Merge commit 'b52fb5234cd7c11ecfae51897a6f7fa52e8777fc' into clippyup

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diff --git a/src/docs/assign_op_pattern.txt b/src/docs/assign_op_pattern.txt
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+### What it does
+Checks for `a = a op b` or `a = b commutative_op a`
+patterns.
+
+### Why is this bad?
+These can be written as the shorter `a op= b`.
+
+### Known problems
+While forbidden by the spec, `OpAssign` traits may have
+implementations that differ from the regular `Op` impl.
+
+### Example
+```
+let mut a = 5;
+let b = 0;
+// ...
+
+a = a + b;
+```
+
+Use instead:
+```
+let mut a = 5;
+let b = 0;
+// ...
+
+a += b;
+```
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