Add SyntaxContext::hygienic_eq.

This combines multiple `HygieneData::with` calls into one, by combining
parts of `hygienic_eq` and `adjust_ident`.

src/librustc/ty/mod.rs

@@ -3089,7 +3089,8 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         // comparison fails frequently, and we want to avoid the expensive
         // `modern()` calls required for the span comparison whenever possible.
         use_name.name == def_name.name &&
-        self.adjust_ident(use_name, def_parent_def_id).span.ctxt() == def_name.modern().span.ctxt()
+        use_name.span.ctxt().hygienic_eq(def_name.span.ctxt(),
+                                         self.expansion_that_defined(def_parent_def_id))
     }
 
     fn expansion_that_defined(self, scope: DefId) -> Mark {

src/libsyntax_pos/hygiene.rs

@@ -543,6 +543,14 @@ impl SyntaxContext {
         })
     }
 
+    pub fn hygienic_eq(self, other: SyntaxContext, mark: Mark) -> bool {
+        HygieneData::with(|data| {
+            let mut self_modern = data.modern(self);
+            data.adjust(&mut self_modern, mark);
+            self_modern == data.modern(other)
+        })
+    }
+
     #[inline]
     pub fn modern(self) -> SyntaxContext {
         HygieneData::with(|data| data.modern(self))