extend EventArgRecorder into span-aware SpannedEventArgRecorder

The self-profiler's `EventArgRecorder` is general-purpose in its ability to record Strings (and `rustc_span` depends on the crate its defined in, `rustc_data_structure`).

Some generic activities could use recording locations where they happen in the user's code: to allow e.g. to track macro expansions and diagnose performance issues there.

This adds a `SpannedEventArgRecorder` that can record an argument given as a span, rather than a String, since turning spans into Strings can be tricky if you're not happy with its default Debug output. This way the recorder can have a `record_arg_spanned` method which will do that.

compiler/rustc_span/src/lib.rs

@@ -59,6 +59,8 @@ pub use symbol::{sym, Symbol};
 mod analyze_source_file;
 pub mod fatal_error;
 
+pub mod profiling;
+
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::{Lock, Lrc};
 

compiler/rustc_span/src/profiling.rs

@@ -0,0 +1,35 @@
+use std::borrow::Borrow;
+
+use rustc_data_structures::profiling::EventArgRecorder;
+
+/// Extension trait for self-profiling purposes: allows to record spans within a generic activity's
+/// event arguments.
+pub trait SpannedEventArgRecorder {
+    /// Records the following event arguments within the current generic activity being profiled:
+    /// - the provided `event_arg`
+    /// - a string representation of the provided `span`
+    ///
+    /// Note: when self-profiling with costly event arguments, at least one argument
+    /// needs to be recorded. A panic will be triggered if that doesn't happen.
+    fn record_arg_with_span<A>(&mut self, event_arg: A, span: crate::Span)
+    where
+        A: Borrow<str> + Into<String>;
+}
+
+impl SpannedEventArgRecorder for EventArgRecorder<'_> {
+    fn record_arg_with_span<A>(&mut self, event_arg: A, span: crate::Span)
+    where
+        A: Borrow<str> + Into<String>,
+    {
+        self.record_arg(event_arg);
+
+        let span_arg = crate::with_session_globals(|session_globals| {
+            if let Some(source_map) = &*session_globals.source_map.borrow() {
+                source_map.span_to_embeddable_string(span)
+            } else {
+                format!("{:?}", span)
+            }
+        });
+        self.record_arg(span_arg);
+    }
+}