Remove SPAN_DEBUG global

The only difference between the default and rustc_interface set version
is that the default accesses the source map from SESSION_GLOBALS while
the rustc_interface version accesses the source map from the global
TyCtxt. SESSION_GLOBALS is always set while running the compiler while
the global TyCtxt is not always set. If the global TyCtxt is set, it's
source map is identical to the one in SESSION_GLOBALS

compiler/rustc_span/src/lib.rs

@@ -1013,37 +1013,25 @@ pub fn with_source_map<T, F: FnOnce() -> T>(source_map: Lrc<SourceMap>, f: F) ->
     f()
 }
 
-pub fn debug_with_source_map(
-    span: Span,
-    f: &mut fmt::Formatter<'_>,
-    source_map: &SourceMap,
-) -> fmt::Result {
-    write!(f, "{} ({:?})", source_map.span_to_diagnostic_string(span), span.ctxt())
-}
-
-pub fn default_span_debug(span: Span, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-    with_session_globals(|session_globals| {
-        if let Some(source_map) = &*session_globals.source_map.borrow() {
-            debug_with_source_map(span, f, source_map)
-        } else {
-            f.debug_struct("Span")
-                .field("lo", &span.lo())
-                .field("hi", &span.hi())
-                .field("ctxt", &span.ctxt())
-                .finish()
-        }
-    })
-}
-
 impl fmt::Debug for Span {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (*SPAN_DEBUG)(*self, f)
+        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")
+                    .field("lo", &self.lo())
+                    .field("hi", &self.hi())
+                    .field("ctxt", &self.ctxt())
+                    .finish()
+            }
+        })
     }
 }
 
 impl fmt::Debug for SpanData {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (*SPAN_DEBUG)(Span::new(self.lo, self.hi, self.ctxt, self.parent), f)
+        fmt::Debug::fmt(&Span::new(self.lo, self.hi, self.ctxt, self.parent), f)
     }
 }
 
@@ -2003,8 +1991,6 @@ pub struct FileLines {
     pub lines: Vec<LineInfo>,
 }
 
-pub static SPAN_DEBUG: AtomicRef<fn(Span, &mut fmt::Formatter<'_>) -> fmt::Result> =
-    AtomicRef::new(&(default_span_debug as fn(_, &mut fmt::Formatter<'_>) -> _));
 pub static SPAN_TRACK: AtomicRef<fn(LocalDefId)> = AtomicRef::new(&((|_| {}) as fn(_)));
 
 // _____________________________________________________________________________