Decoder: show struct field in type mismatch errors (#684)

The goal is to provide some context as to why the type were mismatched. This
change only works for that case, on structs. This is the same a encoding/json. A
more general solution would be great, but this would require a broader change in
the decoder, which I don't think is necessary at the moment.

Fixes #628

unmarshaler.go

@@ -131,6 +131,23 @@ type decoder struct {
 
 	// Strict mode
 	strict strict
+
+	// Current context for the error.
+	errorContext *errorContext
+}
+
+type errorContext struct {
+	Struct reflect.Type
+	Field  []int
+}
+
+func (d *decoder) typeMismatchError(toml string, target reflect.Type) error {
+	if d.errorContext != nil && d.errorContext.Struct != nil {
+		ctx := d.errorContext
+		f := ctx.Struct.FieldByIndex(ctx.Field)
+		return fmt.Errorf("toml: cannot decode TOML %s into struct field %s.%s of type %s", toml, ctx.Struct, f.Name, f.Type)
+	}
+	return fmt.Errorf("toml: cannot decode TOML %s into a Go value of type %s", toml, target)
 }
 
 func (d *decoder) expr() *ast.Node {
@@ -444,12 +461,20 @@ func (d *decoder) handleKeyPart(key ast.Iterator, v reflect.Value, nextFn handle
 			v.SetMapIndex(mk, mv)
 		}
 	case reflect.Struct:
-		f, found := structField(v, string(key.Node().Data))
+		path, found := structFieldPath(v, string(key.Node().Data))
 		if !found {
 			d.skipUntilTable = true
 			return reflect.Value{}, nil
 		}
 
+		if d.errorContext == nil {
+			d.errorContext = new(errorContext)
+		}
+		t := v.Type()
+		d.errorContext.Struct = t
+		d.errorContext.Field = path
+
+		f := v.FieldByIndex(path)
 		x, err := nextFn(key, f)
 		if err != nil || d.skipUntilTable {
 			return reflect.Value{}, err
@@ -457,6 +482,8 @@ func (d *decoder) handleKeyPart(key ast.Iterator, v reflect.Value, nextFn handle
 		if x.IsValid() {
 			f.Set(x)
 		}
+		d.errorContext.Field = nil
+		d.errorContext.Struct = nil
 	case reflect.Interface:
 		if v.Elem().IsValid() {
 			v = v.Elem()
@@ -657,7 +684,7 @@ func (d *decoder) unmarshalArray(array *ast.Node, v reflect.Value) error {
 	default:
 		// TODO: use newDecodeError, but first the parser needs to fill
 		//   array.Data.
-		return fmt.Errorf("toml: cannot store array in Go type %s", v.Kind())
+		return d.typeMismatchError("array", v.Type())
 	}
 
 	elemType := v.Type().Elem()
@@ -904,7 +931,7 @@ func (d *decoder) unmarshalInteger(value *ast.Node, v reflect.Value) error {
 	case reflect.Interface:
 		r = reflect.ValueOf(i)
 	default:
-		return fmt.Errorf("toml: cannot store TOML integer into a Go %s", v.Kind())
+		return d.typeMismatchError("integer", v.Type())
 	}
 
 	if !r.Type().AssignableTo(v.Type()) {
@@ -1007,12 +1034,20 @@ func (d *decoder) handleKeyValuePart(key ast.Iterator, value *ast.Node, v reflec
 			v.SetMapIndex(mk, mv)
 		}
 	case reflect.Struct:
-		f, found := structField(v, string(key.Node().Data))
+		path, found := structFieldPath(v, string(key.Node().Data))
 		if !found {
 			d.skipUntilTable = true
 			break
 		}
 
+		if d.errorContext == nil {
+			d.errorContext = new(errorContext)
+		}
+		t := v.Type()
+		d.errorContext.Struct = t
+		d.errorContext.Field = path
+
+		f := v.FieldByIndex(path)
 		x, err := d.handleKeyValueInner(key, value, f)
 		if err != nil {
 			return reflect.Value{}, err
@@ -1021,6 +1056,8 @@ func (d *decoder) handleKeyValuePart(key ast.Iterator, value *ast.Node, v reflec
 		if x.IsValid() {
 			f.Set(x)
 		}
+		d.errorContext.Struct = nil
+		d.errorContext.Field = nil
 	case reflect.Interface:
 		v = v.Elem()
 
@@ -1078,7 +1115,7 @@ type fieldPathsMap = map[string][]int
 
 var globalFieldPathsCache atomic.Value // map[danger.TypeID]fieldPathsMap
 
-func structField(v reflect.Value, name string) (reflect.Value, bool) {
+func structFieldPath(v reflect.Value, name string) ([]int, bool) {
 	t := v.Type()
 
 	cache, _ := globalFieldPathsCache.Load().(map[danger.TypeID]fieldPathsMap)
@@ -1105,12 +1142,7 @@ func structField(v reflect.Value, name string) (reflect.Value, bool) {
 	if !ok {
 		path, ok = fieldPaths[strings.ToLower(name)]
 	}
-
-	if !ok {
-		return reflect.Value{}, false
-	}
-
-	return v.FieldByIndex(path), true
+	return path, ok
 }
 
 func forEachField(t reflect.Type, path []int, do func(name string, path []int)) {

unmarshaler_test.go

@@ -1790,6 +1790,20 @@ func TestUnmarshalOverflows(t *testing.T) {
 	}
 }
 
+func TestUnmarshalErrors(t *testing.T) {
+	type mystruct struct {
+		Bar string
+	}
+
+	data := `bar = 42`
+
+	s := mystruct{}
+	err := toml.Unmarshal([]byte(data), &s)
+	require.Error(t, err)
+
+	require.Equal(t, "toml: cannot decode TOML integer into struct field toml_test.mystruct.Bar of type string", err.Error())
+}
+
 func TestUnmarshalInvalidTarget(t *testing.T) {
 	x := "foo"
 	err := toml.Unmarshal([]byte{}, x)