Make impl Debug for Span not panic on not having session globals.

compiler/rustc_span/src/lib.rs

@@ -1044,17 +1044,26 @@ impl fmt::Debug for Span {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // Use the global `SourceMap` to print the span. If that's not
         // available, fall back to printing the raw values.
+
+        fn fallback(span: Span, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            f.debug_struct("Span")
+                .field("lo", &span.lo())
+                .field("hi", &span.hi())
+                .field("ctxt", &span.ctxt())
+                .finish()
+        }
+        
+        if SESSION_GLOBALS.is_set() {
             with_session_globals(|session_globals| {
                 if let Some(source_map) = &*session_globals.source_map.borrow() {
                     write!(f, "{} ({:?})", source_map.span_to_diagnostic_string(*self), self.ctxt())
                 } else {
-                f.debug_struct("Span")
+                    fallback(*self, f)
-                    .field("lo", &self.lo())
-                    .field("hi", &self.hi())
-                    .field("ctxt", &self.ctxt())
-                    .finish()
                 }
             })
+        } else {
+            fallback(*self, f)
+        }
     }
 }