Crafting Compile-Time Computations: Implementing a const fn in Rust

Rust's const fn allows functions to be evaluated at compile time, enabling performance optimizations and the creation of complex compile-time constants. This challenge will test your understanding of const fn by requiring you to implement a useful compile-time function. You will focus on creating a const fn that calculates a specific mathematical property.

Problem Description

Your task is to implement a const fn in Rust named fibonacci_const that calculates the nth Fibonacci number. The Fibonacci sequence is defined as follows:

• F(0) = 0
• F(1) = 1
• F(n) = F(n-1) + F(n-2) for n > 1

The fibonacci_const function should take a single u32 argument representing the index n in the Fibonacci sequence and return the corresponding Fibonacci number as a u32.

Key Requirements:

• The function must be a const fn.
• It should correctly calculate Fibonacci numbers for non-negative integers.
• It must handle the base cases F(0) and F(1).

Expected Behavior: The function should behave like a standard recursive or iterative Fibonacci implementation but be entirely evaluated at compile time.

Edge Cases:

• n = 0 should return 0.
• n = 1 should return 1.
• Consider potential overflow for larger n, although for the given constraints, u32 should suffice.

Examples

Example 1:

const FIB_5: u32 = fibonacci_const(5);
// FIB_5 should be 5

Explanation: F(0) = 0 F(1) = 1 F(2) = F(1) + F(0) = 1 + 0 = 1 F(3) = F(2) + F(1) = 1 + 1 = 2 F(4) = F(3) + F(2) = 2 + 1 = 3 F(5) = F(4) + F(3) = 3 + 2 = 5

Example 2:

const FIB_0: u32 = fibonacci_const(0);
// FIB_0 should be 0

Explanation: The base case fibonacci_const(0) should return 0.

Example 3:

const FIB_10: u32 = fibonacci_const(10);
// FIB_10 should be 55

Explanation: Continuing the sequence: F(6) = 8 F(7) = 13 F(8) = 21 F(9) = 34 F(10) = 55

Constraints

• The input n will be a u32 such that 0 <= n <= 30. (Note: F(30) is within u32 limits).
• The output must be a u32.
• The function must be a const fn.

Notes

• You can implement this using either recursion or iteration. However, be mindful that const fn has some limitations regarding heap allocation and certain complex control flow.
• Consider how to handle the base cases efficiently within a const fn.
• The goal is to have the calculation guaranteed to happen at compile time, so you should be able to use the result in other const contexts.