Avoiding Clones

Q: When is clone OK?

At IO boundaries, spawning tasks needing 'static, or cheap Copy types.

Q: Async and borrowed data?

Spawned tasks need 'static; use owned String or Arc at spawn boundary once.

Q: How find redundant clones?

cargo clippy -- -W clippy::redundant_clone and profilers.

Q: Clone vs copy?

Copy types (integers, refs) are bitwise; Clone may heap-allocate.

Q: Iterator collect clones?

collect::>() clones if iterating references; use into_iter() when consuming.

Q: sqlx Row get clones?

Use try_get to typed values; fetch &str only with compatible lifetime patterns or copy once.

Q: Polars clone columns?

Lazy plan shares; clone() on DataFrame is data copy - minimize collects.

Q: Builder pattern clones?

Builder moves fields; avoid #[derive(Clone)] on large configs.

Q: Review rule of thumb?

One clone() per request OK; clone per row in million-row loop needs justification.

clone() is explicit and often expensive. Design APIs around borrows, Cow, and moved ownership so copies happen only at intentional boundaries.

Recipe

use std::borrow::Cow;
 
fn greet(name: Cow<'_, str>) {
    println!("Hello, {name}");
}
 
greet(Cow::Borrowed("Ada"));
greet(Cow::Owned("Bob".into()));

When to reach for this: Clippy redundant_clone fires in hot paths or review spots clone() in loops.

Working Example

struct Request<'a> {
    path: &'a str,
    body: Cow<'a, [u8]>,
}
 
fn handle(req: Request<'_>) -> Status {
    if req.path == "/health" {
        return Status::Ok;
    }
    process_body(req.body.as_ref())
}

What this demonstrates:

&str / &[u8] borrow when caller owns data
Cow accepts borrowed or owned without forcing clone upfront
Move String once into struct instead of clone per use
Arc<str> for shared immutable strings across threads

Deep Dive

Strategies

Pattern	Use
`&T` / `&str` parameters	Read-only hot paths
`Cow<'a, T>`	Sometimes needs ownership
Move by value	Single consumer
`Arc<T>`	Shared read-heavy data
`Rc<T>`	Single-thread share

Axum Example

async fn handler(Json(body): Json<CreateOrder>) -> impl IntoResponse {
    // Json<T> owns body once; pass references into domain layer
    service.create(&body).await
}

Gotchas

Clone to satisfy borrow checker - band-aid. Fix: restructure lifetimes, use owned data in async tasks, or Arc.
Cloning Arc thinking it's cheap data clone - Arc::clone is cheap; inner String clone is not. Fix: distinguish pointer clone vs data clone.
Cow everywhere - API complexity. Fix: use where both borrowed and owned callers exist.
Implicit clone in to_owned() in loop - same cost as clone(). Fix: batch or borrow.
serde always owned - use #[serde(borrow)] on &str fields when deserializing from &[u8] buffer.

Alternatives

Alternative	Use When	Don't Use When
`Arc<str>`	Immutable shared strings	Unique mutation needed
`Pin<Box<T>>`	Self-referential	Simple moved values
`&'static str`	Compile-time literals	Dynamic user input

FAQs

When is clone OK?

At IO boundaries, spawning tasks needing 'static, or cheap Copy types.

Cow performance?

Small enum discriminant; branch cost negligible vs heap clone saved.

Async and borrowed data?

Spawned tasks need 'static; use owned String or Arc at spawn boundary once.

How find redundant clones?

cargo clippy -- -W clippy::redundant_clone and profilers.

Clone vs copy?

Copy types (integers, refs) are bitwise; Clone may heap-allocate.

Iterator collect clones?

collect::<Vec<_>>() clones if iterating references; use into_iter() when consuming.

sqlx Row get clones?

Use try_get to typed values; fetch &str only with compatible lifetime patterns or copy once.

Polars clone columns?

Lazy plan shares; clone() on DataFrame is data copy - minimize collects.

Builder pattern clones?

Builder moves fields; avoid #[derive(Clone)] on large configs.

Review rule of thumb?

One clone() per request OK; clone per row in million-row loop needs justification.

Understanding Allocations - heap cost
Cow, Clone on Write - smart pointer detail
Common Anti-Patterns - over-cloning
30 Performance Rules

Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide.
Clippy Book / Lints - Official linter documentation.
Rust for Rustaceans (Amazon) - Jon Gjengset.
Programming Rust: Fast, Safe Systems Development (Amazon) - Blandy, Orendorff, Tindall.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Recipe

use std::borrow::Cow;
 
fn greet(name: Cow<'_, str>) {
    println!("Hello, {name}");
}
 
greet(Cow::Borrowed("Ada"));
greet(Cow::Owned("Bob".into()));

When to reach for this: Clippy redundant_clone fires in hot paths or review spots clone() in loops.

Working Example

struct Request<'a> {
    path: &'a str,
    body: Cow<'a, [u8]>,
}
 
fn handle(req: Request<'_>) -> Status {
    if req.path == "/health" {
        return Status::Ok;
    }
    process_body(req.body.as_ref())
}

What this demonstrates:

&str / &[u8] borrow when caller owns data
Cow accepts borrowed or owned without forcing clone upfront
Move String once into struct instead of clone per use
Arc<str> for shared immutable strings across threads

Deep Dive

Strategies

Pattern	Use
`&T` / `&str` parameters	Read-only hot paths
`Cow<'a, T>`	Sometimes needs ownership
Move by value	Single consumer
`Arc<T>`	Shared read-heavy data
`Rc<T>`	Single-thread share

Axum Example

async fn handler(Json(body): Json<CreateOrder>) -> impl IntoResponse {
    // Json<T> owns body once; pass references into domain layer
    service.create(&body).await
}

Gotchas

Clone to satisfy borrow checker - band-aid. Fix: restructure lifetimes, use owned data in async tasks, or Arc.
Cloning Arc thinking it's cheap data clone - Arc::clone is cheap; inner String clone is not. Fix: distinguish pointer clone vs data clone.
Cow everywhere - API complexity. Fix: use where both borrowed and owned callers exist.
Implicit clone in to_owned() in loop - same cost as clone(). Fix: batch or borrow.
serde always owned - use #[serde(borrow)] on &str fields when deserializing from &[u8] buffer.

Alternatives

Alternative	Use When	Don't Use When
`Arc<str>`	Immutable shared strings	Unique mutation needed
`Pin<Box<T>>`	Self-referential	Simple moved values
`&'static str`	Compile-time literals	Dynamic user input

FAQs

When is clone OK?

At IO boundaries, spawning tasks needing 'static, or cheap Copy types.

Cow performance?

Small enum discriminant; branch cost negligible vs heap clone saved.

Async and borrowed data?

Spawned tasks need 'static; use owned String or Arc at spawn boundary once.

How find redundant clones?

cargo clippy -- -W clippy::redundant_clone and profilers.

Clone vs copy?

Copy types (integers, refs) are bitwise; Clone may heap-allocate.

Iterator collect clones?

collect::<Vec<_>>() clones if iterating references; use into_iter() when consuming.

sqlx Row get clones?

Use try_get to typed values; fetch &str only with compatible lifetime patterns or copy once.

Polars clone columns?

Lazy plan shares; clone() on DataFrame is data copy - minimize collects.

Builder pattern clones?

Builder moves fields; avoid #[derive(Clone)] on large configs.

Review rule of thumb?

One clone() per request OK; clone per row in million-row loop needs justification.

Understanding Allocations - heap cost
Cow, Clone on Write - smart pointer detail
Common Anti-Patterns - over-cloning
30 Performance Rules

Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide.
Clippy Book / Lints - Official linter documentation.
Rust for Rustaceans (Amazon) - Jon Gjengset.
Programming Rust: Fast, Safe Systems Development (Amazon) - Blandy, Orendorff, Tindall.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Avoiding Clones

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Avoiding Clones