reflect: Add FieldByNameFunc

This adds FieldByNameFunc, which some libraries like reflect2 need.

For my usecase I could also just stub FieldByNameFunc to panic, but
figured that it would work OK to just make it work. I'm not sure if
the overhead to FieldByName using a closure is acceptable.

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>

src/reflect/type.go

@@ -360,7 +360,7 @@ type Type interface {
 	// and FieldByNameFunc returns no match.
 	// This behavior mirrors Go's handling of name lookup in
 	// structs containing embedded fields.
-	//FieldByNameFunc(match func(string) bool) (StructField, bool)
+	FieldByNameFunc(match func(string) bool) (StructField, bool)
 
 	// In returns the type of a function type's i'th input parameter.
 	// It panics if the type's Kind is not Func.
@@ -722,11 +722,11 @@ func (t *rawType) rawField(n int) rawStructField {
 	return rawStructFieldFromPointer(descriptor, field.fieldType, data, flagsByte, name, offset)
 }
 
-// rawFieldByName returns nearly the same value as FieldByName but without converting the
+// rawFieldByNameFunc returns nearly the same value as FieldByNameFunc but without converting the
 // Type member to an interface.
 //
 // For internal use only.
-func (t *rawType) rawFieldByName(n string) (rawStructField, []int, bool) {
+func (t *rawType) rawFieldByNameFunc(match func(string) bool) (rawStructField, []int, bool) {
 	if t.Kind() != Struct {
 		panic(errTypeField)
 	}
@@ -769,7 +769,7 @@ func (t *rawType) rawFieldByName(n string) (rawStructField, []int, bool) {
 
 				name := readStringZ(data)
 				data = unsafe.Add(data, len(name))
-				if name == n {
+				if match(name) {
 					found = append(found, result{
 						rawStructFieldFromPointer(descriptor, field.fieldType, data, flagsByte, name, offset),
 						append(ll.index, int(i)),
@@ -1099,7 +1099,28 @@ func (t *rawType) FieldByName(name string) (StructField, bool) {
 		panic(errTypeFieldByName)
 	}
 
-	field, index, ok := t.rawFieldByName(name)
+	field, index, ok := t.rawFieldByNameFunc(func(n string) bool { return n == name })
+	if !ok {
+		return StructField{}, false
+	}
+
+	return StructField{
+		Name:      field.Name,
+		PkgPath:   field.PkgPath,
+		Type:      field.Type, // note: converts rawType to Type
+		Tag:       field.Tag,
+		Anonymous: field.Anonymous,
+		Offset:    field.Offset,
+		Index:     index,
+	}, true
+}
+
+func (t *rawType) FieldByNameFunc(match func(string) bool) (StructField, bool) {
+	if t.Kind() != Struct {
+		panic(TypeError{"FieldByNameFunc"})
+	}
+
+	field, index, ok := t.rawFieldByNameFunc(match)
 	if !ok {
 		return StructField{}, false
 	}

src/reflect/value.go

@@ -1846,6 +1846,17 @@ func (v Value) FieldByName(name string) Value {
 	return Value{}
 }
 
+func (v Value) FieldByNameFunc(match func(string) bool) Value {
+	if v.Kind() != Struct {
+		panic(&ValueError{"FieldByName", v.Kind()})
+	}
+
+	if field, ok := v.typecode.FieldByNameFunc(match); ok {
+		return v.FieldByIndex(field.Index)
+	}
+	return Value{}
+}
+
 //go:linkname hashmapMake runtime.hashmapMakeUnsafePointer
 func hashmapMake(keySize, valueSize uintptr, sizeHint uintptr, alg uint8) unsafe.Pointer
 

src/reflect/value_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/base64"
 	. "reflect"
 	"sort"
+	"strings"
 	"testing"
 )
 
@@ -452,10 +453,20 @@ func TestTinyStruct(t *testing.T) {
 		t.Errorf("StrucTag for Foo=%v, want %v", got, want)
 	}
 
+	q, ok = reffb.FieldByNameFunc(func(s string) bool { return strings.ToLower(s) == "bar" })
+	if q.Name != "Bar" || !ok {
+		t.Errorf("FieldByNameFunc(bar)=%v,%v, want Bar, true", q.Name, ok)
+	}
+
 	q, ok = reffb.FieldByName("Snorble")
 	if q.Name != "" || ok {
 		t.Errorf("FieldByName(Snorble)=%v,%v, want ``, false", q.Name, ok)
 	}
+
+	q, ok = reffb.FieldByNameFunc(func(s string) bool { return strings.ToLower(s) == "snorble" })
+	if q.Name != "" || ok {
+		t.Errorf("FieldByName(snorble)=%v,%v, want ``, false", q.Name, ok)
+	}
 }
 
 func TestTinyZero(t *testing.T) {