Rust Auto Traits: Ensuring Type Safety at Compile Time

Auto traits in Rust are powerful compile-time checks that guarantee certain properties about a type without requiring explicit implementation. This challenge will test your understanding of how to define and utilize custom auto traits to enforce specific safety guarantees within your Rust code, leading to more robust and maintainable programs.

Problem Description

Your task is to implement a custom auto trait in Rust called NoUnsafeCode. This auto trait should automatically be implemented for any type that does not contain any unsafe blocks within its methods or associated functions. The goal is to create a mechanism to identify and flag types that might require extra scrutiny due to the presence of unsafe operations.

You will need to:

1. Define the NoUnsafeCode auto trait: This trait should be an auto trait.
2. Implement NoUnsafeCode for primitive types and standard library types that are known to be safe (e.g., i32, String, Vec). You'll need to carefully consider which standard library types can be safely assumed to implement NoUnsafeCode without further inspection.
3. Demonstrate that your NoUnsafeCode trait correctly identifies types with unsafe blocks: Create a type that explicitly uses an unsafe block and show that it does not implement NoUnsafeCode.
4. Demonstrate that your NoUnsafeCode trait correctly identifies types without unsafe blocks: Create a type that contains only safe operations and show that it does implement NoUnsafeCode.
5. Show how to use #[cfg(not(feature = "no_unsafe"))] or similar conditional compilation to conditionally remove or enable code based on the presence of unsafe blocks (though the primary focus is on defining the auto trait).

Expected Behavior:

• A type T should implement NoUnsafeCode if and only if all its methods, associated functions, and any associated types within its definition contain no unsafe blocks.
• The compiler should enforce this auto trait. Attempts to use a type that should implement NoUnsafeCode (but doesn't due to unsafe code) in a context requiring NoUnsafeCode should result in a compile-time error.

Edge Cases to Consider:

• Types that wrap other types: If a wrapper type only calls safe methods on its inner type, it should also implement NoUnsafeCode. If it calls unsafe methods on the inner type, it should not.
• Closures: Consider how unsafe code within closures passed to methods might affect the auto trait implementation of the outer type. (For this challenge, focus on unsafe directly within the type's own definitions).
• External crates: You cannot inspect the source code of external crates to determine their unsafe content for this challenge. Assume that standard library types that are inherently safe (e.g., i32, f64) do not rely on unsafe blocks in their core implementation unless a specific common pattern suggests otherwise.

Examples

Example 1: A Safe Struct

struct SafePoint {
    x: i32,
    y: i32,
}

impl SafePoint {
    fn new(x: i32, y: i32) -> Self {
        SafePoint { x, y }
    }

    fn distance_from_origin(&self) -> f64 {
        ((self.x.pow(2) + self.y.pow(2)) as f64).sqrt()
    }
}

// Assume NoUnsafeCode is correctly implemented.
// This assertion should pass at compile time.
// assert!(is_safe::<SafePoint>());

Output (Conceptual): The compiler should verify that SafePoint implements NoUnsafeCode.

Explanation: SafePoint contains no unsafe blocks. Its methods new and distance_from_origin are entirely safe.

Example 2: An Unsafe Struct

use std::ptr;

struct UnsafeProcessor {
    data: *mut u8,
    len: usize,
}

impl UnsafeProcessor {
    fn new(size: usize) -> Self {
        // Allocation might involve unsafe, but we're focusing on explicit blocks
        let layout = std::alloc::Layout::from_size_align(size, 1).unwrap();
        let data = unsafe { std::alloc::alloc(layout) as *mut u8 };
        UnsafeProcessor { data, len: size }
    }

    fn write(&mut self, index: usize, value: u8) {
        if index < self.len {
            // This is an explicit unsafe block
            unsafe {
                ptr::write(self.data.add(index), value);
            }
        } else {
            panic!("Index out of bounds");
        }
    }

    // ... other methods ...
}

// Assume NoUnsafeCode is correctly implemented.
// This assertion should fail at compile time if UnsafeProcessor doesn't implement NoUnsafeCode.
// assert!(is_safe::<UnsafeProcessor>());

Output (Conceptual): The compiler should detect the unsafe block in the write method and determine that UnsafeProcessor does not implement NoUnsafeCode. The conceptual assertion assert!(is_safe::<UnsafeProcessor>()) would fail.

Explanation: The UnsafeProcessor struct has an unsafe block within its write method, making it inherently unsafe and thus not eligible for the NoUnsafeCode trait.

Example 3: Primitive Type

// Assume NoUnsafeCode is correctly implemented for primitives.
// This assertion should pass at compile time.
// assert!(is_safe::<i32>());

Output (Conceptual): The compiler should verify that i32 implements NoUnsafeCode.

Explanation: Primitive integer types in Rust are designed to be safe and do not require unsafe blocks for their fundamental operations.

Constraints

• Your solution must be written in Rust.
• You must define a new auto trait named NoUnsafeCode.
• You are expected to make reasonable assumptions about the safety of standard library types.
• The primary goal is to correctly implement the auto trait such that it accurately reflects the absence or presence of unsafe blocks within a type's definition.
• You do not need to implement a runtime function like is_safe<T>() that checks if a type implements the trait. The emphasis is on the compiler enforcing the trait itself.

Notes

• Auto traits are a compile-time feature. Their implementation and verification happen during compilation.
• The Rust compiler implicitly implements auto traits for types where the trait's criteria are met. You will typically not write explicit impl AutoTrait for MyType blocks for auto traits unless you are special-casing certain types or overriding default behavior. Instead, you define the auto trait and rely on the compiler to infer implementations.
• Think about how the compiler determines if a type satisfies the conditions of an auto trait. It often involves checking the fields of a struct, the methods of an impl block, and so on.
• For this challenge, you can assume that basic operations on primitives and standard library collections that don't involve raw pointers or manual memory management are "safe." You'll need to identify types that explicitly use unsafe { ... } blocks.