Fix E0384: Cannot Borrow as Mutable Because `self` is Also Borrowed as Immutable

Sun, 14 Jun 2026 19:56:04 +0800

1. Symptoms

When attempting to compile Rust code that returns a mutable reference to self from a method that takes an immutable &self parameter, the compiler emits error E0384.

Typical error messages you’ll encounter:

error[E0384]: cannot borrow `self` as mutable because it is also borrowed as immutable
  --> src/main.rs:10:9
   |
9  |     fn get_mut(&self) -> &mut Self {
   |                      ----- first immutable borrow occurs here
10 |         &mut *self
   |         ^^^^^^^^^ second borrow occurs here due to use of `self` in the return type
   |
   = note: this error occurs in the generation of the v-table for trait implementation
   = note: the old error code E0501 has been superseded
---

```text
error[E0384]: cannot borrow `self` as mutable because it is also borrowed as immutable
  --> src/lib.rs:15:13
   |
14 |     fn push(&mut self, elem: T) -> &mut T {
   |             --------- first borrow occurs due to use of `self` in the return type
15 |         let elem_ref = &mut self.data[self.len];
   |                        ^^^^^^^^^^^^^^^^^^^^^ second borrow occurs here

The error specifically states that you cannot borrow self as mutable because it is already borrowed as immutable. This is a fundamental violation of Rust’s borrowing rules, which enforce that you cannot have both an immutable and mutable reference to the same data at the same time.

Self-Borrow on ErrorVault — Developer Error Code Dictionary

Fix E0384: Cannot Borrow as Mutable Because `self` is Also Borrowed as Immutable

1. Symptoms