Documenting https://github.com/rust-lang-nursery/rust-clippy/issues/1439 until it gets fixed.
/// **Why is this bad?** Needlessly creating a closure adds code for no benefit
/// and gives the optimizer more work.
///
-/// **Known problems:** None.
+/// **Known problems:** If creating the closure inside the closure has a side-
+/// effect then moving the closure creation out will change when that side-
+/// effect runs.
+/// See https://github.com/rust-lang-nursery/rust-clippy/issues/1439 for more
+/// details.
///
/// **Example:**
/// ```rust