internal/mcp: unify json tag parsing

Do a thorough job of parsing json struct tags.

Move the code to a place where both mcp and jsonschema can use it.

Change-Id: I6c86308834706b12a1e22cd96ae120590f49d3f4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678757
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/util/util.go

@@ -7,7 +7,9 @@ package util
 import (
 	"cmp"
 	"iter"
+	"reflect"
 	"slices"
+	"strings"
 )
 
 // Helpers below are copied from gopls' moremaps package.
@@ -34,3 +36,40 @@ func KeySlice[M ~map[K]V, K comparable, V any](m M) []K {
 	}
 	return r
 }
+
+type JSONInfo struct {
+	Omit     bool            // unexported or first tag element is "-"
+	Name     string          // Go field name or first tag element. Empty if Omit is true.
+	Settings map[string]bool // "omitempty", "omitzero", etc.
+}
+
+// FieldJSONInfo reports information about how encoding/json
+// handles the given struct field.
+// If the field is unexported, JSONInfo.Omit is true and no other JSONInfo field
+// is populated.
+// If the field is exported and has no tag, then Name is the field's name and all
+// other fields are false.
+// Otherwise, the information is obtained from the tag.
+func FieldJSONInfo(f reflect.StructField) JSONInfo {
+	if !f.IsExported() {
+		return JSONInfo{Omit: true}
+	}
+	info := JSONInfo{Name: f.Name}
+	if tag, ok := f.Tag.Lookup("json"); ok {
+		name, rest, found := strings.Cut(tag, ",")
+		// "-" means omit, but "-," means the name is "-"
+		if name == "-" && !found {
+			return JSONInfo{Omit: true}
+		}
+		if name != "" {
+			info.Name = name
+		}
+		if len(rest) > 0 {
+			info.Settings = map[string]bool{}
+			for _, s := range strings.Split(rest, ",") {
+				info.Settings[s] = true
+			}
+		}
+	}
+	return info
+}

internal/util/util_test.go

@@ -0,0 +1,38 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package util
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestJSONInfo(t *testing.T) {
+	type S struct {
+		A int
+		B int `json:","`
+		C int `json:"-"`
+		D int `json:"-,"`
+		E int `json:"echo"`
+		F int `json:"foxtrot,omitempty"`
+		g int `json:"golf"`
+	}
+	want := []JSONInfo{
+		{Name: "A"},
+		{Name: "B"},
+		{Omit: true},
+		{Name: "-"},
+		{Name: "echo"},
+		{Name: "foxtrot", Settings: map[string]bool{"omitempty": true}},
+		{Omit: true},
+	}
+	tt := reflect.TypeFor[S]()
+	for i := range tt.NumField() {
+		got := FieldJSONInfo(tt.Field(i))
+		if !reflect.DeepEqual(got, want[i]) {
+			t.Errorf("got %+v, want %+v", got, want[i])
+		}
+	}
+}