I have a function

pub fn write(this: *This, byte: u8) !void {
    const gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer std.debug.assert(gpa.deinit() == .ok);

    const allocator = gpa.allocator();

    if (this.capacity < this.count + 1) {
        const old = this.capacity;
        this.capacity = increaseCapacity(old);
        this.code = allocator.realloc(this.code, this.capacity);
    }

    this.code[this.count] = byte;
    this.count += 1;
}

after running this function in test

test "writes byte to the code" {
    const chunk: *Chunk = try Chunk.init(testing_allocator);
    const expected: u8 = 0x1;
    const added: usize = 1;

    try chunk.write(expected);

    const actual = chunk.code[0];

    try std.testing.expectEqual(chunk.code.len, CAPACITY_THRESHOLD);
    try std.testing.expectEqual(chunk.count, added);
    try std.testing.expect(actual == expected);
}

I’m getting an error, that confuses me a bit:

error: expected type '*heap.general_purpose_allocator.GeneralPurposeAllocator(.{ .stack_trace_frames = 10, .enable_memory_limit = false, .safety = true, .thread_safe = true, .MutexType = null, .never_unmap = false, .retain_metadata = false, .verbose_log = false })', found '*const heap.general_purpose_allocator.GeneralPurposeAllocator(.{ .stack_trace_frames = 10, .enable_memory_limit = false, .safety = true, .thread_safe = true, .MutexType = null, .never_unmap = false, .retain_metadata = false, .verbose_log = false })'

in const allocator = gpa.allocator();

Can someone explain what happens and why?