Add a DiffReader for diffing two io.Readers

test.go

@@ -391,6 +391,30 @@ func DiffBytes(tb testing.TB, got, want []byte) {
 	}
 }
 
+// DiffReader reads data from both got and want [io.Reader] and provides
+// a rich unified diff of the two for easy comparison.
+//
+// If either got or want do not end in a newline, one is added to avoid
+// a "No newline at end of file" warning in the diff which is visually distracting.
+func DiffReader(tb testing.TB, got, want io.Reader) {
+	tb.Helper()
+
+	gotData, err := io.ReadAll(got)
+	if err != nil {
+		tb.Fatalf("DiffReader: could not read from got: %v\n", err)
+	}
+
+	wantData, err := io.ReadAll(want)
+	if err != nil {
+		tb.Fatalf("DiffReader: could not read from want: %v\n", err)
+	}
+
+	gotData = fixNL(gotData)
+	wantData = fixNL(wantData)
+
+	DiffBytes(tb, gotData, wantData)
+}
+
 // CaptureOutput captures and returns data printed to [os.Stdout] and [os.Stderr] by the provided function fn, allowing
 // you to test functions that write to those streams and do not have an option to pass in an [io.Writer].
 //

test_test.go

@@ -451,6 +451,26 @@ func TestTest(t *testing.T) {
 			},
 			wantFail: true,
 		},
+		{
+			name: "DiffReader/pass",
+			fn: func(tb testing.TB) {
+				got := []byte("Some\nstuff here in this file\nlines as well wow\nsome more stuff\n")
+				want := []byte("Some\nstuff here in this file\nlines as well wow\nsome more stuff\n")
+
+				test.DiffReader(tb, bytes.NewReader(got), bytes.NewReader(want))
+			},
+			wantFail: false,
+		},
+		{
+			name: "DiffReader/fail",
+			fn: func(tb testing.TB) {
+				got := []byte("Some\nstuff here in this file\nlines as well wow\nsome more stuff\n")
+				want := []byte("Some\ndifferent stuff here in this file\nthis line is different\nsome more stuff\n")
+
+				test.DiffReader(tb, bytes.NewReader(got), bytes.NewReader(want))
+			},
+			wantFail: true,
+		},
 	}
 
 	for _, tt := range tests {

testdata/snapshots/TestTest/DiffReader/fail.snap

@@ -0,0 +1,18 @@
+source: test_test.go
+expression: buf.String()
+---
+|2+
+
+  Diff
+  ----
+  diff want got
+  --- want
+  +++ got
+  @@ -1,4 +1,4 @@
+    Some
+  - different stuff here in this file
+  - this line is different
+  + stuff here in this file
+  + lines as well wow
+    some more stuff
+