Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::format_collect: use of `format!` to build up a string from an iterator

What it meansChecks for usage of .map(|_| format!(..)).collect::<String>(). It is in the pedantic group and off by default.

What you see

warning: use of `format!` to build up a string from an iterator
 --> main.rs:4:5
  |
4 |     bytes.iter().map(|b| format!("{b:02X}")).collect()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: call `fold` instead
 --> main.rs:4:18
  |
4 |     bytes.iter().map(|b| format!("{b:02X}")).collect()
  |                  ^^^
help: ... and use the `write!` macro here
 --> main.rs:4:26
  |
4 |     bytes.iter().map(|b| format!("{b:02X}")).collect()
  |                          ^^^^^^^^^^^^^^^^^^
  = note: this can be written more efficiently by appending to a `String` directly
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#format_collect
  = note: `-W clippy::format-collect` implied by `-W clippy::pedantic`

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?

This allocates a new string for every element in the iterator. This can be done more efficiently by creating the String once and appending to it in Iterator::fold, using either the write! macro which supports exactly the same syntax as the format! macro, or concatenating with + in case the iterator yields &str/String. Note also that write!-ing into a String can never fail, despite the return type of write! being std::fmt::Result, so it can be safely ignored or unwrapped.

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. Change the code by hand:

Flagged

fn hex_encode(bytes: &[u8]) -> String {
    bytes.iter().map(|b| format!("{b:02X}")).collect()
}

Write this instead

use std::fmt::Write;
fn hex_encode(bytes: &[u8]) -> String {
    bytes.iter().fold(String::new(), |mut output, b| {
        let _ = write!(output, "{b:02X}");
        output
    })
}

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_collect = "warn"

In code

#![warn(clippy::format_collect)]

One run

cargo clippy -- -W clippy::format-collect

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

1.73.0added in Rust

Clippy's own documentation

What it does

Checks for usage of .map(|_| format!(..)).collect::<String>().

Why is this bad?

This allocates a new string for every element in the iterator. This can be done more efficiently by creating the String once and appending to it in Iterator::fold, using either the write! macro which supports exactly the same syntax as the format! macro, or concatenating with + in case the iterator yields &str/String.

Note also that write!-ing into a String can never fail, despite the return type of write! being std::fmt::Result, so it can be safely ignored or unwrapped.

Example

fn hex_encode(bytes: &[u8]) -> String {
    bytes.iter().map(|b| format!("{b:02X}")).collect()
}

Use instead:

use std::fmt::Write;
fn hex_encode(bytes: &[u8]) -> String {
    bytes.iter().fold(String::new(), |mut output, b| {
        let _ = write!(output, "{b:02X}");
        output
    })
}

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

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