syntax/rustc: move mark_span_with_reason back.

src/libsyntax/source_map.rs

@@ -935,27 +935,6 @@ impl SourceMap {
 
         None
     }
-
-    /// Reuses the span but adds information like the kind of the desugaring and features that are
-    /// allowed inside this span.
-    pub fn mark_span_with_reason(
-        &self,
-        reason: hygiene::CompilerDesugaringKind,
-        span: Span,
-        allow_internal_unstable: Option<Lrc<[symbol::Symbol]>>,
-    ) -> Span {
-        let mark = Mark::fresh(Mark::root());
-        mark.set_expn_info(ExpnInfo {
-            call_site: span,
-            def_site: Some(span),
-            format: CompilerDesugaring(reason),
-            allow_internal_unstable,
-            allow_internal_unsafe: false,
-            local_inner_macros: false,
-            edition: edition::Edition::from_session(),
-        });
-        span.with_ctxt(SyntaxContext::empty().apply_mark(mark))
-    }
 }
 
 impl SourceMapper for SourceMap {