Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::unused_io_amount: written amount is not handled

What it meansChecks for unused written/read amount. It is in the correctness group and denied by default (the build fails).

What you see

error: written amount is not handled
 --> main.rs:5:5
  |
5 |     w.write(b"foo")?;
  |     ^^^^^^^^^^^^^^^^
  |
  = help: use `Write::write_all` instead, or handle partial writes
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unused_io_amount
  = note: `#[deny(clippy::unused_io_amount)]` 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?

io::Write::write(_vectored) and io::Read::read(_vectored) are not guaranteed to process the entire buffer. They return how many bytes were processed, which might be smaller than a given buffer’s length. If you don’t need to deal with partial-write/read, use write_all/read_exact instead. When working with asynchronous code (either with the futures crate or with tokio), a similar issue exists for AsyncWriteExt::write() and AsyncReadExt::read() : these functions are also not guaranteed to process the entire buffer. Your code should either handle partial-writes/reads, or call the write_all/read_exact methods on those traits instead.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

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

Flagged

use std::io;
fn foo<W: io::Write>(w: &mut W) -> io::Result<()> {
    w.write(b"foo")?;
    Ok(())
}

Write this instead

use std::io;
fn foo<W: io::Write>(w: &mut W) -> io::Result<()> {
    w.write_all(b"foo")?;
    Ok(())
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unused_io_amount = "allow"

One run

cargo clippy -- -A clippy::unused-io-amount

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.

Correctnessgroup

denydefault level

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for unused written/read amount.

Why is this bad?

io::Write::write(_vectored) and io::Read::read(_vectored) are not guaranteed to process the entire buffer. They return how many bytes were processed, which might be smaller than a given buffer’s length. If you don’t need to deal with partial-write/read, use write_all/read_exact instead.

When working with asynchronous code (either with the futures crate or with tokio), a similar issue exists for AsyncWriteExt::write() and AsyncReadExt::read() : these functions are also not guaranteed to process the entire buffer. Your code should either handle partial-writes/reads, or call the write_all/read_exact methods on those traits instead.

Known problems

Detects only common patterns.

Examples

use std::io;
fn foo<W: io::Write>(w: &mut W) -> io::Result<()> {
    w.write(b"foo")?;
    Ok(())
}

Use instead:

use std::io;
fn foo<W: io::Write>(w: &mut W) -> io::Result<()> {
    w.write_all(b"foo")?;
    Ok(())
}

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

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