Merge commit 'b52fb5234cd7c11ecfae51897a6f7fa52e8777fc' into clippyup

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diff --git a/src/docs/extend_with_drain.txt b/src/docs/extend_with_drain.txt
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+### What it does
+Checks for occurrences where one vector gets extended instead of append
+
+### Why is this bad?
+Using `append` instead of `extend` is more concise and faster
+
+### Example
+```
+let mut a = vec![1, 2, 3];
+let mut b = vec![4, 5, 6];
+
+a.extend(b.drain(..));
+```
+
+Use instead:
+```
+let mut a = vec![1, 2, 3];
+let mut b = vec![4, 5, 6];
+
+a.append(&mut b);
+```
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