Clippy lint · Performance · checked with Clippy 0.1.99

clippy::unbuffered_bytes: calling .bytes() is very inefficient when data is not in memory

What it meansChecks for calls to Read::bytes on types which don’t implement BufRead. It is in the performance group and warns by default.

What you see

warning: calling .bytes() is very inefficient when data is not in memory
 --> main.rs:6:1
  |
6 | file.bytes();
  | ^^^^^^^^^^^^
  |
  = help: consider using `BufReader`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unbuffered_bytes
  = note: `#[warn(clippy::unbuffered_bytes)]` 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 default implementation calls read for each byte, which can be very inefficient for data that’s not in memory, such as File.

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

use std::io::Read;
use std::fs::File;
let file = File::open("./bytes.txt").unwrap();
file.bytes();

Write this instead

use std::io::{BufReader, Read};
use std::fs::File;
let file = BufReader::new(File::open("./bytes.txt").unwrap());
file.bytes();

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unbuffered_bytes = "allow"

One run

cargo clippy -- -A clippy::unbuffered-bytes

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

1.87.0added in Rust

Clippy's own documentation

What it does

Checks for calls to Read::bytes on types which don’t implement BufRead.

Why is this bad?

The default implementation calls read for each byte, which can be very inefficient for data that’s not in memory, such as File.

Example

use std::io::Read;
use std::fs::File;
let file = File::open("./bytes.txt").unwrap();
file.bytes();

Use instead:

use std::io::{BufReader, Read};
use std::fs::File;
let file = BufReader::new(File::open("./bytes.txt").unwrap());
file.bytes();

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

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