expand: Do not overwrite existing ExpnInfo when injecting derive markers

Create a fresh expansion for them instead - this is the usual way to allow unstable features for generated/desugared code.
Fixes rust-lang#52363

Author: petrochenkov

src/libsyntax/ext/derive.rs

@@ -8,7 +8,7 @@ use crate::symbol::{Symbol, sym};
 use crate::errors::Applicability;
 
 use syntax_pos::Span;
-
+use syntax_pos::hygiene::{Mark, SyntaxContext};
 use rustc_data_structures::fx::FxHashSet;
 
 pub fn collect_derives(cx: &mut ExtCtxt<'_>, attrs: &mut Vec<ast::Attribute>) -> Vec<ast::Path> {
@@ -55,12 +55,13 @@ pub fn add_derived_markers<T>(cx: &mut ExtCtxt<'_>, span: Span, traits: &[ast::P
         names.insert(unwrap_or!(path.segments.get(0), continue).ident.name);
     }
 
-    cx.current_expansion.mark.set_expn_info(ExpnInfo::with_unstable(
+    let mark = Mark::fresh(cx.current_expansion.mark);
+    mark.set_expn_info(ExpnInfo::with_unstable(
         ExpnKind::Macro(MacroKind::Derive, Symbol::intern(&pretty_name)), span,
         cx.parse_sess.edition, &[sym::rustc_attrs, sym::structural_match],
     ));
 
-    let span = span.with_ctxt(cx.backtrace());
+    let span = span.with_ctxt(SyntaxContext::empty().apply_mark(mark));
     item.visit_attrs(|attrs| {
         if names.contains(&sym::Eq) && names.contains(&sym::PartialEq) {
             let meta = cx.meta_word(span, sym::structural_match);

src/libsyntax_pos/hygiene.rs

@@ -117,7 +117,11 @@ impl Mark {
 
     #[inline]
     pub fn set_expn_info(self, info: ExpnInfo) {
-        HygieneData::with(|data| data.marks[self.0 as usize].expn_info = Some(info))
+        HygieneData::with(|data| {
+            let old_info = &mut data.marks[self.0 as usize].expn_info;
+            assert!(old_info.is_none(), "expansion info is reset for a mark");
+            *old_info = Some(info);
+        })
     }
 
     pub fn is_descendant_of(self, ancestor: Mark) -> bool {