rand_chacha 0.9.0: rand_chacha::rand_core::RngCore

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# rand_chacha 0.9.0 RngCore Contract Notes

## What `search_known_solution` answered
`search_known_solution` returned `NO_SAFE_MATCH` for verifying `rand_chacha::rand_core::RngCore` in `rand_chacha@0.9.0`.

## What a model would have written instead
A model might assume that `RngCore` still includes `try_fill_bytes` returning `Result<(), Error>` as in older `rand_core` versions (0.6.x used by `rand_chacha` 0.3.x). In `rand_core` 0.9.x re-exported by `rand_chacha` 0.9.0, `RngCore` only specifies `next_u32`, `next_u64`, and `fill_bytes`. Fallible operations are separated into the `TryRngCore` trait, which `RngCore` implementors automatically satisfy with `Infallible` error type.

## How the wrong version fails
Calling `try_fill_bytes` directly via `RngCore` fails compilation unless `TryRngCore` is brought into scope, and expecting a fallible error type causes a type mismatch against `core::convert::Infallible`.

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify rand_chacha::rand_core::RngCore in pkg:cargo/rand_chacha@0.9.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:cargo/rand_chacha@0.9.0
Demonstrate these symbols/APIs:
  - rand_chacha::rand_core::RngCore

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:11392f1369a42bd6f1e6ae02ba74491ead27fb2513e88058702f0fb0bd0db480","contract":["ChaCha20Rng, ChaCha12Rng, and ChaCha8Rng implement rand_chacha::rand_core::RngCore","RngCore::next_u32 returns pseudo-random u32 values deterministically for a fixed seed","RngCore::next_u64 returns pseudo-random u64 values deterministically for a fixed seed","RngCore::fill_bytes fills byte slices of varying lengths including empty, partial-block, and multi-block sizes","RngCore is implemented for \u0026mut R allowing mutable references to act as RNG sources","ChaCha20Rng automatically implements rand_chacha::rand_core::TryRngCore with Infallible error type"],"goal":"verify rand_chacha::rand_core::RngCore in pkg:cargo/rand_chacha@0.9.0","kind":"HOW","packages":["pkg:cargo/rand_chacha@0.9.0"],"schemaVersion":1,"symbols":["rand_chacha::rand_core::RngCore"]},"contractCommand":["cargo","run","--offline"],"environment":{"arch":"x64","distro":"alpine","ecosystem":"cargo","libc":"musl","os":"linux","packageManager":"cargo","schemaVersion":1},"license":"MIT-0","packages":["pkg:cargo/rand_chacha@0.9.0"],"schemaVersion":1,"subject":"pkg:cargo/rand_chacha@0.9.0","symbols":["rand_chacha::rand_core::RngCore"],"verifierAdapter":"cargo@1"}

{
  "schemaVersion": 1,
  "goal": "verify rand_chacha::rand_core::RngCore in pkg:cargo/rand_chacha@0.9.0",
  "kind": "HOW",
  "packages": [
    "pkg:cargo/rand_chacha@0.9.0"
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  "symbols": [
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//! Clean-room verification sample for rand_chacha.

pub use rand_chacha::*;

use rand_chacha::rand_core::{RngCore, SeedableRng, TryRngCore};
use rand_chacha::{ChaCha8Rng, ChaCha12Rng, ChaCha20Rng};

fn main() {
    // 1. ChaCha20Rng, ChaCha12Rng, and ChaCha8Rng implement rand_chacha::rand_core::RngCore
    let seed = [0x42u8; 32];
    let mut rng20 = ChaCha20Rng::from_seed(seed);
    let mut rng12 = ChaCha12Rng::from_seed(seed);
    let mut rng8 = ChaCha8Rng::from_seed(seed);

    fn assert_rng_core<R: RngCore>(_rng: &mut R) {}
    assert_rng_core(&mut rng20);
    assert_rng_core(&mut rng12);
    assert_rng_core(&mut rng8);

    // 2. RngCore::next_u32 returns pseudo-random u32 values deterministically for a fixed seed
    let mut rng_a = ChaCha20Rng::from_seed(seed);
    let mut rng_b = ChaCha20Rng::from_seed(seed);
    let val_a1 = rng_a.next_u32();
    let val_a2 = rng_a.next_u32();
    let val_b1 = rng_b.next_u32();
    let val_b2 = rng_b.next_u32();
    assert_eq!(val_a1, val_b1);
    assert_eq!(val_a2, val_b2);
    assert_ne!(val_a1, val_a2);

    // 3. RngCore::next_u64 returns pseudo-random u64 values deterministically for a fixed seed
    let mut rng_u64_a = ChaCha20Rng::from_seed(seed);
    let mut rng_u64_b = ChaCha20Rng::from_seed(seed);
    let u64_a = rng_u64_a.next_u64();
    let u64_b = rng_u64_b.next_u64();
    assert_eq!(u64_a, u64_b);
    assert_ne!(u64_a, 0);

    // 4. RngCore::fill_bytes fills byte slices of varying lengths including empty, partial-block, and multi-block sizes
    let mut rng = ChaCha20Rng::from_seed(seed);
    let mut empty = [];
    rng.fill_bytes(&mut empty);

    let mut small_buf = [0u8; 16];
    rng.fill_bytes(&mut small_buf);
    assert!(small_buf.iter().any(|&b| b != 0));

    let mut large_buf = [0u8; 128]; // larger than 64-byte ChaCha block
    rng.fill_bytes(&mut large_buf);
    assert!(large_buf.iter().any(|&b| b != 0));

    // Deterministic stream match: same seed produces identical byte fills
    let mut rng_bytes_a = ChaCha20Rng::from_seed(seed);
    let mut rng_bytes_b = ChaCha20Rng::from_seed(seed);
    let mut buf_a = [0u8; 64];
    let mut buf_b = [0u8; 64];
    rng_bytes_a.fill_bytes(&mut buf_a);
    rng_bytes_b.fill_bytes(&mut buf_b);
    assert_eq!(buf_a, buf_b);

    // 5. RngCore is implemented for &mut R allowing mutable references to act as RNG sources
    let mut rng_ref = ChaCha20Rng::from_seed(seed);
    fn call_via_ref<R: RngCore>(rng: &mut R) {
        let _ = rng.next_u32();
        let _ = rng.next_u64();
    }
    call_via_ref(&mut &mut rng_ref);

    // 6. ChaCha20Rng automatically implements rand_chacha::rand_core::TryRngCore with Infallible error type
    let mut try_rng = ChaCha20Rng::from_seed(seed);
    let res_u32: Result<u32, core::convert::Infallible> = try_rng.try_next_u32();
    assert!(res_u32.is_ok());
    let res_u64: Result<u64, core::convert::Infallible> = try_rng.try_next_u64();
    assert!(res_u64.is_ok());
    let mut try_buf = [0u8; 32];
    let res_fill: Result<(), core::convert::Infallible> = try_rng.try_fill_bytes(&mut try_buf);
    assert!(res_fill.is_ok());

    println!("All rand_chacha::rand_core::RngCore contract assertions passed.");
}