diff --git a/src/librustc_typeck/check_unused.rs b/src/librustc_typeck/check_unused.rs
index b867a655b4a6a..f2f1e2938cb12 100644
--- a/src/librustc_typeck/check_unused.rs
+++ b/src/librustc_typeck/check_unused.rs
@@ -75,6 +75,25 @@ pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     tcx.hir.krate().visit_all_item_likes(&mut visitor);
 
     for &(def_id, span) in tcx.maybe_unused_extern_crates(LOCAL_CRATE).iter() {
+        // The `def_id` here actually was calculated during resolution (at least
+        // at the time of this writing) and is being shipped to us via a side
+        // channel of the tcx. There may have been extra expansion phases,
+        // however, which ended up removing the `def_id` *after* expansion such
+        // as the `ReplaceBodyWithLoop` pass (which is a bit of a hack, but hey)
+        //
+        // As a result we need to verify that `def_id` is indeed still valid for
+        // our AST and actually present in the HIR map. If it's not there then
+        // there's safely nothing to warn about, and otherwise we carry on with
+        // our execution.
+        //
+        // Note that if we carry through to the `extern_mod_stmt_cnum` query
+        // below it'll cause a panic because `def_id` is actually bogus at this
+        // point in time otherwise.
+        if let Some(id) = tcx.hir.as_local_node_id(def_id) {
+            if tcx.hir.find(id).is_none() {
+                continue
+            }
+        }
         let cnum = tcx.extern_mod_stmt_cnum(def_id).unwrap();
         if tcx.is_compiler_builtins(cnum) {
             continue
diff --git a/src/test/rustdoc/issue-46271.rs b/src/test/rustdoc/issue-46271.rs
new file mode 100644
index 0000000000000..cc3be08c5688d
--- /dev/null
+++ b/src/test/rustdoc/issue-46271.rs
@@ -0,0 +1,15 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// hopefully this doesn't cause an ICE
+
+pub fn foo() {
+    extern crate std;
+}