Скачать или смотреть Solving the Expected enum std::result::Result, found struct std::vec::Vec Error in Rust

Learn how to address the type mismatch error in Rust when trying to return a vector from a function. We'll provide step-by-step solutions and best practices.
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This video is based on the question https://stackoverflow.com/q/62417235/ asked by the user 'daniel' ( https://stackoverflow.com/u/11766292/ ) and on the answer https://stackoverflow.com/a/62418981/ provided by the user 'Brent Kerby' ( https://stackoverflow.com/u/4704553/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Rust's Type Mismatch Error

When working with Rust, you might encounter various errors due to its strict type system. One common problem is the error message that states: "expected enum std::result::Result, found struct std::vec::Vec." This can occur when your function's declared return type does not match the actual returned value type. Let's explore this problem in detail and learn how to resolve it effectively.

The Problem Explained

Imagine you are trying to create a function that finds the divisors of a given integer and returns those values in a vector. However, as you attempt to compile your function, you encounter the following error:

[[See Video to Reveal this Text or Code Snippet]]

Why Does This Error Occur?

The crux of the issue is the mismatch between the return type specified for your function and the type of data being returned.

Expected Return Type: Your function is declared to return a Result<Vec<u32>, String>.

Actual Return Type: You are returning a Vec<i32> at the end of the function.

Rust's Strict Type System

Rust enforces strict type checking, meaning it does not automatically convert types for you. Therefore, you must ensure your return types align with the function's signature.

Solution Steps

To eliminate the error, follow these structured steps:

Step 1: Use the Ok Constructor

You need to wrap your vector inside the Ok variant to return the correct type. Modify the return statement as follows:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Adjust the Vector Type

Next, notice that the types still do not match. The function's return type expects Vec<u32>, while you've declared v as Vec<i32>. To fix this, you need to ensure the vector type matches:

Change the declaration of v to Vec<u32>.

Change the values pushed into v to u32 type.

Here’s the revised function:

[[See Video to Reveal this Text or Code Snippet]]

Summary of Changes

Declared Vector: Changed from Vec<i32> to Vec<u32>.

Cast Values: Cast each divisor found from i32 to u32 using as u32.

Conclusion

Addressing type mismatches is a common hurdle in Rust, especially for newcomers. Understanding the relationship between the expected return types and actual returned values is crucial for coding in Rust. By following the steps outlined above, you can resolve the expected enum std::result::Result, found struct std::vec::Vec error effectively.

With practice, handling such errors will become second nature, allowing you to write more robust and error-free Rust code.