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reflect: add VisibleFields function
When writing code that reflects over a struct type, it's a common requirement to know the full set of struct fields, including fields available due to embedding of anonymous members while excluding fields that are erased because they're at the same level as another field with the same name. The logic to do this is not that complex, but it's a little subtle and easy to get wrong. This CL adds a new `VisibleFields` function to the reflect package that returns the full set of effective fields that apply in a given struct type. Performance isn't a prime consideration, as it's common to cache results by type. Fixes #42782 Change-Id: I7f1af76cecff9b8a2490f17eec058826e396f660 Reviewed-on: https://go-review.googlesource.com/c/go/+/281233 Reviewed-by: Ian Lance Taylor <iant@golang.org> Run-TryBot: Ian Lance Taylor <iant@golang.org> TryBot-Result: Go Bot <gobot@golang.org> Trust: Daniel Martí <mvdan@mvdan.cc>
This commit is contained in:
Roger Peppe
roger peppe
parent
f901ea701d
commit
009bfeae86
101
src/reflect/visiblefields.go
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101
src/reflect/visiblefields.go
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@ -0,0 +1,101 @@
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package reflect
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// VisibleFields returns all the visible fields in t, which must be a
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// struct type. A field is defined as visible if it's accessible
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// directly with a FieldByName call. The returned fields include fields
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// inside anonymous struct members and unexported fields. They follow
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// the same order found in the struct, with anonymous fields followed
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// immediately by their promoted fields.
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//
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// For each element e of the returned slice, the corresponding field
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// can be retrieved from a value v of type t by calling v.FieldByIndex(e.Index).
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func VisibleFields(t Type) []StructField {
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if t == nil {
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panic("reflect: VisibleFields(nil)")
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}
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if t.Kind() != Struct {
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panic("reflect.VisibleFields of non-struct type")
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}
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w := &visibleFieldsWalker{
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byName: make(map[string]int),
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visiting: make(map[Type]bool),
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fields: make([]StructField, 0, t.NumField()),
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index: make([]int, 0, 2),
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}
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w.walk(t)
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// Remove all the fields that have been hidden.
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// Use an in-place removal that avoids copying in
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// the common case that there are no hidden fields.
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j := 0
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for i := range w.fields {
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f := &w.fields[i]
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if f.Name == "" {
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continue
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}
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if i != j {
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// A field has been removed. We need to shuffle
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// all the subsequent elements up.
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w.fields[j] = *f
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}
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j++
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}
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return w.fields[:j]
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}
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type visibleFieldsWalker struct {
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byName map[string]int
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visiting map[Type]bool
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fields []StructField
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index []int
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}
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// walk walks all the fields in the struct type t, visiting
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// fields in index preorder and appending them to w.fields
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// (this maintains the required ordering).
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// Fields that have been overridden have their
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// Name field cleared.
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func (w *visibleFieldsWalker) walk(t Type) {
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if w.visiting[t] {
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return
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}
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w.visiting[t] = true
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for i := 0; i < t.NumField(); i++ {
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f := t.Field(i)
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w.index = append(w.index, i)
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add := true
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if oldIndex, ok := w.byName[f.Name]; ok {
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old := &w.fields[oldIndex]
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if len(w.index) == len(old.Index) {
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// Fields with the same name at the same depth
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// cancel one another out. Set the field name
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// to empty to signify that has happened, and
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// there's no need to add this field.
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old.Name = ""
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add = false
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} else if len(w.index) < len(old.Index) {
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// The old field loses because it's deeper than the new one.
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old.Name = ""
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} else {
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// The old field wins because it's shallower than the new one.
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add = false
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}
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}
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if add {
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// Copy the index so that it's not overwritten
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// by the other appends.
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f.Index = append([]int(nil), w.index...)
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w.byName[f.Name] = len(w.fields)
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w.fields = append(w.fields, f)
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}
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if f.Anonymous {
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if f.Type.Kind() == Ptr {
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f.Type = f.Type.Elem()
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}
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if f.Type.Kind() == Struct {
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w.walk(f.Type)
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}
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}
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w.index = w.index[:len(w.index)-1]
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}
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delete(w.visiting, t)
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}
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326
src/reflect/visiblefields_test.go
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326
src/reflect/visiblefields_test.go
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@ -0,0 +1,326 @@
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package reflect_test
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import (
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. "reflect"
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"testing"
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)
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type structField struct {
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name string
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index []int
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}
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var fieldsTests = []struct {
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testName string
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val interface{}
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expect []structField
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}{{
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testName: "SimpleStruct",
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val: struct {
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A int
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B string
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C bool
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}{},
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expect: []structField{{
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name: "A",
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index: []int{0},
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}, {
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name: "B",
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index: []int{1},
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}, {
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name: "C",
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index: []int{2},
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}},
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}, {
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testName: "NonEmbeddedStructMember",
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val: struct {
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A struct {
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X int
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}
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}{},
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expect: []structField{{
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name: "A",
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index: []int{0},
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}},
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}, {
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testName: "EmbeddedExportedStruct",
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val: struct {
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SFG
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}{},
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expect: []structField{{
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name: "SFG",
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index: []int{0},
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}, {
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name: "F",
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index: []int{0, 0},
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}, {
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name: "G",
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index: []int{0, 1},
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}},
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}, {
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testName: "EmbeddedUnexportedStruct",
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val: struct {
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sFG
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}{},
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expect: []structField{{
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name: "sFG",
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index: []int{0},
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}, {
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name: "F",
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index: []int{0, 0},
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}, {
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name: "G",
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index: []int{0, 1},
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}},
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}, {
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testName: "TwoEmbeddedStructsWithCancellingMembers",
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val: struct {
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SFG
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SF
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}{},
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expect: []structField{{
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name: "SFG",
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index: []int{0},
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}, {
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name: "G",
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index: []int{0, 1},
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}, {
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name: "SF",
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index: []int{1},
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}},
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}, {
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testName: "EmbeddedStructsWithSameFieldsAtDifferentDepths",
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val: struct {
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SFGH3
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SG1
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SFG2
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SF2
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L int
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}{},
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expect: []structField{{
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name: "SFGH3",
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index: []int{0},
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}, {
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name: "SFGH2",
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index: []int{0, 0},
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}, {
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name: "SFGH1",
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index: []int{0, 0, 0},
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}, {
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name: "SFGH",
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index: []int{0, 0, 0, 0},
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}, {
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name: "H",
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index: []int{0, 0, 0, 0, 2},
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}, {
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name: "SG1",
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index: []int{1},
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}, {
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name: "SG",
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index: []int{1, 0},
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}, {
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name: "G",
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index: []int{1, 0, 0},
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}, {
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name: "SFG2",
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index: []int{2},
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}, {
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name: "SFG1",
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index: []int{2, 0},
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}, {
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name: "SFG",
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index: []int{2, 0, 0},
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}, {
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name: "SF2",
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index: []int{3},
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}, {
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name: "SF1",
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index: []int{3, 0},
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}, {
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name: "SF",
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index: []int{3, 0, 0},
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}, {
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name: "L",
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index: []int{4},
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}},
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}, {
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testName: "EmbeddedPointerStruct",
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val: struct {
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*SF
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}{},
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expect: []structField{{
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name: "SF",
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index: []int{0},
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}, {
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name: "F",
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index: []int{0, 0},
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}},
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}, {
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testName: "EmbeddedNotAPointer",
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val: struct {
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M
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}{},
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expect: []structField{{
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name: "M",
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index: []int{0},
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}},
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}, {
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testName: "RecursiveEmbedding",
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val: Rec1{},
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expect: []structField{{
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name: "Rec2",
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index: []int{0},
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}, {
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name: "F",
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index: []int{0, 0},
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}, {
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name: "Rec1",
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index: []int{0, 1},
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}},
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}, {
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testName: "RecursiveEmbedding2",
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val: Rec2{},
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expect: []structField{{
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name: "F",
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index: []int{0},
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}, {
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name: "Rec1",
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index: []int{1},
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}, {
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name: "Rec2",
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index: []int{1, 0},
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}},
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}, {
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testName: "RecursiveEmbedding3",
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val: RS3{},
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expect: []structField{{
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name: "RS2",
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index: []int{0},
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}, {
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name: "RS1",
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index: []int{1},
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}, {
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name: "i",
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index: []int{1, 0},
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}},
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}}
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type SFG struct {
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F int
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G int
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}
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type SFG1 struct {
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SFG
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}
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type SFG2 struct {
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SFG1
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}
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type SFGH struct {
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F int
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G int
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H int
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}
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type SFGH1 struct {
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SFGH
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}
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type SFGH2 struct {
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SFGH1
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}
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type SFGH3 struct {
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SFGH2
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}
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type SF struct {
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F int
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}
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type SF1 struct {
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SF
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}
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type SF2 struct {
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SF1
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}
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type SG struct {
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G int
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}
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type SG1 struct {
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SG
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}
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type sFG struct {
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F int
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G int
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}
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type RS1 struct {
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i int
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}
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type RS2 struct {
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RS1
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}
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type RS3 struct {
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RS2
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RS1
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}
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type M map[string]interface{}
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type Rec1 struct {
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*Rec2
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}
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type Rec2 struct {
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F string
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*Rec1
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}
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func TestFields(t *testing.T) {
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for _, test := range fieldsTests {
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test := test
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t.Run(test.testName, func(t *testing.T) {
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typ := TypeOf(test.val)
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fields := VisibleFields(typ)
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if got, want := len(fields), len(test.expect); got != want {
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t.Fatalf("unexpected field count; got %d want %d", got, want)
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}
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for j, field := range fields {
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expect := test.expect[j]
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t.Logf("field %d: %s", j, expect.name)
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gotField := typ.FieldByIndex(field.Index)
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// Unfortunately, FieldByIndex does not return
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// a field with the same index that we passed in,
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// so we set it to the expected value so that
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// it can be compared later with the result of FieldByName.
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gotField.Index = field.Index
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expectField := typ.FieldByIndex(expect.index)
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// ditto.
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expectField.Index = expect.index
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if !DeepEqual(gotField, expectField) {
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t.Fatalf("unexpected field result\ngot %#v\nwant %#v", gotField, expectField)
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}
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// Sanity check that we can actually access the field by the
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// expected name.
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gotField1, ok := typ.FieldByName(expect.name)
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if !ok {
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t.Fatalf("field %q not accessible by name", expect.name)
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}
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if !DeepEqual(gotField1, expectField) {
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t.Fatalf("unexpected FieldByName result; got %#v want %#v", gotField1, expectField)
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}
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}
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})
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}
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}
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