Understanding Rust Error E0716: Temporary Value Dropped While Borrow is Active

Introduction to Rust Error E0716

Rust Error E0716 occurs when a temporary value is dropped while a borrow is still in active use. This error is mainly associated with memory safety and preventing dangling references, which are references pointing to deallocated memory. In this article, we will analyze and discuss how to solve this error.

Erroneous Code Example

Let's begin by examining the erroneous code example given in the official documentation:

#![allow(unused)]
fn main() {
  fn foo() -> i32 { 22 }
  fn bar(x: &i32) -> &i32 { x }
  let p = bar(&foo());
  let q = *p;
}

Analysis of the Error

In this code sample, the expression &foo() borrows the result of the function call foo(). Since foo() is a function call and not a variable, this creates a temporary value to store the result of foo() so that it can be borrowed. A temporary value is created automatically and dropped (freed) according to fixed rules, usually at the end of the enclosing statement.

To fix Rust Error E0716, we need to ensure that the borrowed value lives long enough. This can be achieved by creating a local variable to store the value, rather than relying on a temporary.

Corrected Code Example

Here's a corrected version of the code that creates a local variable to store the value:

#![allow(unused)]
fn main() {
  fn foo() -> i32 { 22 }
  fn bar(x: &i32) -> &i32 { x }
  let value = foo();
  let p = bar(&value);
  let q = *p;
}

Alternative Solution

Another way to fix Rust Error E0716 is to store the reference to the temporary directly into a variable, as shown in the following example:

#![allow(unused)]
fn main() {
  fn foo() -> i32 { 22 }
  fn bar(x: &i32) -> &i32 { x }
  let value = &foo();
  let p = bar(value);
  let q = *p;
}

Conclusion

Rust Error E0716 is a result of a temporary value being dropped while a borrow is still active. To fix this error, ensure that the borrowed value has a long enough lifetime, either by creating a local variable to store the value or by storing the reference to the temporary directly into a variable. By understanding the fundamentals behind Rust's ownership and borrowing system, you can prevent this error and write safer, more efficient code.