Error E0015

Error E0015 occurs because Rust requires constant values to be computable at compile time, but you are calling a function that isn't marked as const inside a constant definition. To fix this, ensure the function you are calling is defined with the const keyword so the compiler can evaluate it during compilation.

const fn gives_ownership() -> String {
    String::from("yours")
}

const MY_STRING: String = gives_ownership();

Note: As of current Rust versions, String cannot be created in a const context. You must use a &'static str instead.

const fn get_message() -> &'static str {
    "yours"
}

const MY_STRING: &'static str = get_message();

If you cannot make the function const (e.g., it uses non-const features), you must move the logic out of the const definition and into a regular let variable inside a function like main.

Error E0015 occurs when you try to calculate a value that must be known before the program runs using a function that only executes during runtime. Think of it like trying to bake a cake before you have the ingredients; the oven (compiler) needs everything ready beforehand. You must either use a function that works in the 'pre-baking' phase or move the calculation to the 'cooking' phase (runtime).