clippy::type_id_on_box: calling `.type_id()` on `Box<dyn Any>`

What you see

warning: calling `.type_id()` on `Box<dyn Any>`
 --> main.rs:6:12
  |
6 | assert_eq!(any_box.type_id(), TypeId::of::<i32>()); // ⚠️ this fails!
  |            -------^^^^^^^^^^
  |            |
  |            help: consider dereferencing first: `(*any_box)`
  |
  = note: this returns the type id of the literal type `Box<_>` instead of the type id of the boxed value, which is most likely not what you want
  = note: if this is intentional, use `TypeId::of::<Box<dyn Any>>()` instead, which makes it more clear
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#type_id_on_box
  = note: `#[warn(clippy::type_id_on_box)]` on by default

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

This almost certainly does not do what the user expects and can lead to subtle bugs. Calling .type_id() on a Box<dyn Trait> returns a fixed TypeId of the Box itself, rather than returning the TypeId of the underlying type behind the trait object. For Box<dyn Any> specifically (and trait objects that have Any as its supertrait), this lint will provide a suggestion, which is to dereference the receiver explicitly to go from Box<dyn Any> to dyn Any. This makes sure that .type_id() resolves to a dynamic call on the trait object and not on the box. If the fixed TypeId of the Box is the intended behavior, it’s better to be explicit about it and write TypeId::of::<Box<dyn Trait>>(): this makes it clear that a fixed TypeId is returned and not the TypeId of the implementor.

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

use std::any::{Any, TypeId};

let any_box: Box<dyn Any> = Box::new(42_i32);
assert_eq!(any_box.type_id(), TypeId::of::<i32>()); // ⚠️ this fails!

Write this instead

use std::any::{Any, TypeId};

let any_box: Box<dyn Any> = Box::new(42_i32);
assert_eq!((*any_box).type_id(), TypeId::of::<i32>());
//          ^ dereference first, to call `type_id` on `dyn Any`

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

#[allow(clippy::type_id_on_box)]
fn my_function() { /* ... */ }

[lints.clippy]
type_id_on_box = "allow"

cargo clippy -- -A clippy::type-id-on-box

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.