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Handle mapping to Option in `map_flatten` lint Fixes #4496 The existing [`map_flatten`](https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten) lint suggests changing `expr.map(...).flatten()` to `expr.flat_map(...)` when `expr` is `Iterator`. This PR changes suggestion to `filter_map` instead of `flat_map` when mapping to `Option`, because it is more natural Also here are some questions: * If expression has type which implements `Iterator` trait (`match_trait_method(cx, expr, &paths::ITERATOR) == true`), how can I get type of iterator elements? Currently I use return type of closure inside `map`, but probably it is not good way * I would like to change suggestion range to cover only `.map(...).flatten()`, that is from: ``` let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect(); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `flat_map` instead: `vec![5_i8; 6].into_iter().flat_map ``` to ``` let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect(); ^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `flat_map` instead: `.flat_map(|x| 0..x)` ``` Is it ok? * Is `map_flatten` lint intentionally in `pedantic` category, or could it be moved to `complexity`? changelog: Handle mapping to Option in [`map_flatten`](https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten) lint |
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