Why can't container level mutable variables be comptime?

[zigo]

The following code compiles. But if the n declaration is not local, then it fails to compile.

const std = @import("std");

//comptime var n: comptime_int = 0; // error

pub fn main() void {
    comptime var n: comptime_int = 0; // okay
    
    n += 1;
    std.debug.print("{}\n", .{ n });
    n += 1;
    std.debug.print("{}\n", .{ n });
}

[jumpnbrownweasel]

Here’s an earlier answer:

Comptime function call counting

You can’t have a global comptime var. This is because comptime is designed such that the comptime evaluation of one function can’t affect that of another. There have been some hacks to allow mutable global state at comptime (like exploiting error codes), but they aren’t intended and can’t be relied upon. You could keep track of the number of calls to add at comptime by passing a pointer to a comptime var as a parameter, but you would only have the final call count after the calls to add, and yo…