Robust Resource Management with RAII and Drop in Rust

Exception safety, while not directly mirroring traditional exceptions in languages like C++ or Java, is crucial in Rust for ensuring resources are properly cleaned up even in the face of panics or early returns. This challenge focuses on implementing robust resource management using Rust's Resource Acquisition Is Initialization (RAII) principles and the Drop trait to guarantee cleanup, even when errors occur. You'll be designing a system that manages a file resource and ensuring it's always closed correctly, regardless of how the program flow diverges.

Problem Description

You are tasked with creating a FileGuard struct that encapsulates a std::fs::File. The FileGuard should:

1. Acquire the File: Take a file path as a string slice (&str) in its new function and attempt to open the file. If the file cannot be opened, the new function should return a Result containing an std::io::Error.
2. RAII: The FileGuard should hold the std::fs::File within its struct.
3. Guaranteed Cleanup: Implement the Drop trait for FileGuard. The drop method must close the file when the FileGuard goes out of scope, regardless of whether the program exits normally or panics.
4. Read Access: Provide a method read_contents that attempts to read the entire contents of the file into a String. If an error occurs during reading, return a Result containing an std::io::Error. The file should remain open after this operation, even if reading fails.
5. Error Handling: All file operations should return Result types to handle potential errors gracefully.

Examples

Example 1:

Input: File path "test.txt" (file exists and contains "Hello, world!")
Output: Ok("Hello, world!")
Explanation: The `FileGuard` opens "test.txt", reads its contents ("Hello, world!"), and returns the contents as a `String`. The file is closed when the `FileGuard` goes out of scope.

Example 2:

Input: File path "nonexistent.txt" (file does not exist)
Output: Err(std::io::Error)
Explanation: The `FileGuard` attempts to open "nonexistent.txt" but fails. The `new` function returns an `Err` containing the `std::io::Error`. The file is *not* opened, so there's nothing to close.

Example 3:

Input: File path "test.txt" (file exists and is empty)
Output: Ok("")
Explanation: The `FileGuard` opens "test.txt", reads its contents (which is empty), and returns an empty `String`. The file is closed when the `FileGuard` goes out of scope.

Constraints

• The file path will be a string slice (&str).
• The read_contents method should read the entire file into a String.
• The Drop implementation must close the file. Failure to do so will be considered a critical error.
• Assume the file path is valid UTF-8.
• The program should compile and run without panics (other than those explicitly handled and returned as Results).

Notes

• Rust's RAII (Resource Acquisition Is Initialization) is key to solving this problem. The file resource is acquired when the FileGuard is created and released when it's dropped.
• The Drop trait is the mechanism for guaranteeing cleanup.
• Consider using std::fs::File::open and std::fs::File::read for file operations.
• Think carefully about error handling at each step. Returning Results allows the caller to handle errors gracefully.
• The read_contents method should not consume the FileGuard. It should only read from the file and return the contents. The FileGuard should remain valid after the call to read_contents.