attempting to call a function that allocates memory at compile time

test.zig

const mem = @import("std").mem;
const assert = @import("std").debug.assert;

const AllocState = struct {
    some_mem: [100 * 1024]u8,
    index: usize,

    fn alloc(a: &mem.Allocator, n: usize) -> %[]u8 {
        const self = @ptrCast(&AllocState, a.context);
        const result = self.some_mem[self.index ... self.index + n];
        self.index += n;
        return result;
    }

    fn realloc(a: &mem.Allocator, old_mem: []u8, new_size: usize) -> %[]u8 {
        const result = %return alloc(a, new_size);
        @memcpy(result.ptr, old_mem.ptr, old_mem.len);
        return result;
    }

    fn free(a: &mem.Allocator, old_mem: []u8) { }
};

fn foo() {
    comptime {
        var state = AllocState {
            .some_mem = undefined,
            .index = 0,
        };
        var some_mem: [100 * 1024]u8 = undefined;
        var allocator = mem.Allocator {
            .allocFn = AllocState.alloc,
            .reallocFn = AllocState.realloc,
            .freeFn = AllocState.free,
            .context = @ptrCast(&mem.Allocator.Context, &state),
        };
        const ptr = bar(&allocator);
        assert(*ptr == 1234);
    }
}

fn bar(a: &mem.Allocator) -> %i32 {
    const ptr = a.create(i32);
    *ptr = 1234;
    return ptr;
}