Add and use SyntaxContext::outer_and_expn_info.

This combines two `HygieneData::with` calls into one on a hot path.

Author: nnethercote

src/librustc/ty/query/on_disk_cache.rs

@@ -844,9 +844,8 @@ impl<'enc, 'a, 'tcx, E> SpecializedEncoder<Span> for CacheEncoder<'enc, 'a, 'tcx
         if span_data.ctxt == SyntaxContext::empty() {
             TAG_NO_EXPANSION_INFO.encode(self)
         } else {
-            let mark = span_data.ctxt.outer();
-
-            if let Some(expn_info) = mark.expn_info() {
+            let (mark, expn_info) = span_data.ctxt.outer_and_expn_info();
+            if let Some(expn_info) = expn_info {
                 if let Some(pos) = self.expn_info_shorthands.get(&mark).cloned() {
                     TAG_EXPANSION_INFO_SHORTHAND.encode(self)?;
                     pos.encode(self)

src/libsyntax_pos/hygiene.rs

@@ -565,6 +565,16 @@ impl SyntaxContext {
         HygieneData::with(|data| data.expn_info(data.outer(self)))
     }
 
+    /// `ctxt.outer_and_expn_info()` is equivalent to but faster than
+    /// `{ let outer = ctxt.outer(); (outer, outer.expn_info()) }`.
+    #[inline]
+    pub fn outer_and_expn_info(self) -> (Mark, Option<ExpnInfo>) {
+        HygieneData::with(|data| {
+            let outer = data.outer(self);
+            (outer, data.expn_info(outer))
+        })
+    }
+
     pub fn dollar_crate_name(self) -> Symbol {
         HygieneData::with(|data| data.syntax_contexts[self.0 as usize].dollar_crate_name)
     }