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+#### Note: this error code is no longer emitted by the compiler.
+
+Assignments 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 assignments
+were allowed:
+
+```compile_fail,E0594
+match Some(()) {
+    None => { },
+    option if { option = None; false } => { },
+    Some(_) => { } // When the previous match failed, the option became `None`.
+}
+```