don't ICE on cross-crate associated const type mismatch

Fixes #41549.

src/librustc_typeck/check/compare_method.rs

@@ -788,16 +788,18 @@ pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                              trait",
                                             trait_c.name);
 
-            // Add a label to the Span containing just the type of the item
-            let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id).unwrap();
-            let trait_c_span = match tcx.hir.expect_trait_item(trait_c_node_id).node {
-                TraitItemKind::Const(ref ty, _) => ty.span,
-                _ => bug!("{:?} is not a trait const", trait_c),
-            };
+            let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id);
+            let trait_c_span = trait_c_node_id.map(|trait_c_node_id| {
+                // Add a label to the Span containing just the type of the const
+                match tcx.hir.expect_trait_item(trait_c_node_id).node {
+                    TraitItemKind::Const(ref ty, _) => ty.span,
+                    _ => bug!("{:?} is not a trait const", trait_c),
+                }
+            });
 
             infcx.note_type_err(&mut diag,
                                 &cause,
-                                Some((trait_c_span, format!("type in trait"))),
+                                trait_c_span.map(|span| (span, format!("type in trait"))),
                                 Some(infer::ValuePairs::Types(ExpectedFound {
                                     expected: trait_ty,
                                     found: impl_ty,

src/test/compile-fail/auxiliary/issue_41549.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.
+
+#![feature(associated_consts)]
+
+pub trait Trait {
+    const CONST: u32;
+}

src/test/compile-fail/issue-41549.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// aux-build:issue_41549.rs
+
+#![feature(associated_consts)]
+
+extern crate issue_41549;
+
+struct S;
+
+impl issue_41549::Trait for S {
+    const CONST: () = (); //~ ERROR incompatible type for trait
+}
+
+fn main() {}