A TypeId represents a globally unique identifier for a type.

Each TypeId is an opaque object which does not allow inspection of what's inside but does allow basic operations such as cloning, comparison, printing, and showing.

A TypeId is currently only available for types which ascribe to 'static, but this limitation may be removed in the future.

While TypeId implements Hash, PartialOrd, and Ord, it is worth noting that the hashes and ordering will vary between Rust releases. Beware of relying on them inside of your code!

Layout

Like other Rust-representation types, TypeId's size and layout are unstable. In particular, this means that you cannot rely on the size and layout of TypeId remaining the same between Rust releases; they are subject to change without prior notice between Rust releases.

Danger of Improper Variance

You might think that subtyping is impossible between two static types, but this is false; there exists a static type with a static subtype. To wit, fn(&str), which is short for for<'any> fn(&'any str), and fn(&'static str), are two distinct, static types, and yet, fn(&str) is a subtype of fn(&'static str), since any value of type fn(&str) can be used where a value of type fn(&'static str) is needed.

This means that abstractions around TypeId, despite its 'static bound on arguments, still need to worry about unnecessary and improper variance: it is advisable to strive for invariance first. The usability impact will be negligible, while the reduction in the risk of unsoundness will be most welcome.

Examples

Suppose SubType is a subtype of SuperType, that is, a value of type SubType can be used wherever a value of type SuperType is expected. Suppose also that CoVar<T> is a generic type, which is covariant over T (like many other types, including PhantomData<T> and Vec<T>).

Then, by covariance, CoVar<SubType> is a subtype of CoVar<SuperType>, that is, a value of type CoVar<SubType> can be used wherever a value of type CoVar<SuperType> is expected.

Then if CoVar<SuperType> relies on TypeId::of::<SuperType>() to uphold any invariants, those invariants may be broken because a value of type CoVar<SuperType> can be created without going through any of its methods, like so:

type SubType = fn(&());
type SuperType = fn(&'static ());
type CoVar<T> = Vec<T>; // imagine something more complicated

let sub: CoVar<SubType> = CoVar::new();
// we have a `CoVar<SuperType>` instance without
// *ever* having called `CoVar::<SuperType>::new()`!
let fake_super: CoVar<SuperType> = sub;

The following is an example program that tries to use TypeId::of to implement a generic type Unique<T> that guarantees unique instances for each Unique<T>, that is, and for each type T there can be at most one value of type Unique<T> at any time.

mod unique {
    use std::any::TypeId;
    use std::collections::BTreeSet;
    use std::marker::PhantomData;
    use std::sync::Mutex;

    static ID_SET: Mutex<BTreeSet<TypeId>> = Mutex::new(BTreeSet::new());

    // TypeId has only covariant uses, which makes Unique covariant over TypeAsId 🚨
    #[derive(Debug, PartialEq)]
    pub struct Unique<TypeAsId: 'static>(
        // private field prevents creation without `new` outside this module
        PhantomData<TypeAsId>,
    );

    impl<TypeAsId: 'static> Unique<TypeAsId> {
        pub fn new() -> Option<Self> {
            let mut set = ID_SET.lock().unwrap();
            (set.insert(TypeId::of::<TypeAsId>())).then(|| Self(PhantomData))
        }
    }

    impl<TypeAsId: 'static> Drop for Unique<TypeAsId> {
        fn drop(&mut self) {
            let mut set = ID_SET.lock().unwrap();
            (!set.remove(&TypeId::of::<TypeAsId>())).then(|| panic!("duplicity detected"));
        }
    }
}

use unique::Unique;

// `OtherRing` is a subtype of `TheOneRing`. Both are 'static, and thus have a TypeId.
type TheOneRing = fn(&'static ());
type OtherRing = fn(&());

fn main() {
    let the_one_ring: Unique<TheOneRing> = Unique::new().unwrap();
    assert_eq!(Unique::<TheOneRing>::new(), None);

    let other_ring: Unique<OtherRing> = Unique::new().unwrap();
    // Use that `Unique<OtherRing>` is a subtype of `Unique<TheOneRing>` 🚨
    let fake_one_ring: Unique<TheOneRing> = other_ring;
    assert_eq!(fake_one_ring, the_one_ring);

    std::mem::forget(fake_one_ring);
}