Add enter_trace_span! that checks #[cfg("tracing")]

Includes a custom syntax shortand to enter_trace_span! with NAME::SUBNAME.

MaybeEnteredTraceSpan is `pub use`d in lib.rs to make it available also in bin/, just in case.

src/tools/miri/src/helpers.rs

@@ -1431,3 +1431,44 @@ impl ToU64 for usize {
         self.try_into().unwrap()
     }
 }
+
+/// This struct is needed to enforce `#[must_use]` on values produced by [enter_trace_span] even
+/// when the "tracing" feature is not enabled.
+#[must_use]
+pub struct MaybeEnteredTraceSpan {
+    #[cfg(feature = "tracing")]
+    pub _entered_span: tracing::span::EnteredSpan,
+}
+
+/// Enters a [tracing::info_span] only if the "tracing" feature is enabled, otherwise does nothing.
+/// This is like [rustc_const_eval::enter_trace_span] except that it does not depend on the
+/// [Machine] trait to check if tracing is enabled, because from the Miri codebase we can directly
+/// check whether the "tracing" feature is enabled, unlike from the rustc_const_eval codebase.
+///
+/// In addition to the syntax accepted by [tracing::span!], this macro optionally allows passing
+/// the span name (i.e. the first macro argument) in the form `NAME::SUBNAME` (without quotes) to
+/// indicate that the span has name "NAME" (usually the name of the component) and has an additional
+/// more specific name "SUBNAME" (usually the function name). The latter is passed to the [tracing]
+/// infrastructure as a span field with the name "NAME". This allows not being distracted by
+/// subnames when looking at the trace in <https://ui.perfetto.dev>, but when deeper introspection
+/// is needed within a component, it's still possible to view the subnames directly in the UI by
+/// selecting a span, clicking on the "NAME" argument on the right, and clicking on "Visualize
+/// argument values".
+/// ```rust
+/// // for example, the first will expand to the second
+/// enter_trace_span!(borrow_tracker::on_stack_pop, /* ... */)
+/// enter_trace_span!("borrow_tracker", borrow_tracker = "on_stack_pop", /* ... */)
+/// ```
+#[macro_export]
+macro_rules! enter_trace_span {
+    ($name:ident :: $subname:ident $($tt:tt)*) => {{
+        enter_trace_span!(stringify!($name), $name = %stringify!(subname) $($tt)*)
+    }};
+
+    ($($tt:tt)*) => {
+        $crate::MaybeEnteredTraceSpan {
+            #[cfg(feature = "tracing")]
+            _entered_span: tracing::info_span!($($tt)*).entered()
+        }
+    };
+}

src/tools/miri/src/lib.rs

@@ -139,7 +139,9 @@ pub use crate::eval::{
     AlignmentCheck, BacktraceStyle, IsolatedOp, MiriConfig, MiriEntryFnType, RejectOpWith,
     ValidationMode, create_ecx, eval_entry,
 };
-pub use crate::helpers::{AccessKind, EvalContextExt as _, ToU64 as _, ToUsize as _};
+pub use crate::helpers::{
+    AccessKind, EvalContextExt as _, MaybeEnteredTraceSpan, ToU64 as _, ToUsize as _,
+};
 pub use crate::intrinsics::EvalContextExt as _;
 pub use crate::machine::{
     AllocExtra, DynMachineCallback, FrameExtra, MachineCallback, MemoryKind, MiriInterpCx,