[][src]Crate serde_reflection

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

Quick Start

Very often, Serde traits are only implemented using Serde derive macros. In this case, simply

  • call trace_type on the desired top-level definitions, then
  • add a call to trace_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());
let samples = Samples::new();

// 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() + "\n";
assert_eq!(&data, r#"---
      A: UNIT
      B: UNIT
      C: UNIT
    - bar:
        TYPENAME: Bar
    - choice:
        TYPENAME: Choice


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


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() + "\n";
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.



A named value. Used for named parameters or variants.


User inputs, aka "samples", recorded during serialization. This will help passing user-defined checks during deserialization.


Structure to drive the tracing of Serde serialization and deserialization. This typically aims at computing a Registry.


Configuration object to create a tracer.


A mutable holder for an initially unknown value.



Serde-based serialization format for named "container" types. In Rust, those are enums and structs.


Error type used in this crate.


Serde-based serialization format for anonymous "value" types.


A structured Serde value. Meant to be easily recorded while tracing serialization and easily used while tracing deserialization.


Description of a variant in an enum.



Common methods for nodes in the AST of formats.

Type Definitions


A map of container formats.


Result type used in this crate.