clippy::result_filter_map: `filter` for `Ok` followed by `unwrap`

What you see

warning: `filter` for `Ok` followed by `unwrap`
 --> main.rs:3:47
  |
3 | let _ = std::iter::empty::<Result<i32, ()>>().filter(Result::is_ok).map(Result::unwrap);
  |                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `flatten` instead: `flatten()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#result_filter_map
  = note: `#[warn(clippy::result_filter_map)]` 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?

Result implements IntoIterator<Item = T>. This means that Result can be flattened automatically without suspicious-looking unwrap calls.

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 _ = std::iter::empty::<Result<i32, ()>>().filter(Result::is_ok).map(Result::unwrap);

Write this instead

let _ = std::iter::empty::<Result<i32, ()>>().flatten();

Turn it off, or on

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

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

[lints.clippy]
result_filter_map = "allow"

cargo clippy -- -A clippy::result-filter-map

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.