miniconf: serialize/deserialize/access reflection for trees

miniconf enables lightweight (no_std/no alloc) serialization, deserialization, and access within a tree of heretogeneous types by keys.

Reflection

miniconf enables certain kinds of reflective access to heterogeneous trees. Let's compare it to bevy_reflect which is a comprehensive and mature reflection crate:

bevy_reflect is thoroughly std while miniconf aims at no_std. bevy_reflect uses its Reflect trait to operate on and pass nodes as trait objects. miniconf uses serialized data or Any to access leaf nodes and pure "code" to traverse through internal nodes. The Tree* traits like Reflect thus give access to nodes but unlike Reflect they are all decidedly not object-safe and can not be used as trait objects. This allows miniconf to support non-'static borrowed data (only for TreeAny the leaf nodes need to be 'static) while bevy_reflect requires 'static for Reflect types.

miniconfsupports at least the following reflection features mentioned in the bevy_reflect README:

• ➕ Derive the traits: miniconf has Tree* derive macros and blanket implementations for arrays and Options. Leaf nodes just need some impls of Serialize/Deserialize/Any where desired.
• ➕ Interact with fields using their names
• ➖ "Patch" your types with new values: miniconf only supports limited changes to the tree structure at runtime (Option and custom accessors) while bevy_reflect has powerful dynamic typing tools.
• ➕ Look up nested fields using "path strings": In addition to a superset of JSON path style "path strings" miniconf supports hierarchical indices and bit-packed ordered keys.
• ➕ Iterate over struct fields: miniconf Supports recursive iteration over node keys.
• ➕ Automatically serialize and deserialize via Serde without explicit serde impls: miniconf supports automatic serializing/deserializing into key-value pairs without an explicit container serde impls.
• ➕ Trait "reflection": Together with crosstrait supports building the type registry and enables casting from dyn Any returned by TreeAny to desired trait objects. Together with erased-serde it can be used to implement node serialization/deserialization using miniconf's TreeAny without using TreeSerialize/TreeDeserialize similar to bevy_reflect.

Some tangential crates:

• serde-reflection: extract schemata from serde impls
• typetag: "derive serde for trait objects" (local traits and impls)
• deflect: reflection on trait objects using adjacent DWARF debug info as the type registry
• intertrait: inspiration and source of ideas for crosstrait

Example

See below for an example showing some of the features of the Tree* traits. See also the documentation of the [TreeKey] trait for a detailed description.

use serde::{Deserialize, Serialize};
use miniconf::{Error, JsonCoreSlash, JsonPath, Traversal, Tree, TreeKey, Packed};

#[derive(Deserialize, Serialize, Default)]
enum Either {
    #[default]
    Bad,
    Good,
}

#[derive(Deserialize, Serialize, Default, Tree)]
struct Inner {
    a: i32,
    b: i32,
}

#[derive(Tree, Default)]
struct Settings {
    foo: bool,
    enum_: Either,
    struct_: Inner,
    array: [i32; 2],
    option: Option<i32>,

    // Skipping (`()` is not Deserialize/Serialize)
    #[tree(skip)]
    skipped: (),

    #[tree(depth=1)]
    struct_tree: Inner,
    #[tree(depth=1)]
    array_tree: [i32; 2],
    #[tree(depth=2)]
    array_tree2: [Inner; 2],

    #[tree(depth=1)]
    option_tree: Option<i32>,
    #[tree(depth=2)]
    option_tree2: Option<Inner>,
    #[tree(depth=3)]
    array_option_tree: [Option<Inner>; 2],
}

let mut settings = Settings::default();

// Atomic updates by field name
settings.set_json("/foo", b"true")?;
assert_eq!(settings.foo, true);
settings.set_json("/enum_", br#""Good""#)?;
settings.set_json("/struct_", br#"{"a": 3, "b": 3}"#)?;
settings.set_json("/array", b"[6, 6]")?;
settings.set_json("/option", b"12")?;
settings.set_json("/option", b"null")?;

// Exposing nodes of containers
// ... by field name in a struct
settings.set_json("/struct_tree/a", b"4")?;
// ... or by index in an array
settings.set_json("/array_tree/0", b"7")?;
// ... or by index and then struct field name
settings.set_json("/array_tree2/0/a", b"11")?;
// ... or by hierarchical index
settings.set_json_by_key([7, 0, 1].into_iter(), b"8")?;
// ... or by packed index
let (packed, _depth) = Settings::packed([7, 1, 0]).unwrap();
assert_eq!(packed.into_lsb().get(), 0b1_0111_1_0);
settings.set_json_by_key(packed, b"9")?;
// ... or by JSON path
settings.set_json_by_key(JsonPath::from(".array_tree2[1].b"), b"10")?;

// Hiding paths by setting an Option to `None` at runtime
assert_eq!(settings.set_json("/option_tree", b"13"), Err(Traversal::Absent(1).into()));
settings.option_tree = Some(0);
settings.set_json("/option_tree", b"13")?;
// Hiding a path and descending into the inner `Tree`
settings.option_tree2 = Some(Inner::default());
settings.set_json("/option_tree2/a", b"14")?;
// Hiding items of an array of `Tree`s
settings.array_option_tree[1] = Some(Inner::default());
settings.set_json("/array_option_tree/1/a", b"15")?;

let mut buf = [0; 16];

// Serializing nodes by path
let len = settings.get_json("/struct_", &mut buf).unwrap();
assert_eq!(&buf[..len], br#"{"a":3,"b":3}"#);

// Iterating over all paths
for path in Settings::iter_paths::<String>("/") {
    let path = path.unwrap();
    // Serialize each
    match settings.get_json(&path, &mut buf) {
        // Full round-trip: deserialize and set again
        Ok(len) => { settings.set_json(&path, &buf[..len])?; }
        // Some settings are still `None` and thus their paths are expected to be absent
        Err(Error::Traversal(Traversal::Absent(_))) => {}
        e => { e.unwrap(); }
    }
}

# Ok::<(), Error<serde_json_core::de::Error>>(())

Settings management

One possible use of miniconf is a backend for run-time settings management in embedded devices.

It was originally designed to work with JSON (serde_json_core) payloads over MQTT (minimq) and provides a MQTT settings management client and a Python reference implementation to interact with it. Now it is agnostic of serde backend/format, hierarchy separator, and transport/protocol.

Formats

miniconf can be used with any serde::Serializer/serde::Deserializer backend, and key format.

Currently support for / as the path hierarchy separator and JSON (serde_json_core) is implemented through the [JsonCoreSlash] super trait.

The Postcard super trait supports the postcard wire format with any postcard flavor and any [Keys] type. Combined with the [Packed] key representation, this is a very space-efficient key-serde API.

Blanket implementations are provided for all TreeSerialize+TreeDeserialize types for all formats.

Transport

miniconf is also protocol-agnostic. Any means that can receive or emit serialized key-value data can be used to access nodes by path.

The MqttClient in the miniconf_mqtt crate implements settings management over the MQTT protocol with JSON payloads. A Python reference library is provided that interfaces with it. This example discovers the unique prefix of an application listening to messages under the topic quartiq/application/12345 and set its /foo setting to true.

python -m miniconf -d quartiq/application/+ /foo=true

Derive macros

For structs miniconf offers derive macros for [macro@TreeKey], [macro@TreeSerialize], and [macro@TreeDeserialize]. The macros implements the [TreeKey], [TreeSerialize], and [TreeDeserialize] traits. Fields/items that form internal nodes (non-leaf) need to implement the respective Tree{Key,Serialize,Deserialize} trait. Leaf fields/items need to support the respective [serde] trait (and the desired serde::Serializer/serde::Deserializer backend).

Structs, arrays, and Options can then be cascaded to construct more complex trees. When using the derive macro, the behavior and tree recursion depth can be configured for each struct field using the #[tree(depth(Y))] attribute.

See also the [TreeKey] trait documentation for details.

Keys and paths

Lookup into the tree is done using a [Keys] implementation. A blanket implementation through [IntoKeys] is provided for IntoIterators over [Key] items. The [Key] lookup capability is implemented for usize indices and &str names.

Path iteration is supported with arbitrary separator between names.

Very compact hierarchical indices encodings can be obtained from the [Packed] structure. It implements [Keys].

Limitations

Access to inner fields of some types is not yet supported, e.g. enums other than [Option]. These are still however usable in their atomic serde form as leaf nodes.

Many std smart pointers are not supported or handled in any special way: Box, Rc, Arc.

Features

• json-core: Enable the [JsonCoreSlash] implementation of serializing from and into json slices (using the serde_json_core crate).
• postcard: Enable the Postcard implementation of serializing from and into the postcard compact binary format (using the postcard crate).
• derive: Enable the derive macros in miniconf_derive. Enabled by default.