Clippy lint · Performance · checked with Clippy 0.1.99

clippy::manual_try_fold: usage of `Iterator::fold` on a type that implements `Try`

What it meansChecks for usage of Iterator::fold with a type that implements Try. It is in the performance group and warns by default.

What you see

warning: usage of `Iterator::fold` on a type that implements `Try`
 --> main.rs:3:22
  |
3 | vec![1, 2, 3].iter().fold(Some(0i32), |sum, i| sum?.checked_add(*i));
  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `try_fold` instead: `try_fold(0i32, |sum, i| ...)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_try_fold
  = note: `#[warn(clippy::manual_try_fold)]` 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 code should use try_fold instead, which short-circuits on failure, thus opening the door for additional optimizations not possible with fold as rustc can guarantee the function is never called on None, Err, etc., alleviating otherwise necessary checks. It’s also slightly more idiomatic.

Code that could be written to run faster. 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

vec![1, 2, 3].iter().fold(Some(0i32), |sum, i| sum?.checked_add(*i));

Write this instead

vec![1, 2, 3].iter().try_fold(0i32, |sum, i| sum.checked_add(*i));

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
manual_try_fold = "allow"

One run

cargo clippy -- -A clippy::manual-try-fold

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Performancegroup

warndefault level

nofixed by cargo clippy --fix

1.72.0added in Rust

Clippy's own documentation

What it does

Checks for usage of Iterator::fold with a type that implements Try.

Why is this bad?

The code should use try_fold instead, which short-circuits on failure, thus opening the door for additional optimizations not possible with fold as rustc can guarantee the function is never called on None, Err, etc., alleviating otherwise necessary checks. It’s also slightly more idiomatic.

Known issues

This lint doesn’t take into account whether a function does something on the failure case, i.e., whether short-circuiting will affect behavior. Refactoring to try_fold is not desirable in those cases.

Example

vec![1, 2, 3].iter().fold(Some(0i32), |sum, i| sum?.checked_add(*i));

Use instead:

vec![1, 2, 3].iter().try_fold(0i32, |sum, i| sum.checked_add(*i));

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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