Rollup merge of #5846 - dima74:map_flatten.map_to_option, r=flip1995

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

tests/ui/map_flatten.fixed

@@ -5,6 +5,20 @@
 #![allow(clippy::map_identity)]
 
 fn main() {
+    // mapping to Option on Iterator
+    fn option_id(x: i8) -> Option<i8> {
+        Some(x)
+    }
+    let option_id_ref: fn(i8) -> Option<i8> = option_id;
+    let option_id_closure = |x| Some(x);
+    let _: Vec<_> = vec![5_i8; 6].into_iter().filter_map(option_id).collect();
+    let _: Vec<_> = vec![5_i8; 6].into_iter().filter_map(option_id_ref).collect();
+    let _: Vec<_> = vec![5_i8; 6].into_iter().filter_map(option_id_closure).collect();
+    let _: Vec<_> = vec![5_i8; 6].into_iter().filter_map(|x| x.checked_add(1)).collect();
+
+    // mapping to Iterator on Iterator
     let _: Vec<_> = vec![5_i8; 6].into_iter().flat_map(|x| 0..x).collect();
+
+    // mapping to Option on Option
     let _: Option<_> = (Some(Some(1))).and_then(|x| x);
 }

tests/ui/map_flatten.rs

@@ -5,6 +5,20 @@
 #![allow(clippy::map_identity)]
 
 fn main() {
+    // mapping to Option on Iterator
+    fn option_id(x: i8) -> Option<i8> {
+        Some(x)
+    }
+    let option_id_ref: fn(i8) -> Option<i8> = option_id;
+    let option_id_closure = |x| Some(x);
+    let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id).flatten().collect();
+    let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_ref).flatten().collect();
+    let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_closure).flatten().collect();
+    let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| x.checked_add(1)).flatten().collect();
+
+    // mapping to Iterator on Iterator
     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
+
+    // mapping to Option on Option
     let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
 }

tests/ui/map_flatten.stderr

@@ -1,16 +1,40 @@
-error: called `map(..).flatten()` on an `Iterator`. This is more succinctly expressed by calling `.flat_map(..)`
-  --> $DIR/map_flatten.rs:8:21
+error: called `map(..).flatten()` on an `Iterator`
+  --> $DIR/map_flatten.rs:14:46
    |
-LL |     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(|x| 0..x)`
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id).flatten().collect();
+   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(option_id)`
    |
    = note: `-D clippy::map-flatten` implied by `-D warnings`
 
-error: called `map(..).flatten()` on an `Option`. This is more succinctly expressed by calling `.and_then(..)`
-  --> $DIR/map_flatten.rs:9:24
+error: called `map(..).flatten()` on an `Iterator`
+  --> $DIR/map_flatten.rs:15:46
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_ref).flatten().collect();
+   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(option_id_ref)`
+
+error: called `map(..).flatten()` on an `Iterator`
+  --> $DIR/map_flatten.rs:16:46
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(option_id_closure).flatten().collect();
+   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(option_id_closure)`
+
+error: called `map(..).flatten()` on an `Iterator`
+  --> $DIR/map_flatten.rs:17:46
+   |
+LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| x.checked_add(1)).flatten().collect();
+   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `filter_map` instead: `.filter_map(|x| x.checked_add(1))`
+
+error: called `map(..).flatten()` on an `Iterator`
+  --> $DIR/map_flatten.rs:20:46
+   |
+LL |     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)`
+
+error: called `map(..).flatten()` on an `Option`
+  --> $DIR/map_flatten.rs:23:39
    |
 LL |     let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
-   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `and_then` instead: `(Some(Some(1))).and_then(|x| x)`
+   |                                       ^^^^^^^^^^^^^^^^^^^^^ help: try using `and_then` instead: `.and_then(|x| x)`
 
-error: aborting due to 2 previous errors
+error: aborting due to 6 previous errors