Clippy lint · Correctness · checked with Clippy 0.1.99

clippy::read_line_without_trim: calling `.parse()` on a string without trimming the trailing newline character

What it meansLooks for calls to [Stdin::read_line] to read a line from the standard input into a string, then later attempting to use that string for an operation that will never work for strings with a trailing newline character in it (e.g. parsing into a i32). It is in the correctness group and denied by default (the build fails).

What you see

error: calling `.parse()` on a string without trimming the trailing newline character
 --> main.rs:5:22
  |
5 | let num: i32 = input.parse().expect("Not a number!");
  |                ----- ^^^^^^^
  |                |
  |                help: try: `input.trim_end()`
  |
note: call to `.read_line()` here, which leaves a trailing newline character in the buffer, which in turn will cause the checking to always fail
 --> main.rs:4:1
  |
4 | std::io::stdin().read_line(&mut input).expect("Failed to read a line");
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#read_line_without_trim
  = note: `#[deny(clippy::read_line_without_trim)]` 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 operation will always fail at runtime no matter what the user enters, thus making it a useless operation.

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

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let mut input = String::new();
std::io::stdin().read_line(&mut input).expect("Failed to read a line");
let num: i32 = input.parse().expect("Not a number!");
assert_eq!(num, 42); // we never even get here!

Write this instead

let mut input = String::new();
std::io::stdin().read_line(&mut input).expect("Failed to read a line");
let num: i32 = input.trim_end().parse().expect("Not a number!");
//                  ^^^^^^^^^^^ remove the trailing newline
assert_eq!(num, 42);

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
read_line_without_trim = "allow"

One run

cargo clippy -- -A clippy::read-line-without-trim

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

yesfixed by cargo clippy --fix

1.73.0added in Rust

Clippy's own documentation

What it does

Looks for calls to [Stdin::read_line] to read a line from the standard input into a string, then later attempting to use that string for an operation that will never work for strings with a trailing newline character in it (e.g. parsing into a i32).

Why is this bad?

The operation will always fail at runtime no matter what the user enters, thus making it a useless operation.

Example

let mut input = String::new();
std::io::stdin().read_line(&mut input).expect("Failed to read a line");
let num: i32 = input.parse().expect("Not a number!");
assert_eq!(num, 42); // we never even get here!

Use instead:

let mut input = String::new();
std::io::stdin().read_line(&mut input).expect("Failed to read a line");
let num: i32 = input.trim_end().parse().expect("Not a number!");
//                  ^^^^^^^^^^^ remove the trailing newline
assert_eq!(num, 42);

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

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