all: improve panic messages for better debugability

Change-Id: If3e505e715d5ce2c9a81249c868d26226a25b724
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/232339
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect.go b/internal/impl/message_reflect.go
index 3eb2b13..0f4b8db 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -338,24 +338,27 @@
 	}
 	if fi != nil {
 		if fi.fieldDesc != fd {
-			panic("mismatching field descriptor")
+			if got, want := fd.FullName(), fi.fieldDesc.FullName(); got != want {
+				panic(fmt.Sprintf("mismatching field: got %v, want %v", got, want))
+			}
+			panic(fmt.Sprintf("mismatching field: %v", fd.FullName()))
 		}
 		return fi, nil
 	}
 
 	if fd.IsExtension() {
-		if fd.ContainingMessage().FullName() != mi.Desc.FullName() {
+		if got, want := fd.ContainingMessage().FullName(), mi.Desc.FullName(); got != want {
 			// TODO: Should this be exact containing message descriptor match?
-			panic("mismatching containing message")
+			panic(fmt.Sprintf("extension %v has mismatching containing message: got %v, want %v", fd.FullName(), got, want))
 		}
 		if !mi.Desc.ExtensionRanges().Has(fd.Number()) {
-			panic("invalid extension field")
+			panic(fmt.Sprintf("extension %v extends %v outside the extension range", fd.FullName(), mi.Desc.FullName()))
 		}
 		xtd, ok := fd.(pref.ExtensionTypeDescriptor)
 		if !ok {
-			panic("extension descriptor does not implement ExtensionTypeDescriptor")
+			panic(fmt.Sprintf("extension %v does not implement protoreflect.ExtensionTypeDescriptor", fd.FullName()))
 		}
 		return nil, xtd.Type()
 	}
-	panic("invalid field descriptor")
+	panic(fmt.Sprintf("field %v is invalid", fd.FullName()))
 }
diff --git a/internal/impl/message_reflect_field.go b/internal/impl/message_reflect_field.go
index ea6f755..23124a8 100644
--- a/internal/impl/message_reflect_field.go
+++ b/internal/impl/message_reflect_field.go
@@ -31,13 +31,13 @@
 func fieldInfoForOneof(fd pref.FieldDescriptor, fs reflect.StructField, x exporter, ot reflect.Type) fieldInfo {
 	ft := fs.Type
 	if ft.Kind() != reflect.Interface {
-		panic(fmt.Sprintf("invalid type: got %v, want interface kind", ft))
+		panic(fmt.Sprintf("field %v has invalid type: got %v, want interface kind", fd.FullName(), ft))
 	}
 	if ot.Kind() != reflect.Struct {
-		panic(fmt.Sprintf("invalid type: got %v, want struct kind", ot))
+		panic(fmt.Sprintf("field %v has invalid type: got %v, want struct kind", fd.FullName(), ot))
 	}
 	if !reflect.PtrTo(ot).Implements(ft) {
-		panic(fmt.Sprintf("invalid type: %v does not implement %v", ot, ft))
+		panic(fmt.Sprintf("field %v has invalid type: %v does not implement %v", fd.FullName(), ot, ft))
 	}
 	conv := NewConverter(ot.Field(0).Type, fd)
 	isMessage := fd.Message() != nil
@@ -90,7 +90,7 @@
 		},
 		mutable: func(p pointer) pref.Value {
 			if !isMessage {
-				panic("invalid Mutable on field with non-composite type")
+				panic(fmt.Sprintf("field %v with invalid Mutable call on field with non-composite type", fd.FullName()))
 			}
 			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
 			if rv.IsNil() || rv.Elem().Type().Elem() != ot || rv.Elem().IsNil() {
@@ -114,7 +114,7 @@
 func fieldInfoForMap(fd pref.FieldDescriptor, fs reflect.StructField, x exporter) fieldInfo {
 	ft := fs.Type
 	if ft.Kind() != reflect.Map {
-		panic(fmt.Sprintf("invalid type: got %v, want map kind", ft))
+		panic(fmt.Sprintf("field %v has invalid type: got %v, want map kind", fd.FullName(), ft))
 	}
 	conv := NewConverter(ft, fd)
 
@@ -147,7 +147,7 @@
 			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
 			pv := conv.GoValueOf(v)
 			if pv.IsNil() {
-				panic(fmt.Sprintf("invalid value: setting map field to read-only value"))
+				panic(fmt.Sprintf("map field %v cannot be set with read-only value", fd.FullName()))
 			}
 			rv.Set(pv)
 		},
@@ -167,7 +167,7 @@
 func fieldInfoForList(fd pref.FieldDescriptor, fs reflect.StructField, x exporter) fieldInfo {
 	ft := fs.Type
 	if ft.Kind() != reflect.Slice {
-		panic(fmt.Sprintf("invalid type: got %v, want slice kind", ft))
+		panic(fmt.Sprintf("field %v has invalid type: got %v, want slice kind", fd.FullName(), ft))
 	}
 	conv := NewConverter(reflect.PtrTo(ft), fd)
 
@@ -200,7 +200,7 @@
 			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
 			pv := conv.GoValueOf(v)
 			if pv.IsNil() {
-				panic(fmt.Sprintf("invalid value: setting repeated field to read-only value"))
+				panic(fmt.Sprintf("list field %v cannot be set with read-only value", fd.FullName()))
 			}
 			rv.Set(pv.Elem())
 		},
@@ -225,7 +225,7 @@
 	isBytes := ft.Kind() == reflect.Slice && ft.Elem().Kind() == reflect.Uint8
 	if nullable {
 		if ft.Kind() != reflect.Ptr && ft.Kind() != reflect.Slice {
-			panic(fmt.Sprintf("invalid type: got %v, want pointer", ft))
+			panic(fmt.Sprintf("field %v has invalid type: got %v, want pointer", fd.FullName(), ft))
 		}
 		if ft.Kind() == reflect.Ptr {
 			ft = ft.Elem()
@@ -257,7 +257,7 @@
 			case reflect.String, reflect.Slice:
 				return rv.Len() > 0
 			default:
-				panic(fmt.Sprintf("invalid type: %v", rv.Type())) // should never happen
+				panic(fmt.Sprintf("field %v has invalid type: %v", fd.FullName(), rv.Type())) // should never happen
 			}
 		},
 		clear: func(p pointer) {
@@ -314,7 +314,7 @@
 			messageName := fd.Message().FullName()
 			messageType, _ = preg.GlobalTypes.FindMessageByName(messageName)
 			if messageType == nil {
-				panic(fmt.Sprintf("weak message %v is not linked in", messageName))
+				panic(fmt.Sprintf("weak message %v for field %v is not linked in", messageName, fd.FullName()))
 			}
 		})
 	}
@@ -347,7 +347,10 @@
 			lazyInit()
 			m := v.Message()
 			if m.Descriptor() != messageType.Descriptor() {
-				panic("mismatching message descriptor")
+				if got, want := m.Descriptor().FullName(), messageType.Descriptor().FullName(); got != want {
+					panic(fmt.Sprintf("field %v has mismatching message descriptor: got %v, want %v", fd.FullName(), got, want))
+				}
+				panic(fmt.Sprintf("field %v has mismatching message descriptor: %v", fd.FullName(), m.Descriptor().FullName()))
 			}
 			p.Apply(weakOffset).WeakFields().set(num, m.Interface())
 		},
@@ -402,7 +405,7 @@
 			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
 			rv.Set(conv.GoValueOf(v))
 			if rv.IsNil() {
-				panic("invalid nil pointer")
+				panic(fmt.Sprintf("field %v has invalid nil pointer", fd.FullName()))
 			}
 		},
 		mutable: func(p pointer) pref.Value {
diff --git a/internal/impl/message_reflect_gen.go b/internal/impl/message_reflect_gen.go
index 7d65f5c..741d6e5 100644
--- a/internal/impl/message_reflect_gen.go
+++ b/internal/impl/message_reflect_gen.go
@@ -114,7 +114,7 @@
 	if oi := m.messageInfo().oneofs[od.Name()]; oi != nil && oi.oneofDesc == od {
 		return od.Fields().ByNumber(oi.which(m.pointer()))
 	}
-	panic("invalid oneof descriptor")
+	panic("invalid oneof descriptor " + string(od.FullName()) + " for message " + string(m.Descriptor().FullName()))
 }
 func (m *messageState) GetUnknown() protoreflect.RawFields {
 	m.messageInfo().init()
@@ -234,7 +234,7 @@
 	if oi := m.messageInfo().oneofs[od.Name()]; oi != nil && oi.oneofDesc == od {
 		return od.Fields().ByNumber(oi.which(m.pointer()))
 	}
-	panic("invalid oneof descriptor")
+	panic("invalid oneof descriptor " + string(od.FullName()) + " for message " + string(m.Descriptor().FullName()))
 }
 func (m *messageReflectWrapper) GetUnknown() protoreflect.RawFields {
 	m.messageInfo().init()
diff --git a/internal/impl/pointer_unsafe.go b/internal/impl/pointer_unsafe.go
index 2741643..088aa85 100644
--- a/internal/impl/pointer_unsafe.go
+++ b/internal/impl/pointer_unsafe.go
@@ -148,7 +148,11 @@
 	return pointer{p: unsafe.Pointer(ms)}
 }
 func (ms *messageState) messageInfo() *MessageInfo {
-	return ms.LoadMessageInfo()
+	mi := ms.LoadMessageInfo()
+	if mi == nil {
+		panic("invalid nil message info; this suggests memory corruption due to a race or shallow copy on the message struct")
+	}
+	return mi
 }
 func (ms *messageState) LoadMessageInfo() *MessageInfo {
 	return (*MessageInfo)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&ms.atomicMessageInfo))))