fix: handle zero-length strings in unsafe unmarshalling and update te…

…st logic to suit

features/unmarshal/unmarshal.go

@@ -194,7 +194,11 @@ func (p *unmarshal) mapField(varName string, field *protogen.Field) {
 		p.P(`return `, p.Ident("io", `ErrUnexpectedEOF`))
 		p.P(`}`)
 		if p.unsafe {
+			p.P(`if intStringLen`, varName, ` == 0 {`)
+			p.P(varName, ` = `, p.Ident("unsafe", `String`), `(nil, intStringLen`, varName, `)`)
+			p.P(`} else {`)
 			p.P(varName, ` = `, p.Ident("unsafe", `String`), `(&dAtA[iNdEx], intStringLen`, varName, `)`)
+			p.P(`}`)
 		} else {
 			p.P(varName, ` = `, "string", `(dAtA[iNdEx:postStringIndex`, varName, `])`)
 		}
@@ -420,7 +424,13 @@ func (p *unmarshal) fieldItem(field *protogen.Field, fieldname string, message *
 		p.P(`}`)
 		str := "string(dAtA[iNdEx:postIndex])"
 		if p.unsafe {
-			str = p.Ident("unsafe", `String`) + `(&dAtA[iNdEx], intStringLen)`
+			str = "stringValue"
+			p.P(`var stringValue string`)
+			p.P(`if intStringLen == 0 {`)
+			p.P(`stringValue = `, p.Ident("unsafe", `String`), `(nil, intStringLen)`)
+			p.P(`} else {`)
+			p.P(`stringValue = `, p.Ident("unsafe", `String`), `(&dAtA[iNdEx], intStringLen)`)
+			p.P(`}`)
 		}
 		if oneof {
 			p.P(`m.`, fieldname, ` = &`, field.GoIdent, `{`, field.GoName, ": ", str, `}`)

testproto/unsafe/unsafe_test.go

@@ -14,11 +14,17 @@ import (
 // collector doesn't move memory. To provide guarantee that this test works, consider using
 // https://pkg.go.dev/runtime#Pinner when upgrading Go to >= 1.21.
 func assertStringIsOriginal(t *testing.T, s string, belongs bool, originalData []byte) {
+	start := uintptr(unsafe.Pointer(unsafe.StringData(s)))
+	// empty string has no underlying array, compare pointer to nil
+	if len(s) == 0 {
+		assert.Equal(t, uintptr(unsafe.Pointer(nil)), start)
+		return
+	}
+	end := start + uintptr(len(s)) - 1
+
 	originalStart := uintptr(unsafe.Pointer(unsafe.SliceData(originalData)))
 	originalEnd := originalStart + uintptr(len(originalData)) - 1
 
-	start := uintptr(unsafe.Pointer(unsafe.StringData(s)))
-	end := start + uintptr(len(s)) - 1
 	assert.Equal(t, belongs, start >= originalStart && start < originalEnd)
 	assert.Equal(t, belongs, end > originalStart && end <= originalEnd)
 }