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diff --git a/src/tools/clippy/src/docs/manual_flatten.txt b/src/tools/clippy/src/docs/manual_flatten.txt
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+### What it does
+Check for unnecessary `if let` usage in a for loop
+where only the `Some` or `Ok` variant of the iterator element is used.
+
+### Why is this bad?
+It is verbose and can be simplified
+by first calling the `flatten` method on the `Iterator`.
+
+### Example
+
+```
+let x = vec![Some(1), Some(2), Some(3)];
+for n in x {
+    if let Some(n) = n {
+        println!("{}", n);
+    }
+}
+```
+Use instead:
+```
+let x = vec![Some(1), Some(2), Some(3)];
+for n in x.into_iter().flatten() {
+    println!("{}", n);
+}
+```
+\ No newline at end of file