transform: restore previous test behavior

I think it's much nicer to have the test output inline in the source
file, that way it's much easier to review any changes. For example, when
escape analysis is improved this is visible with removed `// OUT` lines.

This is similar to how LLVM writes its tests, and I like that style.

transform/allocs_test.go

@@ -1,7 +1,9 @@
 package transform_test
 
 import (
+	"fmt"
 	"go/token"
+	"os"
 	"regexp"
 	"sort"
 	"strings"
@@ -49,23 +51,37 @@ func TestAllocs2(t *testing.T) {
 	})
 	sort.Slice(reports, func(i, j int) bool { return reports[i].pos.Line < reports[j].pos.Line })
 
-	// Render every report in each format and diff against its golden file.
-	for _, format := range []struct {
-		name   string
-		render func(report) string
-	}{
-		{"reason", func(r report) string { return transform.FormatAllocReason(r.pos, r.reason) }},
-		{"cover", func(r report) string { return transform.FormatAllocCover(r.pos) }},
-	} {
-		t.Run(format.name, func(t *testing.T) {
-			var got strings.Builder
-			for _, r := range reports {
-				if line := format.render(r); line != "" {
-					got.WriteString(line)
-					got.WriteByte('\n')
-				}
-			}
-			checkGolden(t, goldenFile+"."+format.name, got.String())
-		})
+	// Load expected test output (the OUT: lines).
+	testInput, err := os.ReadFile("./testdata/allocs2.go")
+	if err != nil {
+		t.Fatal("could not read test input:", err)
 	}
+	var expectedTestOutput strings.Builder
+	for i, line := range strings.Split(strings.ReplaceAll(string(testInput), "\r\n", "\n"), "\n") {
+		const prefix = " // OUT: "
+		if idx := strings.Index(line, prefix); idx > 0 {
+			msg := line[idx+len(prefix):]
+			fmt.Fprintf(&expectedTestOutput, "allocs2.go:%d: %s\n", i+1, msg)
+		}
+	}
+
+	// Check whether the '// OUT' lines in allocs2.go match with the output we
+	// got from the test.
+	var actualTestOutput strings.Builder
+	for _, r := range reports {
+		fmt.Fprintf(&actualTestOutput, "allocs2.go:%d: %s\n", r.pos.Line, r.reason)
+	}
+	if actualTestOutput.String() != expectedTestOutput.String() {
+		t.Errorf("expected:\n%s\nactual:\n%s", expectedTestOutput.String(), actualTestOutput.String())
+	}
+
+	// Render the cover report and diff it against its golden file.
+	var got strings.Builder
+	for _, r := range reports {
+		if line := transform.FormatAllocCover(r.pos); line != "" {
+			got.WriteString(line)
+			got.WriteByte('\n')
+		}
+	}
+	checkGolden(t, goldenFile+".cover", got.String())
 }