Lint for chaining flatten after map

This change adds a lint to check for instances of `map(..).flatten()`
that can be trivially shortened to `flat_map(..)`

Closes #3196

2 files changed, 17 insertions, 0 deletions

diff --git a/tests/ui/map_flatten.rs b/tests/ui/map_flatten.rs
new file mode 100644
index 00000000000..c5cf24d9bb0
--- /dev/null
+++ b/tests/ui/map_flatten.rs
@@ -0,0 +1,7 @@
+#![feature(tool_lints)]
+#![warn(clippy::all, clippy::pedantic)]
+#![allow(clippy::missing_docs_in_private_items)]
+
+fn main() {
+    let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
+}
diff --git a/tests/ui/map_flatten.stderr b/tests/ui/map_flatten.stderr
new file mode 100644
index 00000000000..d4ce44490d1
--- /dev/null
+++ b/tests/ui/map_flatten.stderr
@@ -0,0 +1,10 @@
+error: called `map(..).flatten()` on an `Iterator`. This is more succinctly expressed by calling `.flat_map(..)`
+ --> $DIR/map_flatten.rs:6:21
+  |
+6 |     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)`
+  |
+  = note: `-D clippy::map-flatten` implied by `-D warnings`
+
+error: aborting due to previous error
+