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

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index c25a7686bea..0c313ab1489 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)