docs: clarify assert.Zero() is unrelated to encoding/json omitempty/omitzero

assert/assertions.go

@@ -1762,6 +1762,10 @@ func NotRegexp(t TestingT, rx interface{}, str interface{}, msgAndArgs ...interf
 }
 
 // Zero asserts that i is the zero value for its type.
+//
+// This is unrelated to [encoding/json]'s `omitzero` tag and does not
+// use or check the [encoding.TextMarshaler.IsZero] method. It simply
+// compares against [reflect.Zero] for the given type.
 func Zero(t TestingT, i interface{}, msgAndArgs ...interface{}) bool {
 	if h, ok := t.(tHelper); ok {
 		h.Helper()

assert/assertions_test.go

@@ -2579,6 +2579,14 @@ func TestCallerInfoWithAutogeneratedFunctions(t *testing.T) {
 	})
 }
 
+// isZeroStruct demonstrates that assert.Zero uses reflect.Zero and is unrelated
+// to encoding/json's omitempty/omitzero or any IsZero() method.
+type isZeroStruct struct {
+	Value int
+}
+
+func (isZeroStruct) IsZero() bool { return false }
+
 func TestZero(t *testing.T) {
 	t.Parallel()
 
@@ -2593,6 +2601,22 @@ func TestZero(t *testing.T) {
 	}
 }
 
+func TestZeroNotIsZero(t *testing.T) {
+	t.Parallel()
+
+	// Verify that assert.Zero uses reflect.Zero and is unrelated to
+	// encoding/json's omitempty/omitzero or any IsZero() method.
+
+	mockT := new(testing.T)
+
+	// The zero value of the struct is "zero" per reflect.Zero,
+	// even though IsZero() would return false for Value==0.
+	True(t, Zero(mockT, isZeroStruct{Value: 0}))
+
+	// A non-zero struct value is "not zero"
+	False(t, Zero(mockT, isZeroStruct{Value: 42}))
+}
+
 func TestNotZero(t *testing.T) {
 	t.Parallel()