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+An unknown predicate was used inside the `cfg` attribute.
+
+Erroneous code example:
+
+```compile_fail,E0537
+#[cfg(unknown())] // error: invalid predicate `unknown`
+pub fn something() {}
+
+pub fn main() {}
+```
+
+The `cfg` attribute supports only three kinds of predicates:
+
+ * any
+ * all
+ * not
+
+Example:
+
+```
+#[cfg(not(target_os = "linux"))] // ok!
+pub fn something() {}
+
+pub fn main() {}
+```
+
+For more information about the `cfg` attribute, read the section on
+[Conditional Compilation][conditional-compilation] in the Reference.
+
+[conditional-compilation]: https://doc.rust-lang.org/reference/conditional-compilation.html