Cherry pick v1.7 into master

vendor/github.com/fatih/structs/field.go

@@ -0,0 +1,141 @@
+package structs
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+var (
+	errNotExported = errors.New("field is not exported")
+	errNotSettable = errors.New("field is not settable")
+)
+
+// Field represents a single struct field that encapsulates high level
+// functions around the field.
+type Field struct {
+	value      reflect.Value
+	field      reflect.StructField
+	defaultTag string
+}
+
+// Tag returns the value associated with key in the tag string. If there is no
+// such key in the tag, Tag returns the empty string.
+func (f *Field) Tag(key string) string {
+	return f.field.Tag.Get(key)
+}
+
+// Value returns the underlying value of the field. It panics if the field
+// is not exported.
+func (f *Field) Value() interface{} {
+	return f.value.Interface()
+}
+
+// IsEmbedded returns true if the given field is an anonymous field (embedded)
+func (f *Field) IsEmbedded() bool {
+	return f.field.Anonymous
+}
+
+// IsExported returns true if the given field is exported.
+func (f *Field) IsExported() bool {
+	return f.field.PkgPath == ""
+}
+
+// IsZero returns true if the given field is not initialized (has a zero value).
+// It panics if the field is not exported.
+func (f *Field) IsZero() bool {
+	zero := reflect.Zero(f.value.Type()).Interface()
+	current := f.Value()
+
+	return reflect.DeepEqual(current, zero)
+}
+
+// Name returns the name of the given field
+func (f *Field) Name() string {
+	return f.field.Name
+}
+
+// Kind returns the fields kind, such as "string", "map", "bool", etc ..
+func (f *Field) Kind() reflect.Kind {
+	return f.value.Kind()
+}
+
+// Set sets the field to given value v. It returns an error if the field is not
+// settable (not addressable or not exported) or if the given value's type
+// doesn't match the fields type.
+func (f *Field) Set(val interface{}) error {
+	// we can't set unexported fields, so be sure this field is exported
+	if !f.IsExported() {
+		return errNotExported
+	}
+
+	// do we get here? not sure...
+	if !f.value.CanSet() {
+		return errNotSettable
+	}
+
+	given := reflect.ValueOf(val)
+
+	if f.value.Kind() != given.Kind() {
+		return fmt.Errorf("wrong kind. got: %s want: %s", given.Kind(), f.value.Kind())
+	}
+
+	f.value.Set(given)
+	return nil
+}
+
+// Zero sets the field to its zero value. It returns an error if the field is not
+// settable (not addressable or not exported).
+func (f *Field) Zero() error {
+	zero := reflect.Zero(f.value.Type()).Interface()
+	return f.Set(zero)
+}
+
+// Fields returns a slice of Fields. This is particular handy to get the fields
+// of a nested struct . A struct tag with the content of "-" ignores the
+// checking of that particular field. Example:
+//
+//   // Field is ignored by this package.
+//   Field *http.Request `structs:"-"`
+//
+// It panics if field is not exported or if field's kind is not struct
+func (f *Field) Fields() []*Field {
+	return getFields(f.value, f.defaultTag)
+}
+
+// Field returns the field from a nested struct. It panics if the nested struct
+// is not exported or if the field was not found.
+func (f *Field) Field(name string) *Field {
+	field, ok := f.FieldOk(name)
+	if !ok {
+		panic("field not found")
+	}
+
+	return field
+}
+
+// FieldOk returns the field from a nested struct. The boolean returns whether
+// the field was found (true) or not (false).
+func (f *Field) FieldOk(name string) (*Field, bool) {
+	value := &f.value
+	// value must be settable so we need to make sure it holds the address of the
+	// variable and not a copy, so we can pass the pointer to strctVal instead of a
+	// copy (which is not assigned to any variable, hence not settable).
+	// see "https://blog.golang.org/laws-of-reflection#TOC_8."
+	if f.value.Kind() != reflect.Ptr {
+		a := f.value.Addr()
+		value = &a
+	}
+	v := strctVal(value.Interface())
+	t := v.Type()
+
+	field, ok := t.FieldByName(name)
+	if !ok {
+		return nil, false
+	}
+
+	return &Field{
+		field: field,
+		value: v.FieldByName(name),
+	}, true
+}