RS.CLIPPY.OWNED_COW

Needlessly owned Cow type

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: owned_cow. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Detects needlessly owned Cow types.

Why is this bad?

The borrowed types are usually more flexible, in that e.g. a Cow<\'_, str> can accept both &str and String while Cow<\'_, String> can only accept &String and String. In particular, &str is more general, because it allows for string literals while &String can only be borrowed from a heap-owned String).

Known Problems

The lint does not check for usage of the type. There may be external interfaces that require the use of an owned type.

At least the CString type also has a different API than CStr: The former has an as_bytes method which the latter calls to_bytes. There is no guarantee that other types won't gain additional methods leading to a similar mismatch.

In addition, the lint only checks for the known problematic types String, Vec<_>, CString, OsString and PathBuf. Custom types that implement ToOwned will not be detected.

Example

let wrogn: std::borrow::Cow<\'_, Vec<u8>>;

Use instead:

let right: std::borrow::Cow<\'_, [u8]>;

Configuration

avoid-breaking-exported-api: Suppress lints whenever the suggested change would cause breakage for other crates.

(default: true)