant-task-graph.png...

cpp-unit-test-variant-task-graph.png...

		return cr
	}
	return r
}

// Variant represents a registered variant of a language as defined by BCP 47.
type Variant struct {
	ID  uint8
	str string
}

// ParseVariant parses and returns a Variant. An error is returned if s is not
// a valid variant.
func ParseVariant(s string) (v Variant, err error) {
	defer func() {
		if recover() != nil {
			v = Variant{}
			err = ErrSyntax
		}
	}()

As a consequence, if we want our instrumented classes to be used in place of this variant when executing tests, we need to attach similar attributes to our variant.
In fact, the attribute we care about is `org.gradle.libraryelements` which explains _what the variant contains_, so we can setup the variant this way:

.Declaring the variant attributes
====

	// Write the buffer's header.
	if exp == traceNoExperiment {
		w.byte(byte(traceEvEventBatch))
	} else {
		w.byte(byte(traceEvExperimentalBatch))
		w.byte(byte(exp))
	}
	w.varint(uint64(w.gen))
	w.varint(uint64(mID))
	w.varint(uint64(ts))
	w.traceBuf.lenPos = w.varintReserve()
	return w
}

// traceBufQueue is a FIFO of traceBufs.
type traceBufQueue struct {
	head, tail *traceBuf
}

	// HasData is true if the event has trailer consisting of a
	// varint length followed by unencoded bytes of some data.
	//
	// An event may not be both a timed event and have data.
	HasData bool

	// IsStack indicates that the event represents a complete
	// stack trace. Specifically, it means that after the arguments
	// there's a varint length, followed by 4*length varints. Each
	// group of 4 represents the PC, file ID, func ID, and line number

	if needSort {
		sort.Sort(variantsSort{varID, variant})
		k, l := 0, -1
		for i, v := range varID {
			w := int(v)
			if l == w {
				// Remove duplicates.
				continue
			}
			varID[k] = varID[i]
			variant[k] = variant[i]
			k++
			l = w
		}
		if str := bytes.Join(variant[:k], separator); len(str) == 0 {
			end = start - 1
		} else {
			scan.resizeRange(start, end, len(str))