The implementation of a `count_lines` function in Go enhances functionality by allowing developers to count the number of newline characters within an `io.Reader`, a common task when processing text files. Utilizing a Test-Driven Development (TDD) approach, a series of test cases are created first, including scenarios for simple inputs, edge cases like empty files, and inputs without trailing newlines. Initially, a `bufio.Scanner` is employed, but it proves inadequate due to its inherent behavior of counting lines regardless of newlines. Thus, the solution shifts to using a `bufio.Reader`, reading individual runes to accurately tally the newline characters. This adaptive methodology not only reinforces the reliability of the implementation but also makes it clear how effective testing can guide development and ensure functionality aligns with intended outcomes.