Use absolute paths to read source files

command.go

@@ -20,7 +20,7 @@ func command(stdin io.Reader, stdout io.Writer) error {
 	if err != nil {
 		return err
 	}
-	return printDiff(stdout, profiles, moduleInfo.modulePath)
+	return printDiff(stdout, profiles, moduleInfo)
 }
 
 func parseCoverProfiles(stdin io.Reader) ([]*cover.Profile, error) {

command_test.go

@@ -45,6 +45,29 @@ func TestCommand(t *testing.T) {
 		}
 	})
 
+	t.Run("from-file-in-subdirectory", func(t *testing.T) {
+		if err := os.Chdir("testdata"); err != nil {
+			t.Fatal(err)
+		}
+		t.Cleanup(func() {
+			if err := os.Chdir(projectPath); err != nil {
+				t.Fatal(err)
+			}
+		})
+		os.Args[1] = filepath.Join(projectPath, "testdata/coverage.out")
+
+		var stdout bytes.Buffer
+		if err := command(nil, &stdout); err != nil {
+			t.Fatal(err)
+		}
+
+		got := stdout.String()
+		want := readFile(t, "coverdiff.out")
+		if got != want {
+			t.Errorf("got len=%d, want len=%d", len(got), len(want))
+		}
+	})
+
 	t.Run("outside-go-module", func(t *testing.T) {
 		changeDir(t, "..")
 

diff.go

@@ -9,12 +9,11 @@ import (
 	"golang.org/x/tools/cover"
 )
 
-func printDiff(stdout io.Writer, profiles []*cover.Profile, modulePath string) error {
+func printDiff(stdout io.Writer, profiles []*cover.Profile, moduleInfo moduleInfo) error {
 	for _, profile := range profiles {
-		// create file path using absolute path to module
-		filepath := strings.ReplaceAll(profile.FileName, modulePath+"/", "")
-
-		b, err := os.ReadFile(filepath)
+		// use absolute path to module to read source code
+		absPath := strings.ReplaceAll(profile.FileName, moduleInfo.modulePath, moduleInfo.moduleDir)
+		b, err := os.ReadFile(absPath)
 		if err != nil {
 			return fmt.Errorf("os.ReadFile: %w", err)
 		}
@@ -34,10 +33,13 @@ func printDiff(stdout io.Writer, profiles []*cover.Profile, modulePath string) e
 			}
 		}
 
+		// use relative path to source code for diff headers
+		relPath := strings.ReplaceAll(profile.FileName, moduleInfo.modulePath+"/", "")
+
 		// print diff file headers
-		fmt.Fprintf(stdout, "diff --git a/%s b/%s\n", filepath, filepath)
-		fmt.Fprintf(stdout, "--- a/%s\n", filepath)
-		fmt.Fprintf(stdout, "+++ b/%s\n", filepath)
+		fmt.Fprintf(stdout, "diff --git a/%s b/%s\n", relPath, relPath)
+		fmt.Fprintf(stdout, "--- a/%s\n", relPath)
+		fmt.Fprintf(stdout, "+++ b/%s\n", relPath)
 
 		// print diff index header
 		fmt.Fprintf(stdout, "@@ -%d,%d +%d,%d @@ %s\n", 0, 0, len(lines), 0, lines[0])

diff_test.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"path/filepath"
 	"runtime"
-	"runtime/debug"
 	"testing"
 
 	"golang.org/x/tools/cover"
@@ -20,13 +19,13 @@ func TestPrintDiff(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	bi, ok := debug.ReadBuildInfo()
-	if !ok {
-		t.Fatal("binary build information not available")
+	moduleInfo, err := findModuleInfo()
+	if err != nil {
+		t.Fatal(err)
 	}
 
 	var b bytes.Buffer
-	if err := printDiff(&b, profiles, bi.Path); err != nil {
+	if err := printDiff(&b, profiles, moduleInfo); err != nil {
 		t.Fatal(err)
 	}
 }