diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index c25a7686bead0..0c313ab14899f 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -14,7 +14,7 @@ use ast::{self, Attribute, Name, PatKind, MetaItem};
 use attr::HasAttrs;
 use codemap::{self, CodeMap, Spanned, respan};
 use syntax_pos::{Span, MultiSpan, DUMMY_SP};
-use errors::DiagnosticBuilder;
+use errors::{DiagnosticBuilder, DiagnosticId};
 use ext::expand::{self, Expansion, Invocation};
 use ext::hygiene::{Mark, SyntaxContext};
 use fold::{self, Folder};
@@ -841,6 +841,9 @@ impl<'a> ExtCtxt<'a> {
     pub fn span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str) {
         self.parse_sess.span_diagnostic.span_err(sp, msg);
     }
+    pub fn span_err_with_code<S: Into<MultiSpan>>(&self, sp: S, msg: &str, code: DiagnosticId) {
+        self.parse_sess.span_diagnostic.span_err_with_code(sp, msg, code);
+    }
     pub fn mut_span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str)
                         -> DiagnosticBuilder<'a> {
         self.parse_sess.span_diagnostic.mut_span_err(sp, msg)
diff --git a/src/libsyntax_ext/asm.rs b/src/libsyntax_ext/asm.rs
index d1de4dccd0043..e1eabc5cb0145 100644
--- a/src/libsyntax_ext/asm.rs
+++ b/src/libsyntax_ext/asm.rs
@@ -45,6 +45,17 @@ impl State {
     }
 }
 
+macro_rules! span_err_if_not_stage0 {
+    ($cx:expr, $sp:expr, $code:ident, $text:tt) => {
+        #[cfg(not(stage0))] {
+            span_err!($cx, $sp, $code, $text)
+        }
+        #[cfg(stage0)] {
+            $cx.span_err($sp, $text)
+        }
+    }
+}
+
 const OPTIONS: &'static [&'static str] = &["volatile", "alignstack", "intel"];
 
 pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt,
@@ -89,7 +100,7 @@ pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt,
                 if asm_str_style.is_some() {
                     // If we already have a string with instructions,
                     // ending up in Asm state again is an error.
-                    cx.span_err(sp, "malformed inline assembly");
+                    span_err_if_not_stage0!(cx, sp, E0660, "malformed inline assembly");
                     return DummyResult::expr(sp);
                 }
                 // Nested parser, stop before the first colon (see above).
@@ -142,7 +153,8 @@ pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt,
                             Some(Symbol::intern(&format!("={}", ch.as_str())))
                         }
                         _ => {
-                            cx.span_err(span, "output operand constraint lacks '=' or '+'");
+                            span_err_if_not_stage0!(cx, span, E0661,
+                                                    "output operand constraint lacks '=' or '+'");
                             None
                         }
                     };
diff --git a/src/libsyntax_ext/diagnostics.rs b/src/libsyntax_ext/diagnostics.rs
new file mode 100644
index 0000000000000..a840c0392e9fe
--- /dev/null
+++ b/src/libsyntax_ext/diagnostics.rs
@@ -0,0 +1,45 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(non_snake_case)]
+
+// Error messages for EXXXX errors.
+// Each message should start and end with a new line, and be wrapped to 80 characters.
+// In vim you can `:set tw=80` and use `gq` to wrap paragraphs. Use `:set tw=0` to disable.
+register_long_diagnostics! {
+E0660: r##"
+The argument to the `asm` macro is not well-formed.
+
+Erroneous code example:
+
+```compile_fail,E0660
+asm!("nop" "nop");
+```
+
+Considering that this would be a long explanation, we instead recommend you to
+take a look at the unstable book:
+https://doc.rust-lang.org/unstable-book/language-features/asm.html
+"##,
+
+E0661: r##"
+An invalid syntax was passed to the second argument of an `asm` macro line.
+
+Erroneous code example:
+
+```compile_fail,E0661
+let a;
+asm!("nop" : "r"(a));
+```
+
+Considering that this would be a long explanation, we instead recommend you to
+take a look at the unstable book:
+https://doc.rust-lang.org/unstable-book/language-features/asm.html
+"##,
+}
diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index 97e34c554d116..39ad594e5c577 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -18,6 +18,8 @@
 #![feature(decl_macro)]
 #![feature(str_escape)]
 
+#![cfg_attr(not(stage0), feature(rustc_diagnostic_macros))]
+
 extern crate fmt_macros;
 #[macro_use]
 extern crate syntax;
@@ -26,6 +28,9 @@ extern crate proc_macro;
 extern crate rustc_data_structures;
 extern crate rustc_errors as errors;
 
+#[cfg(not(stage0))]
+mod diagnostics;
+
 mod assert;
 mod asm;
 mod cfg;
diff --git a/src/test/compile-fail/issue-21045.rs b/src/test/ui/E0660.rs
similarity index 79%
rename from src/test/compile-fail/issue-21045.rs
rename to src/test/ui/E0660.rs
index 134240f8c8aa3..82ef38e96cded 100644
--- a/src/test/compile-fail/issue-21045.rs
+++ b/src/test/ui/E0660.rs
@@ -7,10 +7,15 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
+
+// ignore-stage1
+
 #![feature(asm)]
 
 fn main() {
     let a;
-    asm!("nop" "nop"); //~ ERROR malformed inline assembly
-    asm!("nop" "nop" : "=r"(a)); //~ ERROR malformed inline assembly
+    asm!("nop" "nop");
+    //~^ ERROR E0660
+    asm!("nop" "nop" : "=r"(a));
+    //~^ ERROR E0660
 }
diff --git a/src/test/ui/E0660.stderr b/src/test/ui/E0660.stderr
new file mode 100644
index 0000000000000..fcf3e9a255273
--- /dev/null
+++ b/src/test/ui/E0660.stderr
@@ -0,0 +1,15 @@
+error[E0660]: malformed inline assembly
+  --> $DIR/E0660.rs:17:5
+   |
+LL |     asm!("nop" "nop");
+   |     ^^^^^^^^^^^^^^^^^^
+
+error[E0660]: malformed inline assembly
+  --> $DIR/E0660.rs:19:5
+   |
+LL |     asm!("nop" "nop" : "=r"(a));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0660`.
diff --git a/src/test/ui/E0661.rs b/src/test/ui/E0661.rs
new file mode 100644
index 0000000000000..7f8a0d8b57420
--- /dev/null
+++ b/src/test/ui/E0661.rs
@@ -0,0 +1,19 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-stage1
+
+#![feature(asm)]
+
+fn main() {
+    let a;
+    asm!("nop" : "r"(a));
+    //~^ ERROR E0661
+}
diff --git a/src/test/ui/E0661.stderr b/src/test/ui/E0661.stderr
new file mode 100644
index 0000000000000..d8b974d424074
--- /dev/null
+++ b/src/test/ui/E0661.stderr
@@ -0,0 +1,9 @@
+error[E0661]: output operand constraint lacks '=' or '+'
+  --> $DIR/E0661.rs:17:18
+   |
+LL |     asm!("nop" : "r"(a));
+   |                  ^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0661`.