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serde-reflection on crates.io Documentation (latest release) License License

This crate provides a way to extract format descriptions for Rust containers that implement the Serialize and/or Deserialize trait(s) of Serde.

Format descriptions are useful in several ways:

  • Stored under version control, formats can be tested to prevent unintended modifications of binary serialization formats (e.g. by changing variant order).
  • Formats can be passed to serde-generate in order to generate class definitions and provide Serde-compatible binary serialization in other languages (C++, python, Java, etc).

Quick Start

Very often, Serde traits are simply implemented using Serde derive macros. In this case, you may obtain format descriptions as follows:

  • call trace_simple_type on the desired top-level container definition(s), then
  • add a call to trace_simple_type for each enum type. (This will fix any MissingVariants error.)
struct Foo {
  bar: Bar,
  choice: Choice,

struct Bar(u64);

enum Choice { A, B, C }

// Start the tracing session.
let mut tracer = Tracer::new(TracerConfig::default());

// Trace the desired top-level type(s).

// Also trace each enum type separately to fix any `MissingVariants` error.

// Obtain the registry of Serde formats and serialize it in YAML (for instance).
let registry = tracer.registry()?;
let data = serde_yaml::to_string(&registry).unwrap();
assert_eq!(&data, r#"---
      A: UNIT
      B: UNIT
      C: UNIT
    - bar:
        TYPENAME: Bar
    - choice:
        TYPENAME: Choice

Features and Limitations

serde_reflection is meant to extract formats for Rust containers (i.e. structs and enums) with "reasonable" implementations of the Serde traits Serialize and Deserialize.

Supported features

  • Plain derived implementations obtained with #[derive(Serialize, Deserialize)] for Rust containers in the Serde data model

  • Customized derived implementations using Serde attributes that are compatible with binary serialization formats, such as #[serde(rename = "Name")].

  • Hand-written implementations of Deserialize that are more restrictive than the derived ones, provided that trace_value is used during tracing to provide sample values for all such constrained types (see the detailed example below).

  • Mutually recursive types provided that the first variant of each enum is recursion-free. (For instance, enum List { None, Some(Box<List>)}.) Note that each enum must be traced separately with trace_type to discover all the variants.

Unsupported idioms

  • Containers sharing the same base name (e.g. Foo) but from different modules. (Work around: use #[serde(rename = ..)])

  • Generic types instantiated multiple times in the same tracing session. (Work around: use the crate serde-name and its adapters SerializeNameAdapter and DeserializeNameAdapter.)

  • Attributes that are not compatible with binary formats (e.g. #[serde(flatten)], #[serde(tag = ..)])

  • Tracing type aliases. (E.g. type Pair = (u32, u64) will not create an entry "Pair".)

  • Mutually recursive types for which picking the first variant of each enum does not terminate. (Work around: re-order the variants. For instance enum List { Some(Box<List>), None} must be rewritten enum List { None, Some(Box<List>)}.)

  • Certain standard types such as std::num::NonZeroU8 may not be tracked as a container and appear simply as their underlying primitive type (e.g. u8) in the formats. This loss of information makes it difficult to use trace_value to work around deserialization invariants (see example below). As a work around, you may override the default for the primitive type using TracerConfig (e.g. let config = TracerConfig::default().default_u8_value(1);).

Security CAVEAT

At this time, HashSet<T> and BTreeSet<T> are treated as sequences (i.e. vectors) by Serde.

Cryptographic applications using BCS must use HashMap<T, ()> and BTreeMap<T, ()> instead. Using HashSet<T> or BTreeSet<T> will compile but BCS-deserialization will not enforce canonicity (meaning unique, well-ordered serialized elements in this case). In the case of HashSet<T>, serialization will additionally be non-deterministic.


The error type used in this crate provides a method error.explanation() to help with troubleshooting during format tracing.

Detailed Example

In the following, more complete example, we extract the Serde formats of two containers Name and Person and demonstrate how to handle a custom implementation of serde::Deserialize for Name.

use serde_reflection::{ContainerFormat, Error, Format, Samples, Tracer, TracerConfig};

#[derive(Serialize, PartialEq, Eq, Debug, Clone)]
struct Name(String);
// impl<'de> Deserialize<'de> for Name { ... }

#[derive(Serialize, Deserialize, PartialEq, Eq, Debug, Clone)]
enum Person {
    FullName { first: Name, last: Name },

// Start a session to trace formats.
let mut tracer = Tracer::new(TracerConfig::default());
// Create a store to hold samples of Rust values.
let mut samples = Samples::new();

// For every type (here `Name`), if a user-defined implementation of `Deserialize` exists and
// is known to perform custom validation checks, use `trace_value` first so that `samples`
// contains a valid Rust value of this type.
let bob = Name("Bob".into());
tracer.trace_value(&mut samples, &bob)?;

// Now, let's trace deserialization for the top-level type `Person`.
// We pass a reference to `samples` so that sampled values are used for custom types.
let (format, values) = tracer.trace_type::<Person>(&samples)?;
assert_eq!(format, Format::TypeName("Person".into()));

// As a byproduct, we have also obtained sample values of type `Person`.
// We can see that the user-provided value `bob` was used consistently to pass
// validation checks for `Name`.
assert_eq!(values[0], Person::NickName(bob.clone()));
assert_eq!(values[1], Person::FullName { first: bob.clone(), last: bob.clone() });

// We have no more top-level types to trace, so let's stop the tracing session and obtain
// a final registry of containers.
let registry = tracer.registry()?;

// We have successfully extracted a format description of all Serde containers under `Person`.
match registry.get("Person").unwrap() {
    ContainerFormat::Enum(variants) => assert_eq!(variants.len(), 2),
     _ => panic!(),

// Export the registry in YAML.
let data = serde_yaml::to_string(&registry).unwrap();
assert_eq!(&data, r#"---
          TYPENAME: Name
          - first:
              TYPENAME: Name
          - last:
              TYPENAME: Name

Tracing Serialization with trace_value

Tracing the serialization of a Rust value v consists of visiting the structural components of v in depth and recording Serde formats for all the visited types.

struct FullName<'a> {
  first: &'a str,
  middle: Option<&'a str>,
  last: &'a str,

let mut tracer = Tracer::new(TracerConfig::default());
let mut samples = Samples::new();
tracer.trace_value(&mut samples, &FullName { first: "", middle: Some(""), last: "" })?;
let registry = tracer.registry()?;
match registry.get("FullName").unwrap() {
    ContainerFormat::Struct(fields) => assert_eq!(fields.len(), 3),
    _ => panic!(),

This approach works well but it can only recover the formats of datatypes for which nontrivial samples have been provided:

  • In enums, only the variants explicitly covered by user samples will be recorded.

  • Providing a None value or an empty vector [] within a sample may result in formats that are partially unknown.

let mut tracer = Tracer::new(TracerConfig::default());
let mut samples = Samples::new();
tracer.trace_value(&mut samples, &FullName { first: "", middle: None, last: "" })?;
assert_eq!(tracer.registry().unwrap_err(), Error::UnknownFormatInContainer("FullName".to_string()));

For this reason, we introduce a complementary set of APIs to trace deserialization of types.

Tracing Deserialization with trace_type<T>

Deserialization-tracing APIs take a type T, the current tracing state, and a reference to previously recorded samples as input.

Core Algorithm and High-Level API

The core algorithm trace_type_once<T> attempts to reconstruct a witness value of type T by exploring the graph of all the types occurring in the definition of T. At the same time, the algorithm records the formats of all the visited structs and enum variants.

For the exploration to be able to terminate, the core algorithm trace_type_once<T> explores each possible recursion point only once (see paragraph below). In particular, if T is an enum, trace_type_once<T> discovers only one variant of T at a time.

For this reason, the high-level API trace_type<T> will repeat calls to trace_type_once<T> until all the variants of T are known. Variant cases of T are explored in sequential order, starting with index 0.

Coverage Guarantees

Under the assumptions listed below, a single call to trace_type<T> is guaranteed to record formats for all the types that T depends on. Besides, if T is an enum, it will record all the variants of T.

(0) Container names must not collide. If this happens, consider using #[serde(rename = "name")], or implementing serde traits manually.

(1) The first variants of mutually recursive enums must be a "base case". That is, defaulting to the first variant for every enum type (along with None for option values and [] for sequences) must guarantee termination of depth-first traversals of the graph of type declarations.

(2) If a type runs custom validation checks during deserialization, sample values must have been provided previously by calling trace_value. Besides, the corresponding registered formats must not contain unknown parts.

Design Considerations

Whenever we traverse the graph of type declarations using deserialization callbacks, the type system requires us to return valid Rust values of type V::Value, where V is the type of a given visitor. This contraint limits the way we can stop graph traversal to only a few cases.

The first 4 cases are what we have called possible recursion points above:

  • while visiting an Option<T> for the second time, we choose to return the value None to stop;
  • while visiting an Seq<T> for the second time, we choose to return the empty sequence [];
  • while visiting an Map<K, V> for the second time, we choose to return the empty map {};
  • while visiting an enum T for the second time, we choose to return the first variant, i.e. a "base case" by assumption (1) above.

In addition to the cases above,

  • while visiting a container, if the container's name is mapped to a recorded value, we MAY decide to use it.

The default configuration TracerConfig:default() always picks the recorded value for a NewTypeStruct and never does in the other cases.

For efficiency reasons, the current algorithm does not attempt to scan the variants of enums other than the parameter T of the main call trace_type<T>. As a consequence, each enum type must be traced separately.


See the CONTRIBUTING file for how to help out.


This project is available under the terms of either the Apache 2.0 license or the MIT license.


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