diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index 010c7f6171a..58bb5b9580f 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -59,7 +59,7 @@ use syntax::ast::*;
 use syntax::errors;
 use syntax::ext::hygiene::{Mark, SyntaxContext};
 use syntax::ptr::P;
-use syntax::codemap::{self, respan, Spanned};
+use syntax::codemap::{self, respan, Spanned, CompilerDesugaringKind};
 use syntax::std_inject;
 use syntax::symbol::{Symbol, keywords};
 use syntax::util::small_vector::SmallVector;
@@ -414,12 +414,14 @@ impl<'a> LoweringContext<'a> {
         Symbol::gensym(s)
     }
 
-    fn allow_internal_unstable(&self, reason: &'static str, mut span: Span) -> Span {
+    fn allow_internal_unstable(&self, reason: CompilerDesugaringKind, mut span: Span)
+        -> Span
+    {
         let mark = Mark::fresh(Mark::root());
         mark.set_expn_info(codemap::ExpnInfo {
             call_site: span,
             callee: codemap::NameAndSpan {
-                format: codemap::CompilerDesugaring(Symbol::intern(reason)),
+                format: codemap::CompilerDesugaring(reason),
                 span: Some(span),
                 allow_internal_unstable: true,
                 allow_internal_unsafe: false,
@@ -1790,7 +1792,8 @@ impl<'a> LoweringContext<'a> {
                 let move_val_init = ["intrinsics", "move_val_init"];
                 let inplace_finalize = ["ops", "InPlace", "finalize"];
 
-                let unstable_span = self.allow_internal_unstable("<-", e.span);
+                let unstable_span =
+                    self.allow_internal_unstable(CompilerDesugaringKind::BackArrow, e.span);
                 let make_call = |this: &mut LoweringContext, p, args| {
                     let path = P(this.expr_std_path(unstable_span, p, ThinVec::new()));
                     P(this.expr_call(e.span, path, args))
@@ -2007,12 +2010,14 @@ impl<'a> LoweringContext<'a> {
                     e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e)))
                     .map(|(s, e)| {
                         let expr = P(self.lower_expr(&e));
-                        let unstable_span = self.allow_internal_unstable("...", e.span);
+                        let unstable_span =
+                            self.allow_internal_unstable(CompilerDesugaringKind::DotFill, e.span);
                         self.field(Symbol::intern(s), expr, unstable_span)
                     }).collect::<P<[hir::Field]>>();
 
                 let is_unit = fields.is_empty();
-                let unstable_span = self.allow_internal_unstable("...", e.span);
+                let unstable_span =
+                    self.allow_internal_unstable(CompilerDesugaringKind::DotFill, e.span);
                 let struct_path =
                     iter::once("ops").chain(iter::once(path))
                     .collect::<Vec<_>>();
@@ -2353,7 +2358,8 @@ impl<'a> LoweringContext<'a> {
                 //                 return Try::from_error(From::from(err)),
                 // }
 
-                let unstable_span = self.allow_internal_unstable("?", e.span);
+                let unstable_span =
+                    self.allow_internal_unstable(CompilerDesugaringKind::QuestionMark, e.span);
 
                 // Try::into_result(<expr>)
                 let discr = {
diff --git a/src/libsyntax_pos/hygiene.rs b/src/libsyntax_pos/hygiene.rs
index 296a3b11aed..919804d7efd 100644
--- a/src/libsyntax_pos/hygiene.rs
+++ b/src/libsyntax_pos/hygiene.rs
@@ -323,8 +323,8 @@ impl NameAndSpan {
     pub fn name(&self) -> Symbol {
         match self.format {
             ExpnFormat::MacroAttribute(s) |
-            ExpnFormat::MacroBang(s) |
-            ExpnFormat::CompilerDesugaring(s) => s,
+            ExpnFormat::MacroBang(s) => s,
+            ExpnFormat::CompilerDesugaring(ref kind) => kind.as_symbol(),
         }
     }
 }
@@ -337,7 +337,27 @@ pub enum ExpnFormat {
     /// e.g. `format!()`
     MacroBang(Symbol),
     /// Desugaring done by the compiler during HIR lowering.
-    CompilerDesugaring(Symbol)
+    CompilerDesugaring(CompilerDesugaringKind)
+}
+
+/// The kind of compiler desugaring.
+#[derive(Clone, Hash, Debug, PartialEq, Eq)]
+pub enum CompilerDesugaringKind {
+    BackArrow,
+    DotFill,
+    QuestionMark,
+}
+
+impl CompilerDesugaringKind {
+    pub fn as_symbol(&self) -> Symbol {
+        use CompilerDesugaringKind::*;
+        let s = match *self {
+            BackArrow => "<-",
+            DotFill => "...",
+            QuestionMark => "?",
+        };
+        Symbol::intern(s)
+    }
 }
 
 impl Encodable for SyntaxContext {
diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index f5449061b87..2385e3509ad 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -47,7 +47,7 @@ extern crate serialize;
 extern crate serialize as rustc_serialize; // used by deriving
 
 pub mod hygiene;
-pub use hygiene::{SyntaxContext, ExpnInfo, ExpnFormat, NameAndSpan};
+pub use hygiene::{SyntaxContext, ExpnInfo, ExpnFormat, NameAndSpan, CompilerDesugaringKind};
 
 pub mod symbol;
 
@@ -153,6 +153,17 @@ impl Span {
         }
     }
 
+    /// Check if this span arises from a compiler desugaring of kind `kind`.
+    pub fn is_compiler_desugaring(&self, kind: CompilerDesugaringKind) -> bool {
+        match self.ctxt.outer().expn_info() {
+            Some(info) => match info.callee.format {
+                ExpnFormat::CompilerDesugaring(k) => k == kind,
+                _ => false,
+            },
+            None => false,
+        }
+    }
+
     /// Check if a span is "internal" to a macro in which `unsafe`
     /// can be used without triggering the `unsafe_code` lint
     //  (that is, a macro marked with `#[allow_internal_unsafe]`).