clippy::coerce_container_to_any: Protects against unintended coercion of references to container types to &dyn Any when the container type dereferences to a dyn Any which could be directly referenced instead

let x: Box<dyn Any> = Box::new(0u32);
let dyn_any_of_box: &dyn Any = &x;

// Fails as we have a &dyn Any to the Box, not the u32
assert_eq!(dyn_any_of_box.downcast_ref::<u32>(), None);

let x: Box<dyn Any> = Box::new(0u32);
let dyn_any_of_u32: &dyn Any = &*x;

// Succeeds since we have a &dyn Any to the inner u32!
assert_eq!(dyn_any_of_u32.downcast_ref::<u32>(), Some(&0u32));

[lints.clippy]
coerce_container_to_any = "warn"

#![warn(clippy::coerce_container_to_any)]

cargo clippy -- -W clippy::coerce-container-to-any

Clippy's own documentation

What it does

Protects against unintended coercion of references to container types to &dyn Any when the container type dereferences to a dyn Any which could be directly referenced instead.

Why is this bad?

The intention is usually to get a reference to the dyn Any the value dereferences to, rather than coercing a reference to the container itself to &dyn Any.

Example

Because Box<dyn Any> itself implements Any, &Box<dyn Any> can be coerced to an &dyn Any which refers to the Box itself, rather than the inner dyn Any.

let x: Box<dyn Any> = Box::new(0u32);
let dyn_any_of_box: &dyn Any = &x;

// Fails as we have a &dyn Any to the Box, not the u32
assert_eq!(dyn_any_of_box.downcast_ref::<u32>(), None);

Use instead:

let x: Box<dyn Any> = Box::new(0u32);
let dyn_any_of_u32: &dyn Any = &*x;

// Succeeds since we have a &dyn Any to the inner u32!
assert_eq!(dyn_any_of_u32.downcast_ref::<u32>(), Some(&0u32));

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

Related lints

• redundant_cloneChecks for a redundant clone() (and its relatives) which clones an owned value that is going to be dropped without further use
• as_ptr_cast_mutcasting the result of `as_ptr` to *mut u8
• branches_sharing_codeChecks if the blocks of an if/else, or the arms of a match, contain shared code that can be moved out of the branches
• clear_with_drain`drain` used to clear a `Vec`
• collection_is_never_readChecks for collections that are never queried
• debug_assert_with_mut_callChecks for function/method calls with a mutable parameter in debug_assert!, debug_assert_eq! and debug_assert_ne! macros
• derive_partial_eq_without_eqChecks for types that derive PartialEq and could implement Eq

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker