The Infinite Recursion Trap: Why Rust’s Drop Trait Refuses to Take Ownership

In Rust, the Drop trait is the magic that makes resource management feel automatic. It’s the cleanup crew that arrives the moment a value goes out of scope. But have you ever looked closely at its signature?

pub trait Drop {
    fn drop(&mut self);
}

It takes &mut self. But why?

In most other contexts, if we want to consume a value and ensure it’s never used again, we pass it by value (self). Since Drop is literally the end of the line for a variable, passing by value seems intuitively correct.

Let’s perform a thought experiment. What would happen if we tried to implement Drop using self (pass-by-value) instead of &mut self?

The Forbidden Signature

Imagine a world where the Rust compiler allowed this code:

struct SmartPointer {
    data: String,
}

// ⚠️ HYPOTHETICAL CODE: This does not compile
impl Drop for SmartPointer {
    fn drop(mut self) { 
        println!("Dropping SmartPointer with data: {}", self.data);
    }
}

If you try to compile this today, you get Error E0046: “method drop has an incompatible type for trait”. But let’s assume the compiler let it slide. We run this logic:

We create a SmartPointer in main.
The scope ends.
Rust decides it’s time to clean up.

The Death Spiral

The moment main tries to clean up our variable, we enter a logical paradox that results in an immediate stack overflow. Here is the step-by-step “Death Spiral”:

The Trigger: Your variable sp goes out of scope in main.
The Call: The compiler invokes Drop::drop(sp).
The Move: Because our hypothetical signature is fn drop(mut self), the variable sp is moved from main’s stack frame into the drop function’s argument slot.
The Execution: The body of drop runs. It prints the message.
The End: The drop function finishes.
The Recursion: Now, the argument self (which is inside the drop function) is about to go out of scope.
The Catch-22: Because self is a SmartPointer going out of scope, Rust must clean it up. How? By calling Drop::drop(self).

We are now back at Step 3. The drop function calls itself, which calls itself, which calls itself—ad infinitum—until the stack blows up.

The Solution: `&mut self`

Rust solves this by mandating fn drop(&mut self).

By taking a mutable reference, the drop function says: “I need to make final changes to this data (like closing a file handle or freeing a pointer), but I am not taking responsibility for deleting the container itself.”

The Trigger: sp goes out of scope.
The Call: Rust calls Drop::drop(&mut sp).
The Execution: The code runs. We have a reference, so no ownership is moved.
The End: The drop function finishes. The reference &mut self goes out of scope. (Dropping a reference is a no-op).
The Real Cleanup: Now that the user-defined logic is done, the compiler steps in to recursively drop the fields (like self.data) and deallocate the memory.

The Language-Level Exception: Aliasing XOR Mutability at Drop Time

There’s a subtle but important detail here. Consider this code:

let ptr = SmartPointer { data: String::from("hello") }; // immutable binding—no `mut`!
// ...
} // At scope end, compiler calls: Drop::drop(&mut ptr)

Wait—how can the compiler create &mut ptr when ptr was declared immutable?

This is a language-level exception to Rust’s normal rules. Normally, obtaining &mut T from an immutable binding is forbidden. But at drop time, the compiler relaxes this restriction because:

No aliasing exists: The borrow checker guarantees no other references to ptr are live at this point
The value is dying: No user code can observe this temporary mutation
It’s compiler-controlled: Only the compiler can invoke Drop::drop()—you cannot call it manually (error E0040)

This is safe because the fundamental principle—aliasing XOR mutability—is still upheld: there’s exactly one reference (&mut self), it’s exclusive, and the value ceases to exist immediately after.

Key Insight: The “exception” isn’t really violating aliasing XOR mutability. It’s leveraging the fact that at destruction time, the borrow checker has already proven that exclusive access is possible. The compiler just creates the &mut reference for you.

Why Not Just Special-Case It?

You might wonder: couldn’t the compiler just detect this infinite recursion at compile time and prevent it?

The Rust team discussed this in early language design. The conclusion was that allowing drop(self) would require too many special rules:

Field Dropping Burden: If drop took ownership, either the implementer would need to manually drop every field, or the compiler would need special logic to “actually drop” fields after the user’s drop code finishes—creating confusing semantics.
Preventing “Perverse” Actions: With drop(self), you could theoretically move self somewhere else and keep it alive—defeating the entire purpose of the destructor.
Complexity vs. Clarity: The &mut self approach keeps things simple: you do your cleanup, the compiler handles the rest.

— Stack Overflow: Why does Drop take &mut self instead of self?

`std::mem::drop` vs `Drop::drop`

An important distinction: you cannot call Drop::drop directly. Attempting to do so results in compiler error E0040.

let x = String::from("hello");
// x.drop();  // ERROR E0040: explicit use of destructor method

If you need to drop a value early, use std::mem::drop:

let x = String::from("hello");
std::mem::drop(x);  // ✅ This works
// x is no longer accessible here

Interestingly, std::mem::drop is hilariously simple:

pub fn drop<T>(_x: T) {}

It takes ownership of the value (T, not &mut T) and does… nothing. The value is dropped when _x goes out of scope at the function’s end—using the normal Drop::drop(&mut self) mechanism internally.

“std::mem::drop is not magic; it is literally defined as pub fn drop<T>(_x: T) {}. Because _x is moved into the function, it is automatically dropped before the function returns.”

A Note on Memory Location: Stack vs Heap

A common follow-up question: “Inside drop, does Rust know if self is on the stack or the heap?”

The answer is No, and that is by design.

The drop function is agnostic to memory location. Whether you write:

let sp = SmartPointer { data: String::new() }; // Stack (the struct)

let sp = Box::new(SmartPointer { data: String::new() }); // Heap

The code executed inside fn drop(&mut self) is identical. The self pointer is simply a memory address. The function instructions operate on that address regardless of where it sits in RAM.

It is the owner of the value (the Box, or the stack frame of main) that knows where the memory lives and how to free it once drop has finished its job.

Moving Fields Out of `drop`

What if you need to consume a field in your drop implementation? For example, calling join() on a JoinHandle?

Since drop takes &mut self, you can’t move fields directly. The solution: wrap fields in Option:

struct ThreadWrapper {
    handle: Option<std::thread::JoinHandle<()>>,
}

impl Drop for ThreadWrapper {
    fn drop(&mut self) {
        if let Some(handle) = self.handle.take() {
            handle.join().expect("Thread panicked");
        }
    }
}

Using Option::take() (or std::mem::take) swaps the value with None, leaving a valid state behind.

This pattern is consistently recommended across Stack Overflow and Reddit for consuming owned values within Drop implementations.

Conclusion

Rust’s ownership model is strict, but usually for a reason. The signature of Drop is a perfect example of system design preventing runtime errors at compile time. By forcing &mut self, Rust:

Prevents infinite recursion in destructors
Maintains clean separation between user cleanup code and compiler-managed deallocation
Eliminates “perverse” scenarios where destructors could keep values alive
Reduces cognitive load by centralizing drop glue in the compiler

It’s the kind of design decision that seems obvious in hindsight—but only because someone thought deeply about what could go wrong.