Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::needless_borrows_for_generic_args: the borrowed expression implements the required traits
What it meansChecks for borrow operations (&) that are used as a generic argument to a function when the borrowed value could be used. It is in the style group and warns by default.
What you see
warning: the borrowed expression implements the required traits --> main.rs:6:3 | 6 | f(&x); | ^^ help: change this to: `x` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#needless_borrows_for_generic_args = note: `#[warn(clippy::needless_borrows_for_generic_args)]` 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?
Suggests that the receiver of the expression borrows the expression.
Code that works but is not idiomatic Rust. 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
fn f(_: impl AsRef<str>) {}
let x = "foo";
f(&x);
Write this instead
fn f(_: impl AsRef<str>) {}
let x = "foo";
f(x);
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::needless_borrows_for_generic_args)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
needless_borrows_for_generic_args = "allow"One run
cargo clippy -- -A clippy::needless-borrows-for-generic-argsThe [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.
Clippy's own documentation
What it does
Checks for borrow operations (&) that are used as a generic argument to a
function when the borrowed value could be used.
Why is this bad?
Suggests that the receiver of the expression borrows the expression.
Known problems
The lint cannot tell when the implementation of a trait
for &T and T do different things. Removing a borrow
in such a case can change the semantics of the code.
Example
fn f(_: impl AsRef<str>) {}
let x = "foo";
f(&x);
Use instead:
fn f(_: impl AsRef<str>) {}
let x = "foo";
f(x);
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
- must_use_unitthis unit-returning function has a `#[must_use]` attribute
- mut_mutex_lockcalling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference
- needless_borrowthis expression creates a reference which is immediately dereferenced by the compiler
- needless_doctest_mainChecks for fn main() {
- needless_elsethis `else` branch is empty
- needless_late_initunneeded late initialization
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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