How to fix Rust E0015 calls in constants are limited

The error E0015 occurs because Rust constants must be evaluated at compile time, but your code attempts to call a function that isn't marked as const. Fix this by adding the const keyword before the function definition to allow it to run during compilation.

const fn calculate_value() -> i32 {
    42
}

const MY_VALUE: i32 = calculate_value();

If you cannot make the function const (e.g., it uses heap allocation or non-const logic), you must move the calculation out of the constant context and into a static variable or a runtime initialization block instead.

Rust constants must be calculated before your program even starts running. If you try to use a regular function inside a constant, the compiler stops you because it can't run that code yet. You fix this by marking the function as const, which tells Rust it is safe to calculate the result while building the program.