Understanding Rust Error E0533

Rust Error E0533

Rust Error E0533 occurs when an item is used as a match pattern, and the item is not a unit struct, a variant, nor a constant. This error is raised at the compiler level to ensure that match patterns use valid expressions.

Example of Erroneous Code

Consider the following example that produces Rust Error E0533:

```rust
#![allow(unused)]
fn main() {
    struct Tortoise;

    impl Tortoise {
        fn turtle(&self) -> u32 { 0 }
    }

    match 0u32 {
        Tortoise::turtle => {}, // Error!
        _ => {}
    }
    if let Tortoise::turtle = 0u32 {} // Same error!
}
```

Explanation of the Error

In the erroneous code above, an attempt to match the integer '0u32' is made against a method, 'Tortoise::turtle', which is an invalid match pattern. Methods are not allowed as match patterns in Rust, as match patterns require unit structs, variants, or constants.

Correcting the Error

To fix the error, bind the value returned by the method and compare it in a match guard. Then the corrected code will function properly:

```rust
#![allow(unused)]
fn main() {
    struct Tortoise;

    impl Tortoise {
        fn turtle(&self) -> u32 { 0 }
    }

    let tortoise = Tortoise;

    match 0u32 {
        x if x == tortoise.turtle() => {}, // Bound into `x` then we compare it!
        _ => {}
    }
}
```

Takeaways