6 files changed, 22 insertions, 12 deletions
diff --git a/compiler/rustc_macros/Cargo.toml b/compiler/rustc_macros/Cargo.toml
index 16c4a8500..17651ce95 100644
--- a/compiler/rustc_macros/Cargo.toml
+++ b/compiler/rustc_macros/Cargo.toml
@@ -8,7 +8,6 @@ proc-macro = true
 
 [dependencies]
 synstructure = "0.13.0"
-# FIXME(Nilstrieb): Updating this causes changes in the diagnostics output.
-syn = { version = "=2.0.8", features = ["full"] }
+syn = { version = "2.0.9", features = ["full"] }
 proc-macro2 = "1"
 quote = "1"
diff --git a/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs b/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs
index 2e6e84ad8..e9a5cd9de 100644
--- a/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs
+++ b/compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs
@@ -203,14 +203,18 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
                 if first && (nested.input.is_empty() || nested.input.peek(Token![,])) {
                     self.slug.set_once(path.clone(), path.span().unwrap());
                     first = false;
-                    return Ok(())
+                    return Ok(());
                 }
 
                 first = false;
 
                 let Ok(nested) = nested.value() else {
-                    span_err(nested.input.span().unwrap(), "diagnostic slug must be the first argument").emit();
-                    return Ok(())
+                    span_err(
+                        nested.input.span().unwrap(),
+                        "diagnostic slug must be the first argument",
+                    )
+                    .emit();
+                    return Ok(());
                 };
 
                 if path.is_ident("code") {
@@ -221,7 +225,9 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
                         #diag.code(rustc_errors::DiagnosticId::Error(#code.to_string()));
                     });
                 } else {
-                    span_err(path.span().unwrap(), "unknown argument").note("only the `code` parameter is valid after the slug").emit();
+                    span_err(path.span().unwrap(), "unknown argument")
+                        .note("only the `code` parameter is valid after the slug")
+                        .emit();
 
                     // consume the buffer so we don't have syntax errors from syn
                     let _ = nested.parse::<TokenStream>();
diff --git a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
index e8dc98691..877e97450 100644
--- a/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
+++ b/compiler/rustc_macros/src/diagnostics/subdiagnostic.rs
@@ -188,7 +188,9 @@ impl<'parent, 'a> SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
         let mut kind_slugs = vec![];
 
         for attr in self.variant.ast().attrs {
-            let Some(SubdiagnosticVariant { kind, slug, no_span }) = SubdiagnosticVariant::from_attr(attr, self)? else {
+            let Some(SubdiagnosticVariant { kind, slug, no_span }) =
+                SubdiagnosticVariant::from_attr(attr, self)?
+            else {
                 // Some attributes aren't errors - like documentation comments - but also aren't
                 // subdiagnostics.
                 continue;
diff --git a/compiler/rustc_macros/src/lib.rs b/compiler/rustc_macros/src/lib.rs
index 904f8eb57..f4593d0fe 100644
--- a/compiler/rustc_macros/src/lib.rs
+++ b/compiler/rustc_macros/src/lib.rs
@@ -7,6 +7,7 @@
 #![allow(rustc::default_hash_types)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![cfg_attr(not(bootstrap), allow(internal_features))]
 #![recursion_limit = "128"]
 
 use synstructure::decl_derive;
diff --git a/compiler/rustc_macros/src/newtype.rs b/compiler/rustc_macros/src/newtype.rs
index 415a89b0f..72b47de1a 100644
--- a/compiler/rustc_macros/src/newtype.rs
+++ b/compiler/rustc_macros/src/newtype.rs
@@ -36,7 +36,8 @@ impl Parse for Newtype {
                     false
                 }
                 "max" => {
-                    let Meta::NameValue(MetaNameValue { value: Expr::Lit(lit), .. }) = &attr.meta else {
+                    let Meta::NameValue(MetaNameValue { value: Expr::Lit(lit), .. }) = &attr.meta
+                    else {
                         panic!("#[max = NUMBER] attribute requires max value");
                     };
 
@@ -47,7 +48,8 @@ impl Parse for Newtype {
                     false
                 }
                 "debug_format" => {
-                    let Meta::NameValue(MetaNameValue { value: Expr::Lit(lit), .. }) = &attr.meta else {
+                    let Meta::NameValue(MetaNameValue { value: Expr::Lit(lit), .. }) = &attr.meta
+                    else {
                         panic!("#[debug_format = FMT] attribute requires a format");
                     };
 
diff --git a/compiler/rustc_macros/src/serialize.rs b/compiler/rustc_macros/src/serialize.rs
index 8d017d149..ba75517d7 100644
--- a/compiler/rustc_macros/src/serialize.rs
+++ b/compiler/rustc_macros/src/serialize.rs
@@ -43,7 +43,7 @@ fn decodable_body(
     let ty_name = s.ast().ident.to_string();
     let decode_body = match s.variants() {
         [] => {
-            let message = format!("`{}` has no variants to decode", ty_name);
+            let message = format!("`{ty_name}` has no variants to decode");
             quote! {
                 panic!(#message)
             }
@@ -59,14 +59,14 @@ fn decodable_body(
                 })
                 .collect();
             let message = format!(
-                "invalid enum variant tag while decoding `{}`, expected 0..{}",
+                "invalid enum variant tag while decoding `{}`, expected 0..{}, actual {{}}",
                 ty_name,
                 variants.len()
             );
             quote! {
                 match ::rustc_serialize::Decoder::read_usize(__decoder) {
                     #match_inner
-                    _ => panic!(#message),
+                    n => panic!(#message, n),
                 }
             }
         }