rand_core

rand_core defines the foundational traits (RngCore, SeedableRng, TryRngCore, and related helpers) used by random number generator implementations throughout the Rust ecosystem. It intentionally covers only the mechanics of generating raw random words and seeding a generator, leaving higher-level functionality — sampling distributions, ranges, and floating-point generation — to the rand crate that builds on top of it.

By factoring these traits into their own minimal, dependency-light crate, RNG algorithm implementations (from cryptographic CSPRNGs to fast non-cryptographic generators) can implement a shared interface without depending on the much larger rand crate, and consumer code can accept any conforming RNG generically. With 1.4B+ total downloads, it is one of the most depended-upon crates in the Rust ecosystem.

What You Get

• RngCore trait defining the core next_u32/next_u64/fill_bytes generator interface
• SeedableRng trait for constructing generators from seeds in a consistent way
• TryRngCore/error-handling helpers for fallible entropy sources
• A block-based generator helper module (block.rs) for RNGs that produce output in fixed-size chunks
• no_std compatibility so RNG implementations can be used in embedded and kernel contexts

Common Use Cases

• Implementing a new RNG algorithm (cryptographic or non-cryptographic) that needs to plug into the wider Rust rand ecosystem
• Writing generic code that accepts impl RngCore so callers can supply any compatible generator
• Building no_std or embedded RNG implementations without pulling in the full rand crate’s dependency surface
• Seeding and constructing RNGs consistently across different generator implementations via SeedableRng

Under The Hood

Architecture The crate is small and trait-centric: src/lib.rs (272 lines) defines the core RngCore/TryRngCore traits, seedable_rng.rs defines SeedableRng and its seeding conventions, block.rs provides a reusable helper for RNGs that generate output in fixed-size blocks rather than word-at-a-time, and word.rs/utils.rs hold small shared helper types. Tech Stack Pure Rust, edition 2024, MSRV 1.85 (notably recent, reflecting a deliberate modernization as part of the rand ecosystem’s ongoing 0.10 development), dual-licensed MIT/Apache-2.0, with lint configuration (missing_docs, missing_debug_implementations, undocumented_unsafe_blocks) enforced at warn level to keep the trait surface well-documented. Code Quality A tests/ directory validates trait behavior independently of any concrete RNG implementation, and the crate’s lints specifically require every unsafe block to carry a safety-justification comment — a meaningful quality bar given that RNG code frequently needs unsafe for performance. API Design The crate deliberately restricts itself to only the traits needed to define “what is an RNG,” explicitly deferring sampling ranges, distributions, and floating-point generation to the downstream rand crate, which keeps its own dependency footprint minimal for algorithm-implementation crates that only need to satisfy the trait contract.