Auto merge of rust-lang#5473 - Toxyxer:map-flatten-for-option, r=flip1995

Map flatten for option

Fixes rust-lang#5175

changelog:
- Trigger the map_flatten lint when map is called on an option
- Add test for such case

Author: bors

clippy_lints/src/methods/mod.rs

@@ -2484,7 +2484,7 @@ fn lint_ok_expect(cx: &LateContext<'_, '_>, expr: &hir::Expr<'_>, ok_args: &[hir
     }
 }
 
-/// lint use of `map().flatten()` for `Iterators`
+/// lint use of `map().flatten()` for `Iterators` and 'Options'
 fn lint_map_flatten<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr<'_>, map_args: &'tcx [hir::Expr<'_>]) {
     // lint if caller of `.map().flatten()` is an Iterator
     if match_trait_method(cx, expr, &paths::ITERATOR) {
@@ -2503,6 +2503,24 @@ fn lint_map_flatten<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, expr: &'tcx hir::Expr<
             Applicability::MachineApplicable,
         );
     }
+
+    // lint if caller of `.map().flatten()` is an Option
+    if match_type(cx, cx.tables.expr_ty(&map_args[0]), &paths::OPTION) {
+        let msg = "called `map(..).flatten()` on an `Option`. \
+                    This is more succinctly expressed by calling `.and_then(..)`";
+        let self_snippet = snippet(cx, map_args[0].span, "..");
+        let func_snippet = snippet(cx, map_args[1].span, "..");
+        let hint = format!("{0}.and_then({1})", self_snippet, func_snippet);
+        span_lint_and_sugg(
+            cx,
+            MAP_FLATTEN,
+            expr.span,
+            msg,
+            "try using `and_then` instead",
+            hint,
+            Applicability::MachineApplicable,
+        );
+    }
 }
 
 /// lint use of `map().unwrap_or_else()` for `Option`s and `Result`s

tests/ui/map_flatten.fixed

@@ -5,4 +5,5 @@
 
 fn main() {
     let _: Vec<_> = vec![5_i8; 6].into_iter().flat_map(|x| 0..x).collect();
+    let _: Option<_> = (Some(Some(1))).and_then(|x| x);
 }

tests/ui/map_flatten.rs

@@ -5,4 +5,5 @@
 
 fn main() {
     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
+    let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
 }

tests/ui/map_flatten.stderr

@@ -6,5 +6,11 @@ LL |     let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().colle
    |
    = note: `-D clippy::map-flatten` implied by `-D warnings`
 
-error: aborting due to previous error
+error: called `map(..).flatten()` on an `Option`. This is more succinctly expressed by calling `.and_then(..)`
+  --> $DIR/map_flatten.rs:8:24
+   |
+LL |     let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
+   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `and_then` instead: `(Some(Some(1))).and_then(|x| x)`
+
+error: aborting due to 2 previous errors