Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::filter_map_bool_then: Checks for usage of bool::then in Iterator::filter_map
What it meansChecks for usage of bool::then in Iterator::filter_map. It is in the style group and warns by default.
Does it matter?
This can be written with filter then map instead, which would reduce nesting and separates the filtering from the transformation phase. This comes with no cost to performance and is just cleaner.
Code that works but is not idiomatic Rust. 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
_ = v.into_iter().filter_map(|i| (i % 2 == 0).then(|| really_expensive_fn(i)));
Write this instead
_ = v.into_iter().filter(|i| i % 2 == 0).map(|i| really_expensive_fn(i));
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::filter_map_bool_then)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
filter_map_bool_then = "allow"One run
cargo clippy -- -A clippy::filter-map-bool-thenThe [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.
Clippy's own documentation
What it does
Checks for usage of bool::then in Iterator::filter_map.
Why is this bad?
This can be written with filter then map instead, which would reduce nesting and
separates the filtering from the transformation phase. This comes with no cost to
performance and is just cleaner.
Limitations
Does not lint bool::then_some, as it eagerly evaluates its arguments rather than lazily.
This can create differing behavior, so better safe than sorry.
Example
_ = v.into_iter().filter_map(|i| (i % 2 == 0).then(|| really_expensive_fn(i)));
Use instead:
_ = v.into_iter().filter(|i| i % 2 == 0).map(|i| really_expensive_fn(i));
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
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All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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