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diff --git a/compiler/rustc_error_codes/src/error_codes/E0054.md b/compiler/rustc_error_codes/src/error_codes/E0054.md
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+It is not allowed to cast to a bool.
+
+Erroneous code example:
+
+```compile_fail,E0054
+let x = 5;
+
+// Not allowed, won't compile
+let x_is_nonzero = x as bool;
+```
+
+If you are trying to cast a numeric type to a bool, you can compare it with
+zero instead:
+
+```
+let x = 5;
+
+// Ok
+let x_is_nonzero = x != 0;
+```