diff --git a/src/comp/driver/rustc.rs b/src/comp/driver/rustc.rs
index aa02afe00cb1..0d5223d87023 100644
--- a/src/comp/driver/rustc.rs
+++ b/src/comp/driver/rustc.rs
@@ -37,6 +37,7 @@ import back::link::output_type;
 tag pp_mode {
     ppm_normal;
     ppm_typed;
+    ppm_identified;
 }
 
 fn default_environment(session::session sess,
@@ -132,6 +133,7 @@ fn pretty_print_input(session::session sess, eval::env env, str input,
             mode = pprust::mo_typed(ty_cx);
         }
         case (ppm_normal) { mode = pprust::mo_untyped; }
+        case (ppm_identified) { mode = pprust::mo_identified; }
     }
 
     pprust::print_file(sess, crate.node.module, input, std::io::stdout(),
@@ -313,9 +315,10 @@ fn build_session(@session::options sopts) -> session::session {
 fn parse_pretty(session::session sess, &str name) -> pp_mode {
     if (str::eq(name, "normal")) { ret ppm_normal; }
     else if (str::eq(name, "typed")) { ret ppm_typed; }
-    else {
-        sess.err("argument to `pretty` must be either `normal` or `typed`");
-    }
+    else if (str::eq(name, "identified")) { ret ppm_identified; }
+
+    sess.err("argument to `pretty` must be one of `normal`, `typed`, or " +
+             "`identified`");
 }
 
 fn main(vec[str] args) {
diff --git a/src/comp/pretty/pprust.rs b/src/comp/pretty/pprust.rs
index 6a957d4553f9..ed73cb936b22 100644
--- a/src/comp/pretty/pprust.rs
+++ b/src/comp/pretty/pprust.rs
@@ -1,3 +1,4 @@
+import std::uint;
 import std::vec;
 import std::str;
 import std::io;
@@ -30,6 +31,7 @@ const uint default_columns = 78u;
 tag mode {
     mo_untyped;
     mo_typed(ty::ctxt);
+    mo_identified;
 }
 
 type ps = @rec(pp::printer s,
@@ -161,6 +163,16 @@ fn bclose(&ps s, common::span span) {
     end(s); // close the outer-box
 }
 
+// Synthesizes a comment that was not textually present in the original source
+// file.
+fn synth_comment(&ps s, str text) {
+    word(s.s, "/*");
+    space(s.s);
+    word(s.s, text);
+    space(s.s);
+    word(s.s, "*/");
+}
+
 fn commasep[IN](&ps s, breaks b, vec[IN] elts, fn(&ps, &IN) op) {
     box(s, 0u, b);
     auto first = true;
@@ -503,6 +515,7 @@ fn print_expr(&ps s, &@ast::expr expr) {
     alt (s.mode) {
         case (mo_untyped) { /* no-op */ }
         case (mo_typed(_)) { popen(s); }
+        case (mo_identified) { popen(s); }
     }
 
     alt (expr.node) {
@@ -844,7 +857,7 @@ fn print_expr(&ps s, &@ast::expr expr) {
         }
     }
 
-    // Print the type if necessary.
+    // Print the type or node ID if necessary.
     alt (s.mode) {
         case (mo_untyped) { /* no-op */ }
         case (mo_typed(?tcx)) {
@@ -854,6 +867,11 @@ fn print_expr(&ps s, &@ast::expr expr) {
             word(s.s, ty::ty_to_str(tcx, ty::expr_ty(tcx, expr)));
             pclose(s);
         }
+        case (mo_identified) {
+            space(s.s);
+            synth_comment(s, uint::to_str(ty::expr_ann(expr).id, 10u));
+            pclose(s);
+        }
     }
 
     end(s);
@@ -874,7 +892,7 @@ fn print_decl(&ps s, &@ast::decl decl) {
                 case (_) {
                     word_nbsp(s, "auto");
 
-                    // Print the type if necessary.
+                    // Print the type or node ID if necessary.
                     alt (s.mode) {
                         case (mo_untyped) { /* no-op */ }
                         case (mo_typed(?tcx)) {
@@ -882,6 +900,7 @@ fn print_decl(&ps s, &@ast::decl decl) {
                                 ty::ann_to_type(tcx.node_types, loc.ann);
                             word_space(s, ty::ty_to_str(tcx, lty));
                         }
+                        case (mo_identified) { /* no-op */ }
                     }
                 }
             }