Implement manual flatten lint

tests/ui/for_loops_over_options_or_results.rs

@@ -1,31 +0,0 @@
-#![warn(clippy::for_loops_over_options_or_results)]
-
-fn main() {
-    let x = vec![Some(1), Some(2), Some(3)];
-    for n in x {
-        if let Some(n) = n {
-            println!("{}", n);
-        }
-    }
-
-    let y: Vec<Result<i32, i32>> = vec![];
-    for n in y.clone() {
-        if let Ok(n) = n {
-            println!("{}", n);
-        }
-    }
-
-    // This should not trigger the lint
-    for n in y.clone() {
-        if let Ok(n) = n {
-            println!("{}", n);
-        } else {
-            println!("Oops!");
-        }
-    }
-
-    // This should not trigger the lint
-    for n in vec![Some(1), Some(2), Some(3)].iter().flatten() {
-        println!("{}", n);
-    }
-}

tests/ui/manual_flatten.rs

@@ -0,0 +1,54 @@
+#![warn(clippy::manual_flatten)]
+
+fn main() {
+    let x = vec![Some(1), Some(2), Some(3)];
+    for n in x {
+        if let Some(n) = n {
+            println!("{}", n);
+        }
+    }
+
+    let y: Vec<Result<i32, i32>> = vec![];
+    for n in y.clone() {
+        if let Ok(n) = n {
+            println!("{}", n);
+        }
+    }
+
+    let z = vec![Some(1), Some(2), Some(3)];
+    let z = z.iter();
+    for n in z {
+        if let Some(n) = n {
+            println!("{}", n);
+        }
+    }
+
+    // Using the `None` variant should not trigger the lint
+    let z = vec![Some(1), Some(2), Some(3)];
+    for n in z {
+        if n.is_none() {
+            println!("Nada.");
+        }
+    }
+
+    // Using the `Err` variant should not trigger the lint
+    for n in y.clone() {
+        if let Err(e) = n {
+            println!("Oops: {}!", e);
+        }
+    }
+
+    // Having an else clause should not trigger the lint
+    for n in y.clone() {
+        if let Ok(n) = n {
+            println!("{}", n);
+        } else {
+            println!("Oops!");
+        }
+    }
+
+    // Using manual flatten should not trigger the lint
+    for n in vec![Some(1), Some(2), Some(3)].iter().flatten() {
+        println!("{}", n);
+    }
+}

tests/ui/manual_flatten.stderr

@@ -0,0 +1,51 @@
+error: Unnecessary `if let` since only the `Some` variant of the iterator element is used.
+  --> $DIR/manual_flatten.rs:5:5
+   |
+LL | /     for n in x {
+LL | |         if let Some(n) = n {
+LL | |             println!("{}", n);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = note: `-D clippy::manual-flatten` implied by `-D warnings`
+help: try: `x.into_iter().flatten()`
+  --> $DIR/manual_flatten.rs:5:14
+   |
+LL |     for n in x {
+   |              ^
+
+error: Unnecessary `if let` since only the `Ok` variant of the iterator element is used.
+  --> $DIR/manual_flatten.rs:12:5
+   |
+LL | /     for n in y.clone() {
+LL | |         if let Ok(n) = n {
+LL | |             println!("{}", n);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+help: try: `y.clone().into_iter().flatten()`
+  --> $DIR/manual_flatten.rs:12:14
+   |
+LL |     for n in y.clone() {
+   |              ^^^^^^^^^
+
+error: Unnecessary `if let` since only the `Some` variant of the iterator element is used.
+  --> $DIR/manual_flatten.rs:20:5
+   |
+LL | /     for n in z {
+LL | |         if let Some(n) = n {
+LL | |             println!("{}", n);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+help: try: `z.flatten()`
+  --> $DIR/manual_flatten.rs:20:14
+   |
+LL |     for n in z {
+   |              ^
+
+error: aborting due to 3 previous errors
+