Single-Use Types — Cryptographic Guarantees via Ownership 🟡

// C — nothing stops nonce reuse
uint8_t nonce[12];
generate_nonce(nonce);
encrypt(key, nonce, msg1, out1);   // ✅ first use
encrypt(key, nonce, msg2, out2);   // 🐛 CATASTROPHIC: same nonce

// ring::aead::Nonce is:
// - NOT Clone
// - NOT Copy
// - Consumed by value when used
pub struct Nonce(/* private */);

impl Nonce {
    pub fn try_assume_unique_for_key(value: &[u8]) -> Result<Self, Unspecified> {
        // ...
    }
    // No Clone, no Copy — can only be used once
}

// Pseudocode mirroring ring's API shape
fn seal_in_place(
    key: &SealingKey,
    nonce: Nonce,       // ← moved, not borrowed
    data: &mut Vec<u8>,
) -> Result<(), Error> {
    // ... encrypt data in place ...
    // nonce is consumed — cannot be used again
    Ok(())
}

fn bad_encrypt(key: &SealingKey, data1: &mut Vec<u8>, data2: &mut Vec<u8>) {
    // .unwrap() is safe — a 12-byte array is always a valid nonce.
    let nonce = Nonce::try_assume_unique_for_key(&[0u8; 12]).unwrap();
    seal_in_place(key, nonce, data1).unwrap();  // ✅ nonce moved here
    // seal_in_place(key, nonce, data2).unwrap();
    //                    ^^^^^ ERROR: use of moved value ❌
}

/// A sequence of unique nonces.
/// Not Clone — once bound to a key, cannot be duplicated.
pub trait NonceSequence {
    fn advance(&mut self) -> Result<Nonce, Unspecified>;
}

/// SealingKey wraps a NonceSequence — each seal() auto-advances.
pub struct SealingKey<N: NonceSequence> {
    key: UnboundKey,   // consumed during construction
    nonce_seq: N,
}

impl<N: NonceSequence> SealingKey<N> {
    pub fn new(key: UnboundKey, nonce_seq: N) -> Self {
        // UnboundKey is moved — can't be used for both sealing AND opening
        SealingKey { key, nonce_seq }
    }

    pub fn seal_in_place_append_tag(
        &mut self,       // &mut — exclusive access
        aad: Aad<&[u8]>,
        in_out: &mut Vec<u8>,
    ) -> Result<(), Unspecified> {
        let nonce = self.nonce_seq.advance()?; // auto-generate unique nonce
        // ... encrypt with nonce ...
        Ok(())
    }
}
# pub struct UnboundKey;
# pub struct Aad<T>(T);
# pub struct Unspecified;

/// An ephemeral private key. Not Clone, not Copy.
/// Consumed by agree_ephemeral().
pub struct EphemeralPrivateKey { /* ... */ }

/// Compute shared secret — consumes the private key.
pub fn agree_ephemeral(
    my_private_key: EphemeralPrivateKey,  // ← moved
    peer_public_key: &UnparsedPublicKey,
    error_value: Unspecified,
    kdf: impl FnOnce(&[u8]) -> Result<SharedSecret, Unspecified>,
) -> Result<SharedSecret, Unspecified> {
    // ... DH computation ...
    // my_private_key is consumed — can never be reused
    # kdf(&[])
}
# pub struct UnparsedPublicKey;
# pub struct SharedSecret;
# pub struct Unspecified;

use std::io;

/// A fuse write payload. Not Clone, not Copy.
/// Consumed when the fuse is programmed.
pub struct FusePayload {
    address: u32,
    data: Vec<u8>,
    // private constructor — only created via validated builder
}

/// Proof that the fuse programmer is in the correct state.
pub struct FuseController {
    /* hardware handle */
}

impl FuseController {
    /// Program a fuse — consumes the payload, preventing double-write.
    pub fn program(
        &mut self,
        payload: FusePayload,  // ← moved — can't be used twice
    ) -> io::Result<()> {
        // ... write to OTP hardware ...
        // payload is consumed — trying to program again with the same
        // payload is a compile error
        Ok(())
    }
}

/// Builder with validation — only way to create a FusePayload.
pub struct FusePayloadBuilder {
    address: Option<u32>,
    data: Option<Vec<u8>>,
}

impl FusePayloadBuilder {
    pub fn new() -> Self {
        FusePayloadBuilder { address: None, data: None }
    }

    pub fn address(mut self, addr: u32) -> Self {
        self.address = Some(addr);
        self
    }

    pub fn data(mut self, data: Vec<u8>) -> Self {
        self.data = Some(data);
        self
    }

    pub fn build(self) -> Result<FusePayload, &'static str> {
        let address = self.address.ok_or("address required")?;
        let data = self.data.ok_or("data required")?;
        if data.len() > 32 { return Err("fuse data too long"); }
        Ok(FusePayload { address, data })
    }
}

// Usage:
fn program_board_serial(ctrl: &mut FuseController) -> io::Result<()> {
    let payload = FusePayloadBuilder::new()
        .address(0x100)
        .data(b"SN12345678".to_vec())
        .build()
        .map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;

    ctrl.program(payload)?;      // ✅ payload consumed

    // ctrl.program(payload);    // ❌ ERROR: use of moved value
    //              ^^^^^^^ value used after move

    Ok(())
}

/// Issued once at power-on. Not Clone, not Copy.
pub struct CalibrationToken {
    _private: (),
}

pub struct SensorController {
    calibrated: bool,
}

impl SensorController {
    /// Called once at power-on — returns a calibration token.
    pub fn power_on() -> (Self, CalibrationToken) {
        (
            SensorController { calibrated: false },
            CalibrationToken { _private: () },
        )
    }

    /// Calibrate the sensor — consumes the token.
    pub fn calibrate(&mut self, _token: CalibrationToken) -> io::Result<()> {
        // ... run calibration sequence ...
        self.calibrated = true;
        Ok(())
    }

    /// Read a sensor — only meaningful after calibration.
    ///
    /// **Limitation:** The move-semantics guarantee is *partial*. The caller
    /// can `drop(cal_token)` without calling `calibrate()` — the token will
    /// be destroyed but calibration won't run. The `#[must_use]` annotation
    /// (see below) generates a warning but not a hard error.
    ///
    /// The runtime `self.calibrated` check here is the **safety net** for
    /// that gap. For a fully compile-time solution, see the type-state
    /// pattern in ch05 where `send_command()` only exists on `IpmiSession<Active>`.
    pub fn read(&self) -> io::Result<f64> {
        if !self.calibrated {
            return Err(io::Error::new(io::ErrorKind::Other, "not calibrated"));
        }
        Ok(25.0) // stub
    }
}

fn sensor_workflow() -> io::Result<()> {
    let (mut ctrl, cal_token) = SensorController::power_on();

    // Must use cal_token somewhere — it's not Copy, so dropping it
    // without consuming it generates a warning (or error with #[must_use])
    ctrl.calibrate(cal_token)?;

    // Now reads work:
    let temp = ctrl.read()?;
    println!("Temperature: {temp}°C");

    // Can't calibrate again — token was consumed:
    // ctrl.calibrate(cal_token);  // ❌ use of moved value

    Ok(())
}