Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::map_flatten: called `map(..).flatten()` on `Iterator`

What it meansChecks for usage of _.map(_).flatten(_) on Iterator, Option, and Result. It is in the complexity group and warns by default.

What you see

warning: called `map(..).flatten()` on `Iterator`
 --> main.rs:4:12
  |
4 | vec.iter().map(|x| x.iter()).flatten();
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try replacing `map` with `flat_map` and remove the `.flatten()`: `flat_map(|x| x.iter())`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#map_flatten
  = note: `#[warn(clippy::map_flatten)]` 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?

Readability. This can be written more concisely with Iterator::flat_map, Option::and_then, and Result::and_then.

Code that does something simple in a complicated way. Warns by default.

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 vec = vec![vec![1]];
vec.iter().map(|x| x.iter()).flatten();

let opt = Some(5);
opt.map(|x| Some(x * 2)).flatten();

let res: Result<i32, String> = Ok(5);
res.map(|x| Ok(x * 2)).flatten();

Write this instead

let vec = vec![vec![1]];
vec.iter().flat_map(|x| x.iter());

let opt = Some(5);
opt.and_then(|x| Some(x * 2));

let res: Result<i32, String> = Ok(5);
res.and_then(|x| Ok(x * 2));

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
map_flatten = "allow"

One run

cargo clippy -- -A clippy::map-flatten

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.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.31.0added in Rust

Clippy's own documentation

What it does

Checks for usage of _.map(_).flatten(_) on Iterator, Option, and Result.

Why is this bad?

Readability. This can be written more concisely with Iterator::flat_map, Option::and_then, and Result::and_then.

Example

let vec = vec![vec![1]];
vec.iter().map(|x| x.iter()).flatten();

let opt = Some(5);
opt.map(|x| Some(x * 2)).flatten();

let res: Result<i32, String> = Ok(5);
res.map(|x| Ok(x * 2)).flatten();

Use instead:

let vec = vec![vec![1]];
vec.iter().flat_map(|x| x.iter());

let opt = Some(5);
opt.and_then(|x| Some(x * 2));

let res: Result<i32, String> = Ok(5);
res.and_then(|x| Ok(x * 2));

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

Up next

Also worth your time

Did this answer your question?