Improve replace directive tooling messaging

tooling/go/cmd/local-overrides-enforcer/main.go

@@ -55,13 +55,17 @@ func main() {
 		if err != nil {
 			return fmt.Errorf("failed to read %s: %w", path, err)
 		}
-		expectedContents, ok, err := checker.CheckFile(path, content, parentModule)
+		expectedContents, ok, missingReplaces, err := checker.CheckFile(path, content, parentModule)
 		if err != nil {
 			return fmt.Errorf("failed to check %s: %w", path, err)
 		}
 		if !ok {
 			sawBadFile = true
-			fmt.Printf("⚠️  File at path %s didn't have some local overrides - its contents should be:\n%s\n", path, expectedContents)
+			missingReplacesStr := ""
+			for _, missingReplace := range missingReplaces {
+				missingReplacesStr += " - " + missingReplace + "\n"
+			}
+			fmt.Printf("⚠️  File at path %s didn't have some replace directives.\nMissing replace directives for modules:\n%s\nYou should replace %s with exactly this string:\n%s\n", path, missingReplacesStr, path, expectedContents)
 		}
 		return nil
 	})

tooling/go/internal/local-overrides-enforcer/checker/checker.go

@@ -9,19 +9,19 @@ import (
 
 // CheckFile checks that a go.mod file has local overrides for all children of the passed parent module.
 // It returns one of:
-//   - If the contents was correct: "", true, nil
-//   - If the contents was not correct: the expected contents, false, nil
-//   - If an error occurred: "", false, error
-func CheckFile(goModPath string, contents []byte, parentModule string) (string, bool, error) {
+//   - If the contents was correct: "", true, nil, nil
+//   - If the contents was not correct: the expected contents, false, []string{"missing/override"...}, nil
+//   - If an error occurred: "", false, nil, error
+func CheckFile(goModPath string, contents []byte, parentModule string) (string, bool, []string, error) {
 	gomodFile, err := modfile.Parse(goModPath, contents, nil)
 	if err != nil {
-		return "", false, fmt.Errorf("failed to parse %s as go.mod file: %w", goModPath, err)
+		return "", false, nil, fmt.Errorf("failed to parse %s as go.mod file: %w", goModPath, err)
 	}
 
 	parentModuleWithTrailingSlash := parentModule + "/"
 
 	if !strings.HasPrefix(gomodFile.Module.Mod.Path, parentModuleWithTrailingSlash) {
-		return "", false, fmt.Errorf("module at path %s was named %s which isn't a child of %s", goModPath, gomodFile.Module.Mod.Path, parentModule)
+		return "", false, nil, fmt.Errorf("module at path %s was named %s which isn't a child of %s", goModPath, gomodFile.Module.Mod.Path, parentModule)
 	}
 	slashCount := strings.Count(gomodFile.Module.Mod.Path[len(parentModule):], "/")
 
@@ -30,7 +30,7 @@ func CheckFile(goModPath string, contents []byte, parentModule string) (string,
 		replaces[replace.Old.Path] = struct{}{}
 	}
 
-	missingReplaces := false
+	var missingReplaces []string
 	for _, require := range gomodFile.Require {
 		modPath := require.Mod.Path
 		if !strings.HasPrefix(modPath, parentModuleWithTrailingSlash) {
@@ -39,18 +39,18 @@ func CheckFile(goModPath string, contents []byte, parentModule string) (string,
 		if _, isReplaced := replaces[modPath]; isReplaced {
 			continue
 		}
-		missingReplaces = true
+		missingReplaces = append(missingReplaces, modPath)
 		rel := modPath[len(parentModuleWithTrailingSlash):]
 		if err := gomodFile.AddReplace(modPath, "", strings.Repeat("../", slashCount)+rel, ""); err != nil {
-			return "", false, fmt.Errorf("failed to add replace: %w", err)
+			return "", false, nil, fmt.Errorf("failed to add replace: %w", err)
 		}
 	}
-	if missingReplaces {
+	if len(missingReplaces) != 0 {
 		formatted, err := gomodFile.Format()
 		if err != nil {
-			return "", false, fmt.Errorf("failed to serialize go.mod file: %w", err)
+			return "", false, nil, fmt.Errorf("failed to serialize go.mod file: %w", err)
 		}
-		return string(formatted), false, nil
+		return string(formatted), false, missingReplaces, nil
 	}
-	return "", true, nil
+	return "", true, nil, nil
 }