Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::print_in_format_impl: use of `println!` in `Display` impl

What it meansChecks for usage of println, print, eprintln or eprint in an implementation of a formatting trait. It is in the suspicious group and warns by default.

What you see

warning: use of `println!` in `Display` impl
 --> main.rs:8:9
  |
8 |         println!("S");
  |         ^^^^^^^^^^^^^ help: replace with: `writeln!(f, ..)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#print_in_format_impl
  = note: `#[warn(clippy::print_in_format_impl)]` 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?

Using a print macro is likely unintentional since formatting traits should write to the Formatter, not stdout/stderr.

Code that is most likely wrong or useless. Warns by default.

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::fmt::{Display, Error, Formatter};

struct S;
impl Display for S {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        println!("S");

        Ok(())
    }
}

Write this instead

use std::fmt::{Display, Error, Formatter};

struct S;
impl Display for S {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        writeln!(f, "S");

        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::print_in_format_impl)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
print_in_format_impl = "allow"

One run

cargo clippy -- -A clippy::print-in-format-impl

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.

Suspiciousgroup

warndefault level

nofixed by cargo clippy --fix

1.61.0added in Rust

Clippy's own documentation

What it does

Checks for usage of println, print, eprintln or eprint in an implementation of a formatting trait.

Why is this bad?

Using a print macro is likely unintentional since formatting traits should write to the Formatter, not stdout/stderr.

Example

use std::fmt::{Display, Error, Formatter};

struct S;
impl Display for S {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        println!("S");

        Ok(())
    }
}

Use instead:

use std::fmt::{Display, Error, Formatter};

struct S;
impl Display for S {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        writeln!(f, "S");

        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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