Auto merge of #48173 - GuillaumeGomez:error-codes-libsyntax_ext, r=estebank

Add error codes for libsyntax_ext

I intend to add error codes for `libsyntax_ext` as well. However, they cannot be used at stage 0 directly so I thought it might be possible to enable them at the stage 1 only so we can have access to the macros. However, the error code registration seems to not work this way. Currently I get the following error:

```
error: used diagnostic code E0660 not registered
  --> libsyntax_ext/asm.rs:93:25
   |
93 |                         span_err!(cx, sp, E0660, "malformed inline assembly");
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: used diagnostic code E0661 not registered
   --> libsyntax_ext/asm.rs:151:33
    |
151 | /                                 span_err!(cx, sp, E0661,
152 | |                                           "output operand constraint lacks '=' or '+'");
    | |________________________________________________________________________________________^
    |
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to 2 previous errors

error: Could not compile `syntax_ext`.
```

If anyone has an idea, I'd gladly take it. I'm trying to figure this out on my side as well. I also opened this PR to know if it was worth it to continue (maybe we don't want this?).

Anyway, any answer for both questions is very welcome!

cc @rust-lang/compiler

3 files changed, 64 insertions, 2 deletions

diff --git a/src/libsyntax_ext/asm.rs b/src/libsyntax_ext/asm.rs
index d1de4dccd00..e1eabc5cb01 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 00000000000..a840c0392e9
--- /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 97e34c554d1..39ad594e5c5 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;