Clippy lint · Performance · checked with Clippy 0.1.99

clippy::manual_memcpy: Checks for for-loops that manually copy items between slices that could be optimized by having a memcpy

What it meansChecks for for-loops that manually copy items between slices that could be optimized by having a memcpy. It is in the performance group and warns by default.

Does it matter?

It is not as fast as a memcpy.

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

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

for i in 0..src.len() {
    dst[i + 64] = src[i];
}

Write this instead

dst[64..(src.len() + 64)].clone_from_slice(&src[..]);

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
manual_memcpy = "allow"

One run

cargo clippy -- -A clippy::manual-memcpy

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

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for for-loops that manually copy items between slices that could be optimized by having a memcpy.

Why is this bad?

It is not as fast as a memcpy.

Example

for i in 0..src.len() {
    dst[i + 64] = src[i];
}

Use instead:

dst[64..(src.len() + 64)].clone_from_slice(&src[..]);

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

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