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 += …

if global mutable comptime state existed your code could depend on the order zig evaluates things, a small refactor could drastically change the output of your program (assuming the output depends on the comptime state)