Map flatten for option

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