Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::large_types_passed_by_value: this argument (2048 byte) is passed by value, but might be more efficient if passed by reference (limit: 256 byte)

What it meansChecks for functions taking arguments by value, where the argument type is Copy and large enough to be worth considering passing by reference. Does not trigger if the function is being exported, because that might induce API breakage, if the parameter is declared as mutable, or if the argument is a self. It is in the pedantic group and off by default.

What you see

warning: this argument (2048 byte) is passed by value, but might be more efficient if passed by reference (limit: 256 byte)
 --> main.rs:6:11
  |
6 | fn foo(v: TooLarge) {}
  |           ^^^^^^^^ help: consider passing by reference instead: `&TooLarge`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#large_types_passed_by_value
  = note: `-W clippy::large-types-passed-by-value` implied by `-W clippy::pedantic`

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?

Arguments passed by value might result in an unnecessary shallow copy, taking up more space in the stack and requiring a call to memcpy, which can be expensive.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

#[derive(Clone, Copy)]
struct TooLarge([u8; 2048]);

fn foo(v: TooLarge) {}

Write this instead

fn foo(v: &TooLarge) {}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
large_types_passed_by_value = "warn"

In code

#![warn(clippy::large_types_passed_by_value)]

One run

cargo clippy -- -W clippy::large-types-passed-by-value

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.

Settings in clippy.toml

avoid-breaking-exported-api: Suppress lints whenever the suggested change would cause breakage for other crates.
pass-by-value-size-limit: The minimum size (in bytes) to consider a type for passing by reference instead of by value.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.49.0added in Rust

Clippy's own documentation

What it does

Checks for functions taking arguments by value, where the argument type is Copy and large enough to be worth considering passing by reference. Does not trigger if the function is being exported, because that might induce API breakage, if the parameter is declared as mutable, or if the argument is a self.

Why is this bad?

Arguments passed by value might result in an unnecessary shallow copy, taking up more space in the stack and requiring a call to memcpy, which can be expensive.

Example

#[derive(Clone, Copy)]
struct TooLarge([u8; 2048]);

fn foo(v: TooLarge) {}

Use instead:

fn foo(v: &TooLarge) {}

Configuration

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

(default: true)
pass-by-value-size-limit: The minimum size (in bytes) to consider a type for passing by reference instead of by value.

(default: 256)

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

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