hygiene: Merge a tiny bit of the "share expansion definition data" PR

Author: petrochenkov

src/librustc/hir/lowering.rs

@@ -705,7 +705,6 @@ impl<'a> LoweringContext<'a> {
         allow_internal_unstable: Option<Lrc<[Symbol]>>,
     ) -> Span {
         span.fresh_expansion(ExpnInfo {
-            def_site: span,
             allow_internal_unstable,
             ..ExpnInfo::default(ExpnKind::Desugaring(reason), span, self.sess.edition())
         })

src/libsyntax_pos/hygiene.rs

@@ -603,7 +603,9 @@ pub struct ExpnInfo {
     pub call_site: Span,
 
     // --- The part specific to the macro/desugaring definition.
-    // --- FIXME: Share it between expansions with the same definition.
+    // --- It may be reasonable to share this part between expansions with the same definition,
+    // --- but such sharing is known to bring some minor inconveniences without also bringing
+    // --- noticeable perf improvements (PR #62898).
     /// The span of the macro definition (possibly dummy).
     /// This span serves only informational purpose and is not used for resolution.
     pub def_site: Span,