Merge commit 'b52fb5234cd7c11ecfae51897a6f7fa52e8777fc' into clippyup

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diff --git a/src/docs/missing_spin_loop.txt b/src/docs/missing_spin_loop.txt
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+### What it does
+Check for empty spin loops
+
+### Why is this bad?
+The loop body should have something like `thread::park()` or at least
+`std::hint::spin_loop()` to avoid needlessly burning cycles and conserve
+energy. Perhaps even better use an actual lock, if possible.
+
+### Known problems
+This lint doesn't currently trigger on `while let` or
+`loop { match .. { .. } }` loops, which would be considered idiomatic in
+combination with e.g. `AtomicBool::compare_exchange_weak`.
+
+### Example
+
+```
+use core::sync::atomic::{AtomicBool, Ordering};
+let b = AtomicBool::new(true);
+// give a ref to `b` to another thread,wait for it to become false
+while b.load(Ordering::Acquire) {};
+```
+Use instead:
+```
+while b.load(Ordering::Acquire) {
+    std::hint::spin_loop()
+}
+```
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