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+The `instruction_set` attribute was malformed.
+
+Erroneous code example:
+
+```compile_fail,E0778
+#![feature(isa_attribute)]
+
+#[instruction_set()] // error: expected one argument
+pub fn something() {}
+fn main() {}
+```
+
+The parenthesized `instruction_set` attribute requires the parameter to be
+specified:
+
+```
+#![feature(isa_attribute)]
+
+#[cfg_attr(target_arch="arm", instruction_set(arm::a32))]
+fn something() {}
+```
+
+or:
+
+```
+#![feature(isa_attribute)]
+
+#[cfg_attr(target_arch="arm", instruction_set(arm::t32))]
+fn something() {}
+```
+
+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