Merge commit 'b52fb5234cd7c11ecfae51897a6f7fa52e8777fc' into clippyup

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diff --git a/src/docs/len_zero.txt b/src/docs/len_zero.txt
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+### What it does
+Checks for getting the length of something via `.len()`
+just to compare to zero, and suggests using `.is_empty()` where applicable.
+
+### Why is this bad?
+Some structures can answer `.is_empty()` much faster
+than calculating their length. So it is good to get into the habit of using
+`.is_empty()`, and having it is cheap.
+Besides, it makes the intent clearer than a manual comparison in some contexts.
+
+### Example
+```
+if x.len() == 0 {
+    ..
+}
+if y.len() != 0 {
+    ..
+}
+```
+instead use
+```
+if x.is_empty() {
+    ..
+}
+if !y.is_empty() {
+    ..
+}
+```
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