Restore error

Author: chansuke

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -380,10 +380,38 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                                         unreachable!()
                                     }
                                 }
-                                _ => None,
+                                _ => {
+                                    struct_span_code_err!(
+                                        tcx.dcx(),
+                                        attr.span,
+                                        E0779,
+                                        "invalid instruction set specified",
+                                    )
+                                    .emit();
+                                    None
+                                }
                             }
                         }
-                        _ => None,
+                        [] => {
+                            struct_span_code_err!(
+                                tcx.dcx(),
+                                attr.span,
+                                E0778,
+                                "`#[instruction_set]` requires an argument"
+                            )
+                            .emit();
+                            None
+                        }
+                        _ => {
+                            struct_span_code_err!(
+                                tcx.dcx(),
+                                attr.span,
+                                E0779,
+                                "cannot specify more than one instruction set"
+                            )
+                            .emit();
+                            None
+                        }
                     })
             }
             sym::repr => {