Why can't values of a struct containing function fields be run-time mutable?

Explain
5

Ah, if the function field is annotated with comptime, then it compiles.

const Foo = struct {
    x: bool,
    comptime y: u8 = 123,
    comptime f: fn() void = bar, // compiles okay now
};

fn bar() void {}

pub fn main() !void {
    var foo = Foo{.x = true};
    foo.x = false;
}

I thought function fields are comptime implicitly before. So it looks this understanding is wrong.

I still haven’t got the reason why the code in the first comment fails to compile.