Add TRACING_ENABLED to Machine and add enter_trace_span!()

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -147,6 +147,12 @@ pub trait Machine<'tcx>: Sized {
     /// already been checked before.
     const ALL_CONSTS_ARE_PRECHECKED: bool = true;
 
+    /// Determines whether rustc_const_eval functions that make use of the [Machine] should make
+    /// tracing calls (to the `tracing` library). By default this is `false`, meaning the tracing
+    /// calls will supposedly be optimized out. This flag is set to `true` inside Miri, to allow
+    /// tracing the interpretation steps, among other things.
+    const TRACING_ENABLED: bool = false;
+
     /// Whether memory accesses should be alignment-checked.
     fn enforce_alignment(ecx: &InterpCx<'tcx, Self>) -> bool;
 

compiler/rustc_const_eval/src/interpret/util.rs

@@ -45,3 +45,22 @@ pub(crate) fn create_static_alloc<'tcx>(
     assert!(ecx.memory.alloc_map.insert(alloc_id, (MemoryKind::Stack, alloc)).is_none());
     interp_ok(ecx.ptr_to_mplace(Pointer::from(alloc_id).into(), layout))
 }
+
+/// This struct is needed to enforce `#[must_use]` on [tracing::span::EnteredSpan]
+/// while wrapping them in an `Option`.
+#[must_use]
+pub enum MaybeEnteredSpan {
+    Some(tracing::span::EnteredSpan),
+    None,
+}
+
+#[macro_export]
+macro_rules! enter_trace_span {
+    ($machine:ident, $($tt:tt)*) => {
+        if $machine::TRACING_ENABLED {
+            $crate::interpret::tracing_utils::MaybeEnteredSpan::Some(tracing::info_span!($($tt)*).entered())
+        } else {
+            $crate::interpret::tracing_utils::MaybeEnteredSpan::None
+        }
+    }
+}