loader: better error message on import cycles

loader/errors.go

@@ -1,5 +1,10 @@
 package loader
 
+import (
+	"go/token"
+	"strings"
+)
+
 // Errors contains a list of parser errors or a list of typechecker errors for
 // the given package.
 type Errors struct {
@@ -15,13 +20,20 @@ func (e Errors) Error() string {
 // packages is a list from the root package to the leaf package that imports one
 // of the packages in the list.
 type ImportCycleError struct {
-	Packages []string
+	Packages        []string
+	ImportPositions []token.Position
 }
 
 func (e *ImportCycleError) Error() string {
-	msg := "import cycle: " + e.Packages[0]
-	for _, path := range e.Packages[1:] {
-		msg += " → " + path
+	var msg strings.Builder
+	msg.WriteString("import cycle:\n\t")
+	msg.WriteString(strings.Join(e.Packages, "\n\t"))
+	msg.WriteString("\n at ")
+	for i, pos := range e.ImportPositions {
+		if i > 0 {
+			msg.WriteString(", ")
+		}
+		msg.WriteString(pos.String())
 	}
-	return msg
+	return msg.String()
 }

loader/loader.go

@@ -166,7 +166,9 @@ func (p *Program) Parse() error {
 		err := pkg.importRecursively()
 		if err != nil {
 			if err, ok := err.(*ImportCycleError); ok {
-				err.Packages = append([]string{pkg.ImportPath}, err.Packages...)
+				if pkg.ImportPath != err.Packages[0] {
+					err.Packages = append([]string{pkg.ImportPath}, err.Packages...)
+				}
 			}
 			return err
 		}
@@ -339,10 +341,13 @@ func (p *Package) importRecursively() error {
 			return err
 		}
 		if importedPkg.Importing {
-			return &ImportCycleError{[]string{p.ImportPath, importedPkg.ImportPath}}
+			return &ImportCycleError{[]string{p.ImportPath, importedPkg.ImportPath}, p.ImportPos[to]}
 		}
 		err = importedPkg.importRecursively()
 		if err != nil {
+			if err, ok := err.(*ImportCycleError); ok {
+				err.Packages = append([]string{p.ImportPath}, err.Packages...)
+			}
 			return err
 		}
 		p.Imports[to] = importedPkg