Rollup merge of #143520 - Stypox:enter_trace_span-closure, r=RalfJung

Fix perf regression caused by tracing

See rust-lang/rust#143334, this is another alternative that may be worth benchmarking as suggested in rust-lang/rust#143334 (comment).

r? ``@RalfJung``

Author: tgross35

src/machine.rs

@@ -1014,8 +1014,6 @@ impl<'tcx> Machine<'tcx> for MiriMachine<'tcx> {
 
     const PANIC_ON_ALLOC_FAIL: bool = false;
 
-    const TRACING_ENABLED: bool = cfg!(feature = "tracing");
-
     #[inline(always)]
     fn enforce_alignment(ecx: &MiriInterpCx<'tcx>) -> bool {
         ecx.machine.check_alignment != AlignmentCheck::None
@@ -1827,6 +1825,16 @@ impl<'tcx> Machine<'tcx> for MiriMachine<'tcx> {
         #[cfg(not(target_os = "linux"))]
         MiriAllocParams::Global
     }
+
+    fn enter_trace_span(span: impl FnOnce() -> tracing::Span) -> impl EnteredTraceSpan {
+        #[cfg(feature = "tracing")]
+        { span().entered() }
+        #[cfg(not(feature = "tracing"))]
+        {
+            let _ = span; // so we avoid the "unused variable" warning
+            ()
+        }
+    }
 }
 
 /// Trait for callbacks handling asynchronous machine operations.