Port #[panic_handler] to the new attribute parsers

compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs

@@ -668,6 +668,15 @@ impl<S: Stage> NoArgsAttributeParser<S> for RustcHasIncoherentInherentImplsParse
     const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcHasIncoherentInherentImpls;
 }
 
+pub(crate) struct PanicHandlerParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for PanicHandlerParser {
+    const PATH: &[Symbol] = &[sym::panic_handler];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(ALL_TARGETS); // Targets are checked per lang item in `rustc_passes`
+    const CREATE: fn(Span) -> AttributeKind = |span| AttributeKind::Lang(LangItem::PanicImpl, span);
+}
+
 pub(crate) struct RustcHiddenTypeOfOpaquesParser;
 
 impl<S: Stage> NoArgsAttributeParser<S> for RustcHiddenTypeOfOpaquesParser {

compiler/rustc_attr_parsing/src/context.rs

@@ -254,6 +254,7 @@ attribute_parsers!(
         Single<WithoutArgs<NoMangleParser>>,
         Single<WithoutArgs<NoStdParser>>,
         Single<WithoutArgs<NonExhaustiveParser>>,
+        Single<WithoutArgs<PanicHandlerParser>>,
         Single<WithoutArgs<PanicRuntimeParser>>,
         Single<WithoutArgs<ParenSugarParser>>,
         Single<WithoutArgs<PassByValueParser>>,

compiler/rustc_passes/src/lang_items.rs

@@ -8,7 +8,7 @@
 //! * Functions called by the compiler itself.
 
 use rustc_ast as ast;
-use rustc_ast::fmvisit;
+use rustc_ast::visit;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::lang_items::GenericRequirement;