zerocopy 0.8.56: zerocopy::KnownLayout

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# zerocopy 0.8.56 KnownLayout API Contracts

## What `search_known_solution` answered
`search_known_solution` returned NO_SAFE_MATCH (DECISION: UNKNOWN — no safe verified match). No existing verified sample in the network proved `zerocopy::KnownLayout` in `pkg:cargo/zerocopy@0.8.56`.

## What a model would have written instead
A model might attempt to manually implement `KnownLayout` (which is an unsafe trait guarded by hidden methods that only allow the derive macro to implement) instead of using `#[derive(KnownLayout)]`.
Furthermore, a model might assume that `size_for_metadata` for dynamically sized types (slice DSTs) is a naive calculation `offset + count * elem_size`, forgetting that the total size must be rounded up to the struct's alignment (e.g. 8 bytes header + 10 bytes payload with align 4 yields 20 bytes, not 18 bytes).

## How the wrong version fails
1. Manual trait implementation fails to compile because only `zerocopy-derive` is permitted to implement `KnownLayout`.
2. Asserting `size_for_metadata(10) == Some(18)` causes a test panic because the actual layout computation rounds up to the struct alignment (20).

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 zerocopy::KnownLayout in pkg:cargo/zerocopy@0.8.56
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:cargo/zerocopy@0.8.56
Demonstrate these symbols/APIs:
  - zerocopy::KnownLayout

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.

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{
  "schemaVersion": 1,
  "goal": "verify zerocopy::KnownLayout in pkg:cargo/zerocopy@0.8.56",
  "kind": "HOW",
  "packages": [
    "pkg:cargo/zerocopy@0.8.56"
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  "symbols": [
    "zerocopy::KnownLayout"
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}

//! Clean-room verification sample for zerocopy::KnownLayout.

pub use zerocopy::KnownLayout;

use zerocopy::{FromBytes, Immutable, KnownLayout, Ref};

#[derive(Debug, PartialEq, Eq, KnownLayout, FromBytes, Immutable, Copy, Clone)]
#[repr(C)]
pub struct Header {
    pub magic: u32,
    pub length: u16,
    pub flags: u16,
}

#[derive(Debug, KnownLayout, FromBytes, Immutable)]
#[repr(C)]
pub struct Packet {
    pub header: Header,
    pub payload: [u8],
}

fn assert_known_layout<T: ?Sized + KnownLayout>() {}

fn main() {
    // 1. KnownLayout is implemented for primitive sized types and slices
    assert_known_layout::<u8>();
    assert_known_layout::<u32>();
    assert_known_layout::<[u8]>();
    assert_known_layout::<Header>();
    assert_known_layout::<Packet>();

    // 2. KnownLayout can be derived for repr(C) sized structs with PointerMetadata of ()
    let () = <Header as KnownLayout>::PointerMetadata::default();
    assert_eq!(Header::size_for_metadata(()), Some(8));

    // 3. KnownLayout can be derived for repr(C) slice DST structs with PointerMetadata of usize
    let meta: <Packet as KnownLayout>::PointerMetadata = 10usize;
    assert_eq!(meta, 10usize);

    // 4. size_for_metadata accurately calculates size including alignment rounding for slice DSTs
    assert_eq!(<[u8]>::size_for_metadata(42), Some(42));
    assert_eq!(<[u16]>::size_for_metadata(42), Some(84));
    assert_eq!(Packet::size_for_metadata(0), Some(8));
    assert_eq!(Packet::size_for_metadata(8), Some(16));
    assert_eq!(Packet::size_for_metadata(10), Some(20));

    // 5. size_for_metadata returns None when size calculation overflows usize
    assert_eq!(<[u16]>::size_for_metadata(usize::MAX), None);
    assert_eq!(Packet::size_for_metadata(usize::MAX), None);

    // 6. Ref::from_prefix_with_elems safely accesses slice DST fields with KnownLayout
    let raw_bytes: [u8; 12] = [
        0x78, 0x56, 0x34, 0x12,
        0x04, 0x00,
        0x01, 0x00,
        0xDE, 0xAD, 0xBE, 0xEF,
    ];
    let (packet_ref, rest) = Ref::<&[u8], Packet>::from_prefix_with_elems(&raw_bytes[..], 4)
        .expect("valid slice DST prefix");
    assert_eq!(packet_ref.header.magic, 0x12345678);
    assert_eq!(packet_ref.header.length, 4);
    assert_eq!(packet_ref.header.flags, 1);
    assert_eq!(&packet_ref.payload, &[0xDE, 0xAD, 0xBE, 0xEF]);
    assert!(rest.is_empty());

    println!("All zerocopy::KnownLayout contract assertions passed successfully.");
}