Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::flat_map_option: used `flat_map` where `filter_map` could be used instead

What it meansChecks for usage of Iterator::flat_map() where filter_map() could be used instead. It is in the pedantic group and off by default.

What you see

warning: used `flat_map` where `filter_map` could be used instead
 --> main.rs:3:49
  |
3 | let nums: Vec<i32> = ["1", "2", "whee!"].iter().flat_map(|x| x.parse().ok()).collect();
  |                                                 ^^^^^^^^ help: try: `filter_map`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#flat_map_option
  = note: `-W clippy::flat-map-option` 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?

filter_map() is known to always produce 0 or 1 output items per input item, rather than however many the inner iterator type produces. Therefore, it maintains the upper bound in Iterator::size_hint(), and communicates to the reader that the input items are not being expanded into multiple output items without their having to notice that the mapping function returns an Option.

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

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 nums: Vec<i32> = ["1", "2", "whee!"].iter().flat_map(|x| x.parse().ok()).collect();

Write this instead

let nums: Vec<i32> = ["1", "2", "whee!"].iter().filter_map(|x| x.parse().ok()).collect();

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]
flat_map_option = "warn"

In code

#![warn(clippy::flat_map_option)]

One run

cargo clippy -- -W clippy::flat-map-option

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

yesfixed by cargo clippy --fix

1.53.0added in Rust

Clippy's own documentation

What it does

Checks for usage of Iterator::flat_map() where filter_map() could be used instead.

Why is this bad?

filter_map() is known to always produce 0 or 1 output items per input item, rather than however many the inner iterator type produces. Therefore, it maintains the upper bound in Iterator::size_hint(), and communicates to the reader that the input items are not being expanded into multiple output items without their having to notice that the mapping function returns an Option.

Example

let nums: Vec<i32> = ["1", "2", "whee!"].iter().flat_map(|x| x.parse().ok()).collect();

Use instead:

let nums: Vec<i32> = ["1", "2", "whee!"].iter().filter_map(|x| x.parse().ok()).collect();

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

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