Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::read_zero_byte_vec: reading zero byte data to `Vec`

What it meansThis lint catches reads into a zero-length Vec. Especially in the case of a call to with_capacity, this lint warns that read gets the number of bytes from the Vec’s length, not its capacity. It is in the nursery group and off by default.

What you see

warning: reading zero byte data to `Vec`
 --> main.rs:6:5
  |
6 |     f.read(&mut data).unwrap();
  |     ^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#read_zero_byte_vec
  = note: `-W clippy::read-zero-byte-vec` implied by `-W clippy::nursery`
help: try
  |
6 ~     data.resize(100, 0);
7 ~     f.read(&mut data).unwrap();
  |

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?

Reading zero bytes is almost certainly not the intended behavior.

New lints still being finished. Off by default and may have false positives.

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

use std::io;
fn foo<F: io::Read>(mut f: F) {
    let mut data = Vec::with_capacity(100);
    f.read(&mut data).unwrap();
}

Write this instead

use std::io;
fn foo<F: io::Read>(mut f: F) {
    let mut data = Vec::with_capacity(100);
    data.resize(100, 0);
    f.read(&mut data).unwrap();
}

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]
read_zero_byte_vec = "warn"

In code

#![warn(clippy::read_zero_byte_vec)]

One run

cargo clippy -- -W clippy::read-zero-byte-vec

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.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.63.0added in Rust

Clippy's own documentation

What it does

This lint catches reads into a zero-length Vec. Especially in the case of a call to with_capacity, this lint warns that read gets the number of bytes from the Vec’s length, not its capacity.

Why is this bad?

Reading zero bytes is almost certainly not the intended behavior.

Known problems

In theory, a very unusual read implementation could assign some semantic meaning to zero-byte reads. But it seems exceptionally unlikely that code intending to do a zero-byte read would allocate a Vec for it.

Example

use std::io;
fn foo<F: io::Read>(mut f: F) {
    let mut data = Vec::with_capacity(100);
    f.read(&mut data).unwrap();
}

Use instead:

use std::io;
fn foo<F: io::Read>(mut f: F) {
    let mut data = Vec::with_capacity(100);
    data.resize(100, 0);
    f.read(&mut data).unwrap();
}

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

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