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+An unknown argument was given to the `instruction_set` attribute.
+
+Erroneous code example:
+
+```compile_fail,E0779
+#![feature(isa_attribute)]
+
+#[instruction_set(intel::x64)] // error: invalid argument
+pub fn something() {}
+fn main() {}
+```
+
+The `instruction_set` attribute only supports two arguments currently:
+
+ * arm::a32
+ * arm::t32
+
+All other arguments given to the `instruction_set` attribute will return this
+error. Example:
+
+```
+#![feature(isa_attribute)]
+
+#[cfg_attr(target_arch="arm", instruction_set(arm::a32))] // ok!
+pub fn something() {}
+fn main() {}
+```
+
+For more information see the [`instruction_set` attribute][isa-attribute]
+section of the Reference.
+
+[isa-attribute]: https://doc.rust-lang.org/reference/attributes/codegen.html