From 326b44e4d371644e8d48469087d763085b6c7711 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Thu, 22 Feb 2024 11:53:40 +1100
Subject: [PATCH] Fix panic when compiling `Rocket`.

`Rustc::emit_diagnostic` reconstructs a diagnostic passed in from the
macro machinery. Currently it uses the type `DiagnosticBuilder<'_,
ErrorGuaranteed>`, which is incorrect, because the diagnostic might be a
warning. And if it is a warning, because of the `ErrorGuaranteed` we end
up calling into `emit_producing_error_guaranteed` and the assertion
within that function (correctly) fails because the level is not an error
level.

The fix is simple: change the type to `DiagnosticBuilder<'_, ()>`. Using
`()` works no matter what the diagnostic level is, and we don't need an
`ErrorGuaranteed` here.

The panic was reported in #120576.
---
 compiler/rustc_expand/src/proc_macro_server.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/rustc_expand/src/proc_macro_server.rs b/compiler/rustc_expand/src/proc_macro_server.rs
index b80ecbc9c65..62d0deb2d3a 100644
--- a/compiler/rustc_expand/src/proc_macro_server.rs
+++ b/compiler/rustc_expand/src/proc_macro_server.rs
@@ -510,7 +510,7 @@ fn literal_from_str(&mut self, s: &str) -> Result<Literal<Self::Span, Self::Symb
 
     fn emit_diagnostic(&mut self, diagnostic: Diagnostic<Self::Span>) {
         let message = rustc_errors::DiagnosticMessage::from(diagnostic.message);
-        let mut diag: DiagnosticBuilder<'_, rustc_errors::ErrorGuaranteed> =
+        let mut diag: DiagnosticBuilder<'_, ()> =
             DiagnosticBuilder::new(&self.sess().dcx, diagnostic.level.to_internal(), message);
         diag.span(MultiSpan::from_spans(diagnostic.spans));
         for child in diagnostic.children {