Comparison of two approaches: one not using, one using optionals.

commonPrefixLength-1.swift

/**
 * Determine the common prefix elements of two collections.
 * @param collection1 First collection.
 * @param collection2 Second collection.
 * @return The number of elements common to the start of each collection.
 */
func commonPrefixLength<T: Swift.Collection, U: Swift.Collection where
	T: Sequence, U: Sequence,
	T.GeneratorType.Element: Equatable,
	T.GeneratorType.Element == U.GeneratorType.Element>
	(collection1: T, collection2: U) -> T.IndexType.DistanceType {
		var collection2generator = collection2.generate()
		
		var i: T.IndexType.DistanceType = 0
		
		for element1 in collection1 {
			let element2 = collection2generator.next()
			
			if (element1 != element2) {
				return i
			}
			
			i++
		}
		
		return i
}
commonPrefixLength("abX", "abc")

commonPrefixLength-2.swift

/**
 * Determine the common prefix elements of two collections.
 * @param collection1 First collection.
 * @param collection2 Second collection.
 * @return The number of elements common to the start of each collection.
 */
func commonPrefixLength<T: Swift.Collection, U: Swift.Collection where
	T: Sequence, U: Sequence,
	T.GeneratorType.Element: Equatable,
	T.GeneratorType.Element == U.GeneratorType.Element>
	(collection1: T, collection2: U) -> T.IndexType.DistanceType {
		var collection2generator = collection2.generate()
		
		var i: T.IndexType.DistanceType = 0
		
		for element1 in collection1 {
			let optionalElement2 = collection2generator.next()
			
			if let element2 = optionalElement2 {
				if (element1 != element2) {
					return i
				}
			}
			else {
				break
			}
		}
		
		return i
}
commonPrefixLength("abX", "abc")