PhantomData 2/4: Type-level tagging

Let’s solve the problem from the previous slide by adding a type parameter.

// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

// use std::marker::PhantomData;

pub struct ChatId<T> { id: u64, tag: T }

pub struct UserTag;
pub struct AdminTag;

pub trait ChatUser {/* ... */}
pub trait ChatAdmin {/* ... */}

impl ChatUser for UserTag {/* ... */}
impl ChatUser for AdminTag {/* ... */} // Admins are users
impl ChatAdmin for AdminTag {/* ... */}

// impl <T> Debug for UserTag<T> {/* ... */}
// impl <T> PartialEq for UserTag<T> {/* ... */}
// impl <T> Eq for UserTag<T> {/* ... */}
// And so on ...

impl <T: ChatUser> ChatId<T> {/* All functionality for users and above */}
impl <T: ChatAdmin> ChatId<T> {/* All functionality for only admins */}

fn main() {}

Here we’re using a type parameter and gating permissions behind “tag” types that implement different permission traits.

Tag types, or marker types, are zero-sized types that have some semantic meaning to users and API designers.
Ask: What issues does having it be an actual instance of that type pose?

Answer: If it’s not a zero-sized type (like () or struct MyTag;), then we’re allocating more memory than we need to when all we care for is type information that is only relevant at compile-time.
Demonstrate: remove the tag value entirely, then compile!

This won’t compile, as there’s an unused (phantom) type parameter.

This is where PhantomData comes in!

Demonstrate: Uncomment the PhantomData import, and make ChatId<T> the following:

#![allow(unused)]
fn main() {
// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

pub struct ChatId<T> {
    id: u64,
    tag: PhantomData<T>,
}
}

PhantomData<T> is a zero-sized type with a type parameter. We can construct values of it like other ZSTs with let phantom: PhantomData<UserTag> = PhantomData; or with the PhantomData::default() implementation.

Demonstrate: implement From<u64> for ChatId<T>, emphasizing the construction of PhantomData

#![allow(unused)]
fn main() {
// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0

impl<T> From<u64> for ChatId<T> {
    fn from(value: u64) -> Self {
        ChatId {
            id: value,
            // Or `PhantomData::default()`
            tag: PhantomData,
        }
    }
}
}

PhantomData can be used as part of the Typestate pattern to have data with the same structure but different methods, e.g., have TaggedData<Start> implement methods or trait implementations that TaggedData<End> doesn’t.

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PhantomData 2/4: Type-level tagging