expand: Remove obsolete ExpansionConfig::keep_macs

Maybe it was used before the introduction of placeholders, but now it has no effect.

compiler/rustc_expand/src/expand.rs

@@ -1615,7 +1615,6 @@ pub struct ExpansionConfig<'feat> {
     pub recursion_limit: Limit,
     pub trace_mac: bool,
     pub should_test: bool,          // If false, strip `#[test]` nodes
-    pub keep_macs: bool,
     pub span_debug: bool,           // If true, use verbose debugging for `proc_macro::Span`
     pub proc_macro_backtrace: bool, // If true, show backtraces for proc-macro panics
 }
@@ -1628,7 +1627,6 @@ impl<'feat> ExpansionConfig<'feat> {
             recursion_limit: Limit::new(1024),
             trace_mac: false,
             should_test: false,
-            keep_macs: false,
             span_debug: false,
             proc_macro_backtrace: false,
         }

compiler/rustc_expand/src/placeholders.rs

@@ -371,12 +371,4 @@ impl<'a, 'b> MutVisitor for PlaceholderExpander<'a, 'b> {
             }
         }
     }
-
-    fn visit_mod(&mut self, module: &mut ast::Mod) {
-        noop_visit_mod(module, self);
-        // remove macro definitions
-        module.items.retain(
-            |item| !matches!(item.kind, ast::ItemKind::MacCall(_) if !self.cx.ecfg.keep_macs),
-        );
-    }
 }