A Rust Drop Implementation Keeps Exclusive Access Until Destruction

When a value implements Drop, its destructor receives &mut self at the end of the value's lifetime. A mutable borrow of one field cannot escape across that destructor if drop may access the same field. The failing fixture attempts exactly this and receives E0713.

Consuming the wrapper still runs cleanup

The function takes Guard<'a> by value and tries to return its inner &'a mut String. Before the function can finish, guard must be dropped. Its Drop implementation appends text through the same mutable reference.

Allowing the returned borrow to become active while drop also receives exclusive access would violate the uniqueness guarantee of &mut. The official E0713 explanation describes this overlap.

Ownership of the wrapper does not imply permission to skip its destructor or move arbitrary fields out of a Drop type.

The repaired signature borrows the guard

The repaired fixture follows the documented form: it borrows Guard mutably for the required lifetime and returns a borrow tied to that access. The fixture verifies compilation rather than trying to manufacture a call whose borrow ends while the same guard immediately drops.

This signature is restrictive because it must prevent the destructor from running during the returned borrow. In many application APIs, a closure-based method is easier: the guard lends the field to a callback, and the borrow is known to end before the method returns and later destruction occurs.

Another design exposes operations on the guard itself rather than returning its inner mutable reference. This keeps cleanup and invariant management together.

Drop makes field extraction special

Rust normally lets code destructure owned structs and move fields out. For a type implementing Drop, partial moves would leave the destructor with a partly moved value. Safe Rust prevents this.

ManuallyDrop, raw reads, and related unsafe tools can control destruction, but they transfer detailed responsibilities: every field must be dropped exactly as required, no invalid partly moved value may reach Drop, and panic paths must remain correct. I do not use them to bypass E0713 without a small audited abstraction.

The standard Drop trait and Reference destructor rules are the basis for the proof.

Cleanup behaviour shapes the public API

RAII guards commonly unlock mutexes, return connections, close transactions, restore state, or emit metrics. Letting internal access escape can outlive the resource policy the guard represents.

A transaction guard should not return a mutable database object that remains usable after rollback or commit runs. A lock guard's protected reference must not outlive the lock. E0713 often reveals that the requested return type contradicts the guard abstraction.

I prefer methods that operate while the guard is alive, explicit commit methods consuming the guard and returning an owned outcome, or APIs from established guard types.

Destructor side effects deserve careful design

Drop cannot be async, and errors cannot be returned normally. Critical fallible cleanup should have an explicit method whose result callers handle; Drop becomes a best-effort fallback or invariant-preserving cleanup.

Panicking in Drop can abort the process if another panic is already unwinding. I keep destructors simple, non-panicking, and independent of complex external services.

Tests cover normal scope exit, early return, panic where applicable, explicit completion, and forgotten values when that is possible. The compiler protects aliasing, not successful business cleanup.

My E0713 checklist

• Does the consumed wrapper implement Drop directly or through generated code?
• Which fields can its destructor read or mutate?
• Is a borrow of the same state being returned past destruction?
• Can the caller operate through the guard rather than extracting the reference?
• Would a scoped callback ensure the loan ends before Drop?
• Should an explicit commit consume the guard and return owned data?
• Is unsafe manual destruction genuinely necessary and fully audited?
• Are fallible cleanup and panic paths tested outside the destructor?

The core principle is that Drop is a final exclusive operation on the whole value. A field borrow cannot pretend that operation will not happen. I redesign access so the loan ends first or transfer a truly owned outcome under an explicit cleanup protocol.