clippy::lines_filter_map_ok: Checks for usage of lines.filter_map(Result::ok) or lines.flat_map(Result::ok) when lines has type std::io::Lines

let mut lines = BufReader::new(File::open("some-path")?).lines().filter_map(Result::ok);
// If "some-path" points to a directory, the next statement never terminates:
let first_line: Option<String> = lines.next();

let mut lines = BufReader::new(File::open("some-path")?).lines().map_while(Result::ok);
let first_line: Option<String> = lines.next();

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

[lints.clippy]
lines_filter_map_ok = "allow"

cargo clippy -- -A clippy::lines-filter-map-ok

Clippy's own documentation

What it does

Checks for usage of lines.filter_map(Result::ok) or lines.flat_map(Result::ok) when lines has type std::io::Lines.

Why is this bad?

Lines instances might produce a never-ending stream of Err, in which case filter_map(Result::ok) will enter an infinite loop while waiting for an Ok variant. Calling next() once is sufficient to enter the infinite loop, even in the absence of explicit loops in the user code.

This situation can arise when working with user-provided paths. On some platforms, std::fs::File::open(path) might return Ok(fs) even when path is a directory, but any later attempt to read from fs will return an error.

Known problems

This lint suggests replacing filter_map() or flat_map() applied to a Lines instance in all cases. There are two cases where the suggestion might not be appropriate or necessary:

• If the Lines instance can never produce any error, or if an error is produced only once just before terminating the iterator, using map_while() is not necessary but will not do any harm.
• If the Lines instance can produce intermittent errors then recover and produce successful results, using map_while() would stop at the first error.

Example

let mut lines = BufReader::new(File::open("some-path")?).lines().filter_map(Result::ok);
// If "some-path" points to a directory, the next statement never terminates:
let first_line: Option<String> = lines.next();

Use instead:

let mut lines = BufReader::new(File::open("some-path")?).lines().map_while(Result::ok);
let first_line: Option<String> = lines.next();

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.

Related lints

• iter_out_of_boundsthis `.take()` call takes more items than the iterator will produce
• join_absolute_pathsChecks for calls to Path::join that start with a path separator (\\ or /)
• let_underscore_futurenon-binding `let` on a future
• macro_metavars_in_unsafeLooks for macros that expand metavariables in an unsafe block
• manual_unwrap_or_defaultmatch can be simplified with `.unwrap_or_default()`
• mismatched_bit_width_typepossible buggy implementation of `bit_width`

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker