Sentinel

pub fn Sentinel(comptime T: type, comptime sentinel_val: Elem(T)) type {
    switch (@typeInfo(T)) {
        .pointer => |info| switch (info.size) {
            .one => switch (@typeInfo(info.child)) {
                .array => |array_info| return @Type(.{
                    .pointer = .{
                        .size = info.size,
                        .is_const = info.is_const,
                        .is_volatile = info.is_volatile,
                        .alignment = info.alignment,
                        .address_space = info.address_space,
                        .child = @Type(.{
                            .array = .{
                                .len = array_info.len,
                                .child = array_info.child,
                                .sentinel_ptr = @as(?*const anyopaque, @ptrCast(&sentinel_val)),
                            },
                        }),
                        .is_allowzero = info.is_allowzero,
                        .sentinel_ptr = info.sentinel_ptr,
                    },
                }),
                else => {},
            },
            .many, .slice => return @Type(.{
                .pointer = .{
                    .size = info.size,
                    .is_const = info.is_const,
                    .is_volatile = info.is_volatile,
                    .alignment = info.alignment,
                    .address_space = info.address_space,
                    .child = info.child,
                    .is_allowzero = info.is_allowzero,
                    .sentinel_ptr = @as(?*const anyopaque, @ptrCast(&sentinel_val)),
                },
            }),
            else => {},
        },
        .optional => |info| switch (@typeInfo(info.child)) {
            .pointer => |ptr_info| switch (ptr_info.size) {
                .many => return @Type(.{
                    .optional = .{
                        .child = @Type(.{
                            .pointer = .{
                                .size = ptr_info.size,
                                .is_const = ptr_info.is_const,
                                .is_volatile = ptr_info.is_volatile,
                                .alignment = ptr_info.alignment,
                                .address_space = ptr_info.address_space,
                                .child = ptr_info.child,
                                .is_allowzero = ptr_info.is_allowzero,
                                .sentinel_ptr = @as(?*const anyopaque, @ptrCast(&sentinel_val)),
                            },
                        }),
                    },
                }),
                else => {},
            },
            else => {},
        },
        else => {},
    }
    @compileError("Unable to derive a sentinel pointer type from " ++ @typeName(T));
}