cmd/go: add vet check for json/xml omitempty struct tags on non-basic…

… types

Fixes golang/go#51261

Change-Id: I99c3a0afc5171777b39f8288e2159f8554151e43

cmd/guru/serial/serial.go

@@ -108,7 +108,7 @@ type FreeVar struct {
 // types to which it is assignable, and the set of named/*named types
 // (concrete or non-empty interface) which may be assigned to it.
 type Implements struct {
-	T                 ImplementsType   `json:"type,omitempty"`    // the queried type
+	T                 ImplementsType   `json:"type"`              // the queried type
 	AssignableTo      []ImplementsType `json:"to,omitempty"`      // types assignable to T
 	AssignableFrom    []ImplementsType `json:"from,omitempty"`    // interface types assignable from T
 	AssignableFromPtr []ImplementsType `json:"fromptr,omitempty"` // interface types assignable only from *T

go/analysis/passes/structtag/structtag.go

@@ -13,6 +13,7 @@ import (
 	"go/types"
 	"path/filepath"
 	"reflect"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -83,8 +84,10 @@ func (s *namesSeen) Set(key, name string, level int, pos token.Pos) {
 	(*s)[uniqueName{key, name, level}] = pos
 }
 
-var checkTagDups = []string{"json", "xml"}
-var checkTagSpaces = map[string]bool{"json": true, "xml": true, "asn1": true}
+var (
+	checkTagDups   = []string{"json", "xml"}
+	checkTagSpaces = map[string]bool{"json": true, "xml": true, "asn1": true}
+)
 
 // checkCanonicalFieldTag checks a single struct field tag.
 func checkCanonicalFieldTag(pass *analysis.Pass, field *types.Var, tag string, seen *namesSeen) {
@@ -112,6 +115,25 @@ func checkCanonicalFieldTag(pass *analysis.Pass, field *types.Var, tag string, s
 		return
 	}
 
+	// Check for use of json or xml omitempty tags with unsupported field types.
+	for _, enc := range [...]string{"json", "xml"} {
+		typ := field.Type()
+		switch typ.Underlying().(type) {
+		case *types.Basic, *types.Interface, *types.Map, *types.Pointer, *types.Slice:
+			continue
+		}
+
+		val := reflect.StructTag(tag).Get(enc)
+		pos := strings.IndexRune(val, ',')
+		if pos < 0 {
+			continue
+		}
+
+		if slices.Contains(strings.Split(val[pos+1:], ","), "omitempty") {
+			pass.Reportf(field.Pos(), "struct field %s has %s tag but underlying type %s is not an omittable type", field.Name(), val, typ.String())
+		}
+	}
+
 	if field.Exported() {
 		return
 	}

go/analysis/passes/structtag/testdata/src/a/a.go

@@ -9,6 +9,7 @@ package a
 import (
 	"a/b"
 	"encoding/xml"
+	"time"
 )
 
 type StructTagTest struct {
@@ -44,8 +45,10 @@ type JSONEmbeddedField struct {
 	unexp                     `is:"embedded,notexported" json:"unexp"` // OK for now, see issue 7363
 }
 
-type AnonymousJSON struct{}
-type AnonymousXML struct{}
+type (
+	AnonymousJSON struct{}
+	AnonymousXML  struct{}
+)
 
 type AnonymousJSONField struct {
 	DuplicateAnonJSON int `json:"a"`
@@ -138,3 +141,9 @@ type DuplicateWithAnotherPackage struct {
 	b.AnonymousJSONField
 	AnonymousJSONField2 // want "struct field DuplicateAnonJSON repeats json tag .a. also at b.b.go:8"
 }
+
+type OmitemptyUsage struct {
+	T0 string    `json:",omitempty"`
+	T1 time.Time `json:"omitempty"`
+	T2 time.Time `json:"t2,omitempty"` // want "struct field T2 has t2,omitempty tag but underlying type time.Time is not an omittable type"
+}