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diff --git a/compiler/rustc_error_codes/src/error_codes/E0301.md b/compiler/rustc_error_codes/src/error_codes/E0301.md
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+#### Note: this error code is no longer emitted by the compiler.
+
+Mutable borrows are not allowed in pattern guards, because matching cannot have
+side effects. Side effects could alter the matched object or the environment
+on which the match depends in such a way, that the match would not be
+exhaustive. For instance, the following would not match any arm if mutable
+borrows were allowed:
+
+```compile_fail,E0596
+match Some(()) {
+    None => { },
+    option if option.take().is_none() => {
+        /* impossible, option is `Some` */
+    },
+    Some(_) => { } // When the previous match failed, the option became `None`.
+}
+```