reflect: remove unecessary heap allocations

src/reflect/type.go

@@ -601,6 +601,18 @@ func (t *rawType) Kind() Kind {
 	return Kind(t.meta & kindMask)
 }
 
+var (
+	errTypeElem         = &TypeError{"Elem"}
+	errTypeKey          = &TypeError{"Key"}
+	errTypeField        = &TypeError{"Field"}
+	errTypeBits         = &TypeError{"Bits"}
+	errTypeLen          = &TypeError{"Len"}
+	errTypeNumField     = &TypeError{"NumField"}
+	errTypeChanDir      = &TypeError{"ChanDir"}
+	errTypeFieldByName  = &TypeError{"FieldByName"}
+	errTypeFieldByIndex = &TypeError{"FieldByIndex"}
+)
+
 // Elem returns the element type for channel, slice and array types, the
 // pointed-to value for pointer types, and the key type for map types.
 func (t *rawType) Elem() Type {
@@ -619,14 +631,14 @@ func (t *rawType) elem() *rawType {
 	case Chan, Slice, Array, Map:
 		return (*elemType)(unsafe.Pointer(underlying)).elem
 	default:
-		panic(&TypeError{"Elem"})
+		panic(errTypeElem)
 	}
 }
 
 func (t *rawType) key() *rawType {
 	underlying := t.underlying()
 	if underlying.Kind() != Map {
-		panic(&TypeError{"Key"})
+		panic(errTypeKey)
 	}
 	return (*mapType)(unsafe.Pointer(underlying)).key
 }
@@ -683,7 +695,7 @@ func rawStructFieldFromPointer(descriptor *structType, fieldType *rawType, data
 // For internal use only.
 func (t *rawType) rawField(n int) rawStructField {
 	if t.Kind() != Struct {
-		panic(&TypeError{"Field"})
+		panic(errTypeField)
 	}
 	descriptor := (*structType)(unsafe.Pointer(t.underlying()))
 	if uint(n) >= uint(descriptor.numField) {
@@ -716,7 +728,7 @@ func (t *rawType) rawField(n int) rawStructField {
 // For internal use only.
 func (t *rawType) rawFieldByName(n string) (rawStructField, []int, bool) {
 	if t.Kind() != Struct {
-		panic(&TypeError{"Field"})
+		panic(errTypeField)
 	}
 
 	type fieldWalker struct {
@@ -812,14 +824,14 @@ func (t *rawType) Bits() int {
 	if kind >= Int && kind <= Complex128 {
 		return int(t.Size()) * 8
 	}
-	panic(TypeError{"Bits"})
+	panic(errTypeBits)
 }
 
 // Len returns the number of elements in this array. It panics of the type kind
 // is not Array.
 func (t *rawType) Len() int {
 	if t.Kind() != Array {
-		panic(TypeError{"Len"})
+		panic(errTypeLen)
 	}
 
 	return int((*arrayType)(unsafe.Pointer(t.underlying())).arrayLen)
@@ -829,7 +841,7 @@ func (t *rawType) Len() int {
 // type kinds.
 func (t *rawType) NumField() int {
 	if t.Kind() != Struct {
-		panic(&TypeError{"NumField"})
+		panic(errTypeNumField)
 	}
 	return int((*structType)(unsafe.Pointer(t.underlying())).numField)
 }
@@ -973,7 +985,7 @@ func (t *rawType) isBinary() bool {
 
 func (t *rawType) ChanDir() ChanDir {
 	if t.Kind() != Chan {
-		panic(TypeError{"ChanDir"})
+		panic(errTypeChanDir)
 	}
 
 	dir := int((*elemType)(unsafe.Pointer(t)).numMethod)
@@ -1084,7 +1096,7 @@ func (t *rawType) PkgPath() string {
 
 func (t *rawType) FieldByName(name string) (StructField, bool) {
 	if t.Kind() != Struct {
-		panic(TypeError{"FieldByName"})
+		panic(errTypeFieldByName)
 	}
 
 	field, index, ok := t.rawFieldByName(name)
@@ -1110,7 +1122,7 @@ func (t *rawType) FieldByIndex(index []int) StructField {
 	for _, n := range index {
 		structOrPtrToStruct := ftype.Kind() == Struct || (ftype.Kind() == Pointer && ftype.elem().Kind() == Struct)
 		if !structOrPtrToStruct {
-			panic(&TypeError{"FieldByIndex:" + ftype.Kind().String()})
+			panic(errTypeFieldByIndex)
 		}
 
 		if ftype.Kind() == Pointer {