Understanding Rust's E0055 Error: Cannot Move Out of a Borrowed Content in a Match Guard
When working with Rust, you may encounter the E0055 error, which states: cannot move out of a borrowed content in a match guard. This error is common among those getting accustomed to Rust's ownership and borrowing rules, and it underscores the language’s emphasis on safety and memory management. It typically arises when attempting to move out of a value borrowed in a match guard, leading to ownership issues.
Let’s delve into what's happening, why it's an issue, and how to correct it with practical examples.
Understanding the Problem
To appreciate the error, it’s essential to understand Rust’s principles: a value can only have a single owner at one time (ownership) and when you borrow a value, you create one or more references to that value without taking ownership.
Consider this Rust code:
fn main() {
let numbers = vec![10, 20, 30];
for num in &numbers {
match numbers.contains(num) {
true if num > &10 => println!("{} is greater than 10", num),
_ => println!("{} is less than or equal to 10", num),
}
}
}
Here, the error occurs because the num variable, borrowed from the numbers vector, is being moved out erroneously. When you pass num to numbers.contains(num), it attempts to move the borrowed content, violating Rust's borrowing rules.
Why Does This Error Occur?
In Rust, match guards are designed to form patterns into which the variable matches without moving the value out. The move happens when numbers.contains(num) tries to use the value directly rather than its reference, conflicting with Rust’s borrowing mechanism.
Resolving the Error
To fix this issue, ensure that the match conditions only borrow values rather than cause a move. We can fix the previous error by making sure the match condition only uses references:
fn main() {
let numbers = vec![10, 20, 30];
for num in &numbers {
match *num { // Dereference num to make use of its value in the condition
n if n > 10 => println!("{} is greater than 10", n),
_ => println!("{} is less than or equal to 10", num),
}
}
}
In this fixed version, by using match *num, we’re pattern matching against the dereferenced value directly. Also, note that we use n instead of *num in the println macro within the true branch of the match statement to move values safely.
Additional Fix Strategies
Rust provides alternatives such as borrowing directly within guards or utilizing if let to handle cases where moving is tempting inadvertently.
Utilizing & Reference Instead
Another way is directly borrowing the reference within the guard:
fn main() {
let numbers = vec![10, 20, 30];
for num in &numbers {
if numbers.contains(num) {
println!("{} is indeed in the list!", num);
}
}
}
While the fix above works, this source indicates Rust's powerful reference checking and compilation strategy, ensuring safety limits on application crashes that may arise from mismanaged memory.
Conclusion
Understanding and handling E0055 in match guards requires you to grasp Rust’s borrowing model to implement design patterns avoiding move semantics within a borrowed context. Though at first intimidating, consistent practice and review of borrow checking rust messages enrich Rust safe coding strategies for achieving high-quality software systems.
