clippy::manual_flatten: unnecessary `if let` since only the `Some` variant of the iterator element is used

What you see

warning: unnecessary `if let` since only the `Some` variant of the iterator element is used
 --> main.rs:4:1
  |
4 | / for n in x {
5 | |     if let Some(n) = n {
6 | |         println!("{}", n);
7 | |     }
8 | | }
  | |_^
  |
help: try `.flatten()` and remove the `if let` statement in the for loop
 --> main.rs:5:5
  |
5 | /     if let Some(n) = n {
6 | |         println!("{}", n);
7 | |     }
  | |_____^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_flatten
  = note: `#[warn(clippy::manual_flatten)]` on by default
help: try
  |
4 ~ for n in x.into_iter().flatten() {
5 +     println!("{}", n);
6 + }
  |

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?

It is verbose and can be simplified by first calling the flatten method on the Iterator.

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 x = vec![Some(1), Some(2), Some(3)];
for n in x {
    if let Some(n) = n {
        println!("{}", n);
    }
}

Write this instead

let x = vec![Some(1), Some(2), Some(3)];
for n in x.into_iter().flatten() {
    println!("{}", n);
}

Turn it off, or on

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

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

[lints.clippy]
manual_flatten = "allow"

cargo clippy -- -A clippy::manual-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.

Settings in clippy.toml

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