Uniform enter_trace_span! and add documentation

The macro was uniformed between rustc_const_eval and miri

Author: Stypox

compiler/rustc_const_eval/src/interpret/stack.rs

@@ -397,9 +397,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         // Finish things up.
         M::after_stack_push(self)?;
         self.frame_mut().loc = Left(mir::Location::START);
-        // `tracing_separate_thread` is used to instruct the chrome_tracing [tracing::Layer] in Miri
+        // `tracing_separate_thread` is used to instruct the tracing_chrome [tracing::Layer] in Miri
         // to put the "frame" span on a separate trace thread/line than other spans, to make the
-        // visualization in https://ui.perfetto.dev easier to interpret. It is set to a value of
+        // visualization in <https://ui.perfetto.dev> easier to interpret. It is set to a value of
         // [tracing::field::Empty] so that other tracing layers (e.g. the logger) will ignore it.
         let span = info_span!("frame", tracing_separate_thread = Empty, "{}", instance);
         self.frame_mut().tracing_span.enter(span);

compiler/rustc_const_eval/src/interpret/util.rs

@@ -53,12 +53,63 @@ pub trait EnteredTraceSpan {}
 impl EnteredTraceSpan for () {}
 impl EnteredTraceSpan for tracing::span::EnteredSpan {}
 
-/// Shortand for calling [crate::interpret::Machine::enter_trace_span] on a [tracing::info_span].
+/// Shortand for calling [crate::interpret::Machine::enter_trace_span] on a [tracing::info_span!].
 /// This is supposed to be compiled out when [crate::interpret::Machine::enter_trace_span] has the
 /// default implementation (i.e. when it does not actually enter the span but instead returns `()`).
+/// This macro takes a type implementing the [crate::interpret::Machine] trait as its first argument
+/// and otherwise accepts the same syntax as [tracing::span!] (see some tips below).
 /// Note: the result of this macro **must be used** because the span is exited when it's dropped.
+///
+/// ### Syntax accepted by this macro
+///
+/// The full documentation for the [tracing::span!] syntax can be found at [tracing] under "Using the
+/// Macros". A few possibly confusing syntaxes are listed here:
+/// ```rust
+/// // logs a span named "hello" with a field named "arg" of value 42 (works only because
+/// // 42 implements the tracing::Value trait, otherwise use one of the options below)
+/// let _span = enter_trace_span!(M, "hello", arg = 42);
+/// // logs a field called "my_display_var" using the Display implementation
+/// let _span = enter_trace_span!(M, "hello", %my_display_var);
+/// // logs a field called "my_debug_var" using the Debug implementation
+/// let _span = enter_trace_span!(M, "hello", ?my_debug_var);
+///  ```
+///
+/// ### `NAME::SUBNAME` syntax
+///
+/// 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
+/// let _span = enter_trace_span!(M, borrow_tracker::on_stack_pop, /* ... */)
+/// let _span = enter_trace_span!(M, "borrow_tracker", borrow_tracker = "on_stack_pop", /* ... */)
+/// ```
+///
+/// ### `tracing_separate_thread` parameter
+///
+/// This macro was introduced to obtain better traces of Miri without impacting release performance.
+/// Miri saves traces using the the `tracing_chrome` [tracing::Layer] so that they can be visualized
+/// in <https://ui.perfetto.dev>. To instruct `tracing_chrome` to put some spans on a separate trace
+/// thread/line than other spans when viewed in <https://ui.perfetto.dev>, you can pass
+/// `tracing_separate_thread = tracing::field::Empty` to the tracing macros. This is useful to
+/// separate out spans which just indicate the current step or program frame being processed by the
+/// interpreter. You should use a value of [tracing::field::Empty] so that other tracing layers
+/// (e.g. the logger) will ignore the `tracing_separate_thread` field. For example:
+/// ```rust
+/// let _span = enter_trace_span!(M, step::eval_statement, tracing_separate_thread = tracing::field::Empty)
+/// ```
 #[macro_export]
 macro_rules! enter_trace_span {
+    ($machine:ident, $name:ident :: $subname:ident $($tt:tt)*) => {{
+        $crate::enter_trace_span!($machine, stringify!($name), $name = %stringify!($subname) $($tt)*)
+    }};
+
     ($machine:ident, $($tt:tt)*) => {
         $machine::enter_trace_span(|| tracing::info_span!($($tt)*))
     }

src/tools/miri/src/helpers.rs

@@ -1257,25 +1257,11 @@ pub struct MaybeEnteredTraceSpan {
 /// 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", /* ... */)
-/// ```
+/// Look at [rustc_const_eval::enter_trace_span] for complete documentation, examples and tips.
 #[macro_export]
 macro_rules! enter_trace_span {
     ($name:ident :: $subname:ident $($tt:tt)*) => {{
-        enter_trace_span!(stringify!($name), $name = %stringify!($subname) $($tt)*)
+        $crate::enter_trace_span!(stringify!($name), $name = %stringify!($subname) $($tt)*)
     }};
 
     ($($tt:tt)*) => {