Understanding Rust Error E0618

Overview of Rust Error E0618

Rust Error E0618 occurs when there is an attempt to call something that isn't a function or a method. Only functions and methods can be called using the () syntax. This error is usually triggered when a non-function or non-method type is mistakenly used as the callee.

Erroneous code examples

Example 1: Enum variant mistaken for a function

```rust
    enum X {
        Entry,
    }

    X::Entry(); // Error E0618: expected function, tuple struct or tuple variant, found `X::Entry`
```

In this example, `X::Entry` is an enum variant, and we accidentally try to call it as if it were a function.

Example 2: Integer value mistaken for a function

```rust
    let x = 0i32;
    x(); // Error E0618: expected function, tuple struct or tuple variant, found `i32`
```

In the code above, we mistakenly try to call an integer value `x` using the function call syntax. This causes Rust Error E0618.

Correct code examples

Example 1: Calling a correct function

```rust
    fn example_function() {}

    example_function(); // No error
```

In this case, `example_function` is declared as a function and correctly called, which does not cause Rust Error E0618.

Example 2: Accessing enum variant value properly

```rust
    enum X {
        Entry,
    }

    let entry = X::Entry; // No error
```

Here, enum variant `X::Entry` is accessed without incorrectly trying to treat it as a function.