bun.sh/src/NullableAllocator.zig

//! A nullable allocator the same size as `std.mem.Allocator`.
const std = @import("std");
const NullableAllocator = @This();

ptr: *anyopaque = undefined,
// Utilize the null pointer optimization on the vtable instead of
// the regular ptr because some allocator implementations might tag their
// `ptr` property.
vtable: ?*const std.mem.Allocator.VTable = null,

pub inline fn init(allocator: ?std.mem.Allocator) NullableAllocator {
    return if (allocator) |a| .{
        .ptr = a.ptr,
        .vtable = a.vtable,
    } else .{};
}

pub inline fn isNull(this: NullableAllocator) bool {
    return this.vtable == null;
}

pub inline fn get(this: NullableAllocator) ?std.mem.Allocator {
    return if (this.vtable) |vt| std.mem.Allocator{ .ptr = this.ptr, .vtable = vt } else null;
}

comptime {
    if (@sizeOf(NullableAllocator) != @sizeOf(std.mem.Allocator)) {
        @compileError("Expected the sizes to be the same.");
    }
}