Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::owned_cow: needlessly owned Cow type
What it meansDetects needlessly owned Cow types. It is in the style group and warns by default.
What you see
warning: needlessly owned Cow type --> main.rs:3:33 | 3 | let wrogn: std::borrow::Cow<'_, Vec<u8>>; | ^^^^^^^ help: use: `[u8]` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#owned_cow = note: `#[warn(clippy::owned_cow)]` 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?
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).
Code that works but is not idiomatic Rust. Warns by default.
How to fix it
Clippy does not fix this one automatically. Change the code by hand:
Flagged
let wrogn: std::borrow::Cow<'_, Vec<u8>>;
Write this instead
let right: std::borrow::Cow<'_, [u8]>;
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::owned_cow)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
owned_cow = "allow"One run
cargo clippy -- -A clippy::owned-cowThe [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.
Settings in clippy.toml
avoid-breaking-exported-api: Suppress lints whenever the suggested change would cause breakage for other crates.
Clippy's own documentation
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)
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- needless_returnunneeded `return` statement
- new_without_defaultChecks for public types with a pub fn new() -> Self method and no implementation of Default
- len_zeroChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
- ok_expectChecks for usage of ok().expect(..). Note: This lint only triggers for code marked compatible with versions of the compiler older than Rust 1.82.0
- op_refChecks for arguments to == which have their address taken to satisfy a bound and suggests to dereference the other argument instead
- option_map_or_noneChecks for usage of _.map_or(None, _)
- partialeq_to_nonebinary comparison to literal `Option::None`
- print_literalliteral with an empty format string
- print_with_newlineThis lint warns when you use print!() with a format string that ends in a newline
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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