4. PhantomData — Types That Carry No Data 🔴

use std::marker::PhantomData;

// Without PhantomData:
struct Slice<'a, T> {
    ptr: *const T,
    len: usize,
    // Problem: compiler doesn't know this struct borrows from 'a
    // or that it's associated with T for drop-check purposes
}

// With PhantomData:
struct Slice<'a, T> {
    ptr: *const T,
    len: usize,
    _marker: PhantomData<&'a T>,
    // Now the compiler knows:
    // 1. This struct borrows data with lifetime 'a
    // 2. It's covariant over 'a (lifetimes can shrink)
    // 3. Drop check considers T
}

use std::marker::PhantomData;

/// A handle that's valid only within a specific arena's lifetime
struct ArenaHandle<'arena> {
    index: usize,
    _brand: PhantomData<&'arena ()>,
}

struct Arena {
    data: Vec<String>,
}

impl Arena {
    fn new() -> Self {
        Arena { data: Vec::new() }
    }

    /// Allocate a string and return a branded handle
    fn alloc<'a>(&'a mut self, value: String) -> ArenaHandle<'a> {
        let index = self.data.len();
        self.data.push(value);
        ArenaHandle { index, _brand: PhantomData }
    }

    /// Look up by handle — only accepts handles from THIS arena
    fn get<'a>(&'a self, handle: ArenaHandle<'a>) -> &'a str {
        &self.data[handle.index]
    }
}

fn main() {
    let mut arena1 = Arena::new();
    let handle1 = arena1.alloc("hello".to_string());

    // Can't use handle1 with a different arena — lifetimes won't match
    // let mut arena2 = Arena::new();
    // arena2.get(handle1); // ❌ Lifetime mismatch

    println!("{}", arena1.get(handle1)); // ✅
}

use std::marker::PhantomData;
use std::ops::{Add, Mul};

// Unit marker types (zero-sized)
struct Meters;
struct Seconds;
struct MetersPerSecond;

#[derive(Debug, Clone, Copy)]
struct Quantity<Unit> {
    value: f64,
    _unit: PhantomData<Unit>,
}

impl<U> Quantity<U> {
    fn new(value: f64) -> Self {
        Quantity { value, _unit: PhantomData }
    }
}

// Can only add same units:
impl<U> Add for Quantity<U> {
    type Output = Quantity<U>;
    fn add(self, rhs: Self) -> Self::Output {
        Quantity::new(self.value + rhs.value)
    }
}

// Meters / Seconds = MetersPerSecond (custom trait)
impl std::ops::Div<Quantity<Seconds>> for Quantity<Meters> {
    type Output = Quantity<MetersPerSecond>;
    fn div(self, rhs: Quantity<Seconds>) -> Quantity<MetersPerSecond> {
        Quantity::new(self.value / rhs.value)
    }
}

fn main() {
    let dist = Quantity::<Meters>::new(100.0);
    let time = Quantity::<Seconds>::new(9.58);
    let speed = dist / time; // Quantity<MetersPerSecond>
    println!("Speed: {:.2} m/s", speed.value); // 10.44 m/s

    // let nonsense = dist + time; // ❌ Compile error: can't add Meters + Seconds
}

use std::marker::PhantomData;

// PhantomData<T> — compiler assumes we MIGHT drop a T
// This means T must outlive our struct
struct OwningSemantic<T> {
    ptr: *const T,
    _marker: PhantomData<T>,  // "I logically own a T"
}

// PhantomData<*const T> — compiler assumes we DON'T own T
// More permissive — T doesn't need to outlive us
struct NonOwningSemantic<T> {
    ptr: *const T,
    _marker: PhantomData<*const T>,  // "I just point to T"
}

graph LR
    subgraph Covariant
        direction TB
        A1["&'long T"] -->|"can become"| A2["&'short T"]
    end

    subgraph Contravariant
        direction TB
        B1["fn(&'short T)"] -->|"can become"| B2["fn(&'long T)"]
    end

    subgraph Invariant
        direction TB
        C1["&'a mut T"] ---|"NO substitution"| C2["&'b mut T"]
    end

    style A1 fill:#d4efdf,stroke:#27ae60,color:#000
    style A2 fill:#d4efdf,stroke:#27ae60,color:#000
    style B1 fill:#e8daef,stroke:#8e44ad,color:#000
    style B2 fill:#e8daef,stroke:#8e44ad,color:#000
    style C1 fill:#fadbd8,stroke:#e74c3c,color:#000
    style C2 fill:#fadbd8,stroke:#e74c3c,color:#000

fn print_str(s: &str) {
    println!("{s}");
}

fn main() {
    let owned = String::from("hello");
    // owned lives for the entire function ('long)
    // print_str expects &'_ str ('short — just for the call)
    print_str(&owned); // ✅ Covariance: 'long → 'short is safe
    // A longer-lived reference can always be used where a shorter one is needed.
}

// If &mut T were covariant over T, this would compile:
fn evil(s: &mut &'static str) {
    // We could write a shorter-lived &str into a &'static str slot!
    let local = String::from("temporary");
    // *s = &local; // ← Would create a dangling &'static str
}

// Invariance prevents this: &'static str ≠ &'a str when mutating.
// The compiler rejects the substitution entirely.

use std::marker::PhantomData;

// Covariant over 'a — a Ref<'long> can be used as Ref<'short>
struct Ref<'a, T> {
    ptr: *const T,
    _marker: PhantomData<&'a T>,  // Covariant over 'a, covariant over T
}

// Invariant over T — prevents unsound lifetime shortening of T
struct MutRef<'a, T> {
    ptr: *mut T,
    _marker: PhantomData<&'a mut T>,  // Covariant over 'a, INVARIANT over T
}

// Contravariant over T — useful for callback containers
struct CallbackSlot<T> {
    _marker: PhantomData<fn(T)>,  // Contravariant over T
}

use std::marker::PhantomData;

// A token that brands values with a session lifetime.
// MUST be covariant over 'a — otherwise callers can't shorten
// the lifetime when passing to functions that need a shorter borrow.
struct SessionToken<'a> {
    id: u64,
    _brand: PhantomData<&'a ()>,  // ✅ Covariant — callers can shorten 'a
    // _brand: PhantomData<fn(&'a ())>,  // ❌ Contravariant — breaks ergonomics
    // _brand: PhantomData<&'a mut ()>;  // Still covariant over 'a (invariant over T, but T is fixed as ())
}

fn use_token(token: &SessionToken<'_>) {
    println!("Using token {}", token.id);
}

fn main() {
    let token = SessionToken { id: 42, _brand: PhantomData };
    use_token(&token); // ✅ Works because SessionToken is covariant over 'a
}

use std::marker::PhantomData;
use std::ops::{Add, Mul, Div};

#[derive(Clone, Copy)]
struct Meters;
#[derive(Clone, Copy)]
struct Seconds;
#[derive(Clone, Copy)]
struct Kilograms;
#[derive(Clone, Copy)]
struct SquareMeters;
#[derive(Clone, Copy)]
struct MetersPerSecond;

#[derive(Debug, Clone, Copy)]
struct Qty<U> {
    value: f64,
    _unit: PhantomData<U>,
}

impl<U> Qty<U> {
    fn new(v: f64) -> Self { Qty { value: v, _unit: PhantomData } }
}

impl<U> Add for Qty<U> {
    type Output = Qty<U>;
    fn add(self, rhs: Self) -> Self::Output { Qty::new(self.value + rhs.value) }
}

impl Mul<Qty<Meters>> for Qty<Meters> {
    type Output = Qty<SquareMeters>;
    fn mul(self, rhs: Qty<Meters>) -> Qty<SquareMeters> {
        Qty::new(self.value * rhs.value)
    }
}

impl Div<Qty<Seconds>> for Qty<Meters> {
    type Output = Qty<MetersPerSecond>;
    fn div(self, rhs: Qty<Seconds>) -> Qty<MetersPerSecond> {
        Qty::new(self.value / rhs.value)
    }
}

fn main() {
    let width = Qty::<Meters>::new(5.0);
    let height = Qty::<Meters>::new(3.0);
    let area = width * height; // Qty<SquareMeters>
    println!("Area: {:.1} m²", area.value);

    let dist = Qty::<Meters>::new(100.0);
    let time = Qty::<Seconds>::new(9.58);
    let speed = dist / time;
    println!("Speed: {:.2} m/s", speed.value);

    let sum = width + height; // Same unit ✅
    println!("Sum: {:.1} m", sum.value);

    // let bad = width + time; // ❌ Compile error: can't add Meters + Seconds
}