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facet

@fasterthanlime/.claude
4
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Facet is a serde replacement for Rust using reflection. Use this skill when working with serialization, deserialization, CLI parsing, or any task where you'd normally use serde. Provides crate mappings (serde_json → facet-json, clap → facet-args, etc.) and usage patterns.

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SKILL.md

name facet
description Facet is a serde replacement for Rust using reflection. Use this skill when working with serialization, deserialization, CLI parsing, or any task where you'd normally use serde. Provides crate mappings (serde_json → facet-json, clap → facet-args, etc.) and usage patterns.

Facet - Rust Reflection & Serialization

Facet (https://facet.rs) is a reflection-based alternative to serde for Rust. Instead of Serialize/Deserialize traits, types derive Facet which provides a SHAPE associated const with full type introspection.

Derive Macro

// Instead of:
use serde::{Serialize, Deserialize};
#[derive(Serialize, Deserialize)]
struct Foo { ... }

// Use:
use facet::Facet;
#[derive(Facet)]
struct Foo { ... }

Crate Mapping

Traditional Crate Facet Equivalent Notes
serde facet Core derive macro
serde_json facet-json JSON (de)serialization
toml / serde_toml facet-toml TOML (de)serialization
serde_yaml facet-yaml YAML (de)serialization
clap facet-args CLI argument parsing
rmp-serde / msgpack facet-msgpack MessagePack (de)serialization
postcard facet-postcard Binary format for embedded/no_std
quick-xml / serde_xml_rs facet-xml XML (de)serialization
kdl facet-kdl KDL format support
csv facet-csv CSV (de)serialization
serde_urlencoded facet-urlencoded URL-encoded form data (deser only)
rasn / asn1 facet-asn1 ASN.1 (serialization only)
xdr-codec facet-xdr XDR (serialization only)

Web Framework Integration

Framework Facet Crate Replaces
Axum facet-axum axum::Json, axum::Form

facet-axum provides extractors and responses. Enable features for each format:

[dependencies]
facet-axum = { version = "0.33", features = ["json", "form", "yaml", "toml", "xml", "kdl", "msgpack", "postcard"] }

Utility Crates

Crate Purpose
facet-reflect Build/inspect values dynamically via Peek and Poke
facet-value Dynamic value type (like serde_json::Value)
facet-pretty Pretty-print any Facet type
facet-diff Diff two Facet values
facet-assert Pretty assertions (no PartialEq required)

Typical Cargo.toml

[dependencies]
facet = "0.33"
facet-json = "0.33"  # If you need JSON
facet-toml = "0.33"  # If you need TOML
# etc.

Feature Flags (on facet crate)

Common features to enable for third-party type support:

  • uuid - uuid::Uuid
  • ulid - ulid::Ulid
  • chrono - chrono date/time types
  • time - time crate types
  • jiff02 - jiff date/time types
  • bytes - bytes::Bytes / BytesMut
  • camino - Utf8Path / Utf8PathBuf
  • url - url::Url
  • ordered-float - ordered float types
  • indexmap - IndexMap / IndexSet
  • net - network types (SocketAddr, IpAddr, etc.)
  • nonzero - NonZero<T> types

Code Examples

JSON serialization

use facet::Facet;
use facet_json::{to_string, from_str};

#[derive(Facet)]
struct Config {
    name: String,
    count: u32,
}

let config = Config { name: "test".into(), count: 42 };
let json = to_string(&config);
let parsed: Config = from_str(&json)?;

CLI argument parsing

use facet::Facet;
use facet_args::from_slice;

#[derive(Facet)]
struct Args {
    /// Input file path
    #[facet(positional)]
    input: String,

    /// Enable verbose output
    #[facet(short = 'v')]
    verbose: bool,
}

let args: Args = from_slice(&std::env::args().collect::<Vec<_>>())?;

Axum handlers

use facet::Facet;
use facet_axum::FacetJson;

#[derive(Facet)]
struct Request { id: u64 }

#[derive(Facet)]
struct Response { message: String }

async fn handler(FacetJson(req): FacetJson<Request>) -> FacetJson<Response> {
    FacetJson(Response { message: format!("Got id {}", req.id) })
}

Extended Ecosystem (External)

  • facet-v8 - V8 JavaScript engine integration
  • facet-openapi - OpenAPI schema generation
  • facet_generate - Generate types for Java/Swift/TypeScript

Reference