Rustdex

A compact, queryable index of Rust standard library trait implementations.

What it does

Rustdex reads the machine-readable documentation that ships with your Rust toolchain (rustdoc --output-format json) and extracts every trait implementation into a small binary index. You can then query this index to answer questions like:

• "Does HashMap implement FromIterator?" → Yes
• "What traits does Vec implement?" → IntoIterator, FromIterator, Index, Deref, …
• "Which types implement Display?" → String, i32, bool, …

Why it's useful

If you're building a compiler, transpiler, code generator, or IDE plugin that targets Rust, you often need to know what methods and traits are available on standard types — without parsing the entire standard library yourself. Rustdex gives you that information in a single, fast, <100KB binary file.

Example use cases

• Transpilers (like Elevate) need to know if .collect() can produce a HashMap from an iterator of tuples
• Code generators need to know if a type supports Display to decide how to format it
• Language servers can use the index for auto-completion of trait methods
• Linters can check if a type actually implements a trait before suggesting a method call

Quick start

# Install
cargo install --path .

# Build the index from your current Rust toolchain
rustdex build

# Query: what traits does HashMap implement?
rustdex query HashMap

# Query: what types implement FromIterator?
rustdex query --trait FromIterator

# Check: does Vec implement IntoIterator?
rustdex check Vec IntoIterator
# ✅ Vec implements IntoIterator

# Show statistics
rustdex stats

Library usage

use rustdex::{StdIndex, IndexBuilder};

// Build from current toolchain's std.json
let index = IndexBuilder::from_sysroot()?.build()?;

// Query
if index.implements("HashMap", "FromIterator") {
    println!("HashMap can be collected from an iterator!");
}

// Find all traits for a type
for imp in index.find_impls_for("Vec") {
    println!("Vec: {}", imp.trait_name);
}

// Save for later (bincode, ~80KB)
index.save_to_sysroot()?;

// Load a previously built index (fast — no parsing needed)
let cached = StdIndex::load_from_sysroot()?;

How it works

1. Input: Reads std.json from $(rustc --print sysroot)/share/doc/rust/json/ (requires the rust-docs-json component)
2. Extract: Parses all trait implementation blocks using the rustdoc-types crate
3. Index: Builds a compact representation with trait paths, implementing types, generic parameters, and method names
4. Output: Serializes to bincode (default) or JSON (with --json flag) alongside the source JSON in the sysroot

Prerequisites

Note: rustdex depends on rustdoc JSON features that are available only on the Rust nightly toolchain. Generating or using the rustdex index therefore requires a nightly toolchain with the JSON docs component.

# Install nightly toolchain and the JSON documentation component for nightly
rustup toolchain install nightly
rustup component add rust-docs-json --toolchain nightly

This repository includes a rust-toolchain.toml that requests nightly and the rust-docs-json component; cargo run inside the repo will automatically use that toolchain (rustup will install it if needed).

Index location

The index is saved alongside the rustdoc JSON in your toolchain's sysroot:

~/.rustup/toolchains/<toolchain>/share/doc/rust/json/rustdex_<version>.bin

This is automatically version-specific — each Rust version gets its own index file. Run rustdex path to see the exact location.

Features

Feature	Default	Description
json-export	off	Enables StdIndex::to_json() in the library API

The CLI always supports --json regardless of this feature flag.

License

BSD 3-Clause — see LICENSE.