Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::format_push_string: `format!(..)` appended to existing `String`

What it meansDetects cases where the result of a format! call is appended to an existing String. It is in the pedantic group and off by default.

What you see

warning: `format!(..)` appended to existing `String`
 --> main.rs:4:1
  |
4 | s += &format!("0x{:X}", 1024);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: you may need to import the `std::fmt::Write` trait
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#format_push_string
  = note: `-W clippy::format-push-string` implied by `-W clippy::pedantic`
help: consider using `write!` to avoid the extra allocation
  |
4 - s += &format!("0x{:X}", 1024);
4 + let _ = write!(s, "0x{:X}", 1024);
  |

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?

Introduces an extra, avoidable heap allocation.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

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

let mut s = String::new();
s += &format!("0x{:X}", 1024);
s.push_str(&format!("0x{:X}", 1024));

Write this instead

use std::fmt::Write as _; // import without risk of name clashing

let mut s = String::new();
let _ = write!(s, "0x{:X}", 1024);

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]
format_push_string = "warn"

In code

#![warn(clippy::format_push_string)]

One run

cargo clippy -- -W clippy::format-push-string

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.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.62.0added in Rust

Clippy's own documentation

What it does

Detects cases where the result of a format! call is appended to an existing String.

Why is this bad?

Introduces an extra, avoidable heap allocation.

Known problems

format! returns a String but write! returns a Result. Thus you are forced to ignore the Err variant to achieve the same API.

While using write! in the suggested way should never fail, this isn’t necessarily clear to the programmer.

Example

let mut s = String::new();
s += &format!("0x{:X}", 1024);
s.push_str(&format!("0x{:X}", 1024));

Use instead:

use std::fmt::Write as _; // import without risk of name clashing

let mut s = String::new();
let _ = write!(s, "0x{:X}", 1024);

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

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