Excellent — let’s move deeper into the machine. We now stand between two worlds: the heap allocator and the container logic. To build a truly reusable Vec, we’ll isolate memory management into its own mini-engine — just like std::Vec does internally.

🧩 Reinventing from Scratch: Building Vec<T> the Hard Way

Chapter 4 — The RawVec Abstraction

“Safety isn’t free; it’s layered. And the first layer is a box that knows how to grow without breaking.”

🧭 4.1 Why Split Vec and RawVec ?

A real-world Vec actually has two halves:

Why? Because allocation is orthogonal to initialization. When you grow or shrink, you don’t want to touch every element — you just need a new box.

⚠️ If allocation logic and element logic mingle, you’ll eventually double-free or leak.

🧩 4.2 Our RawVec Blueprint

We already built a rough version in Chapter 3. Now we’ll refactor it to mimic how std::RawVec (the internal one) behaves.

use std::alloc::{self, Layout};

pub struct RawVec<T> {
    ptr: *mut T,
    cap: usize,
}

impl<T> RawVec<T> {
    pub fn new() -> Self {
        Self { ptr: std::ptr::null_mut(), cap: 0 }
    }

    pub fn with_capacity(cap: usize) -> Self {
        if cap == 0 {
            return Self::new();
        }

        let layout = Layout::array::<T>(cap).unwrap();
        let ptr = unsafe { alloc::alloc(layout) } as *mut T;

        if ptr.is_null() {
            panic!("allocation failed");
        }

        Self { ptr, cap }
    }

    pub fn capacity(&self) -> usize {
        self.cap
    }

    pub fn ptr(&self) -> *mut T {
        self.ptr
    }
}

We’ve defined a contract:

“RawVec<T> promises valid ownership of a heap region large enough for cap × T, but doesn’t care what lives inside it.”

💡 Sidenote — The Allocator’s Responsibilities

A healthy RawVec must uphold three invariants:

1. cap == 0 ⇒ ptr is null.
2. Non-null ptr ⇒ points to a valid block sized for cap.
3. Deallocation uses the exact same Layout that allocation used.

Break #3 and you enter UB country.

⚙️ 4.3 Growing and Shrinking — The Engine Room

impl<T> RawVec<T> {
    pub fn grow(&mut self) {
        // Choose new capacity
        let new_cap = if self.cap == 0 { 4 } else { self.cap * 2 };
        let new_layout = Layout::array::<T>(new_cap).unwrap();

        unsafe {
            let new_ptr = if self.cap == 0 {
                alloc::alloc(new_layout)
            } else {
                let old_layout = Layout::array::<T>(self.cap).unwrap();
                alloc::realloc(self.ptr.cast(), old_layout, new_layout.size())
            } as *mut T;

            if new_ptr.is_null() {
                panic!("realloc failed");
            }

            self.ptr = new_ptr;
            self.cap = new_cap;
        }
    }

    pub fn shrink_to_fit(&mut self, used: usize) {
        // optional: reduce wasted memory
        if used == 0 {
            unsafe {
                if self.cap != 0 {
                    let layout = Layout::array::<T>(self.cap).unwrap();
                    alloc::dealloc(self.ptr.cast(), layout);
                }
            }
            *self = Self::new();
        } else {
            let layout = Layout::array::<T>(used).unwrap();
            unsafe {
                let new_ptr = alloc::realloc(self.ptr.cast(),
                    Layout::array::<T>(self.cap).unwrap(),
                    layout.size()) as *mut T;
                self.ptr = new_ptr;
                self.cap = used;
            }
        }
    }
}

⚠️ Unsafe Corner

realloc is deceptively simple but lethal:

• If it fails → your old memory is still valid (but you’ve lost track if you overwrite the pointer).
• If you panic mid-grow and drop old elements twice → 💥 UB.

That’s why the real Vec has complex try_reserve and panic-guard logic. For us mortals, we accept a little unsafety to learn how the gods work.

🧹 4.4 Cleaning Up

impl<T> Drop for RawVec<T> {
    fn drop(&mut self) {
        if self.cap != 0 {
            unsafe {
                let layout = Layout::array::<T>(self.cap).unwrap();
                alloc::dealloc(self.ptr.cast(), layout);
            }
        }
    }
}

✅ RawVec now behaves like a mini-allocator. It knows how to create, resize, and destroy a heap region. It does not know what’s inside — and that’s exactly the point.

📦 4.5 Composing It with MyVec

Now MyVec can delegate all heavy lifting:

pub struct MyVec<T> {
    buf: RawVec<T>,
    len: usize,
}

impl<T> MyVec<T> {
    pub fn new() -> Self {
        Self { buf: RawVec::new(), len: 0 }
    }

    pub fn with_capacity(cap: usize) -> Self {
        Self { buf: RawVec::with_capacity(cap), len: 0 }
    }

    pub fn capacity(&self) -> usize { self.buf.capacity() }
}

We’ve separated capacity management (in RawVec) from element management (in MyVec). That’s modularity and safety.

💡 Visualization

MyVec<T>
 ├── buf: RawVec<T>
 │     ├── ptr → heap block
 │     └── cap = 8
 └── len = 3

When MyVec drops, its RawVec drops automatically, freeing the heap.

⚙️ 4.6 Edge Cases to Fear

🔥 Unsafe Rule #3: Your memory bookkeeping is your life. Miscount once, and you’re in undefined territory forever.

🧠 4.7 Summary

Next → Chapter 5: Constructing Vec<T> We’ll finally implement push, pop, insert, remove and watch our vector come alive with real elements and real drops.

Would you like me to continue directly with Chapter 5: Constructing Vec<T> in the same detailed, illustrated Rustonomicon tone?