Improving PHPUnit's Diff Output

[sebastianbergmann]

PHPUnit's assertion failure diffs are produced by three packages working together:

1. sebastian/diff provides Differ, UnifiedDiffOutputBuilder, and StrictUnifiedDiffOutputBuilder
2. In sebastian/comparator, ComparisonFailure::getDiff() instantiates Differ with a hard-coded UnifiedDiffOutputBuilder
3. phpunit/phpunit instantiates Differ directly in TestCase (global state snapshot comparison) and StringMatchesFormatDescription constraint

Currently, the context line count (3), the output builder choice (UnifiedDiffOutputBuilder), and the hunk header format (@@ @@ without line numbers) are all hard-coded with no user-facing configuration. This makes it difficult to read diffs for large, deeply nested structures, for example.

The goal is to incrementally move toward StrictUnifiedDiffOutputBuilder, which always produces spec-compliant unified diffs with proper hunk headers, while giving users control over context lines along the way.

• Add $contextLines constructor parameter on UnifiedDiffOutputBuilder diff#135
  UnifiedDiffOutputBuilder hardcodes contextLines = 3 with no way for callers to override it. Before PHPUnit can offer a --diff-context option, the underlying builder must accept the value.
• Make diff context lines configurable in PHPUnit #6567
• Make diff context lines configurable comparator#137
  All call sites in PHPUnit that construct ComparisonFailure need to pass the configured $contextLines value through
• Always show line numbers in hunk headers #6568
• Align UnifiedDiffOutputBuilder behavior with StrictUnifiedDiffOutputBuilder diff#136
• Use StrictUnifiedDiffOutputBuilder instead of UnifiedDiffOutputBuilder comparator#138
• Use StrictUnifiedDiffOutputBuilder instead of UnifiedDiffOutputBuilder #6569