Clippy lint · Performance · checked with Clippy 0.1.99

clippy::unnecessary_to_owned: unnecessary use of `to_string`

What it meansChecks for unnecessary calls to ToOwned::to_owned and other to_owned-like functions. It is in the performance group and warns by default.

What you see

warning: unnecessary use of `to_string`
 --> main.rs:4:5
  |
4 | foo(&path.to_string_lossy().to_string());
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `path.to_string_lossy().as_ref()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_to_owned
  = note: `#[warn(clippy::unnecessary_to_owned)]` 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 unnecessary calls result in useless allocations.

Code that could be written to run faster. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let path = std::path::Path::new("x");
foo(&path.to_string_lossy().to_string());
fn foo(s: &str) {}

Write this instead

let path = std::path::Path::new("x");
foo(&path.to_string_lossy());
fn foo(s: &str) {}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unnecessary_to_owned = "allow"

One run

cargo clippy -- -A clippy::unnecessary-to-owned

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.

Performancegroup

warndefault level

yesfixed by cargo clippy --fix

1.59.0added in Rust

Clippy's own documentation

What it does

Checks for unnecessary calls to ToOwned::to_owned and other to_owned-like functions.

Why is this bad?

The unnecessary calls result in useless allocations.

Known problems

unnecessary_to_owned can falsely trigger if IntoIterator::into_iter is applied to an owned copy of a resource and the resource is later used mutably. See #8148.

Example

let path = std::path::Path::new("x");
foo(&path.to_string_lossy().to_string());
fn foo(s: &str) {}

Use instead:

let path = std::path::Path::new("x");
foo(&path.to_string_lossy());
fn foo(s: &str) {}

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

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