Understanding Rust Error E0317: Missing else Block in an if Expression

What is Rust Error E0317?

Rust Error E0317 occurs when an if expression is missing an else block in a context where a type other than () is expected. This error is a type error because an if expression without an else block has the type (). To resolve it, add an else block having the same type as the if block.

Erroneous Code Example

#![allow(unused)]
fn main() {
let x = 5;
let a = if x == 5 {
    1
};
}

In the code example above, the let expression was expecting a value, but since there was no else block, no value was returned. The if expression is missing an else block, resulting in a type error (E0317).

How to Resolve Rust Error E0317

To fix Rust Error E0317, add an else block with the same type as the if block. The corrected code example below demonstrates the appropriate changes:

#![allow(unused)]
fn main() {
let x = 5;
let a = if x == 5 {
    1
} else {
    2
};
}

In the corrected code example, an else block has been added with the same type (integer) as the if block. This resolves the type error and ensures the let expression retrieves the expected value.

Further Considerations

When writing Rust code, ensure that all if expressions have a corresponding else block when a type other than () is expected. This not only helps avoid Rust Error E0317 but also contributes to clearer, more readable code.