Don't ICE just because an impl is missing an associated type. Trust in the other compiler passes.

Fixes #17359.

src/librustc/middle/traits/project.rs

@@ -643,11 +643,12 @@ fn confirm_candidate<'cx,'tcx>(
             match impl_ty {
                 Some(ty) => (ty, impl_vtable.nested.to_vec()),
                 None => {
-                    selcx.tcx().sess.span_bug(
-                        obligation.cause.span,
-                        format!("impl `{}` did not contain projection for `{}`",
-                                impl_vtable.repr(selcx.tcx()),
-                                obligation.repr(selcx.tcx())).as_slice());
+                    // This means that the impl is missing a
+                    // definition for the associated type. This error
+                    // ought to be reported by the type checker method
+                    // `check_impl_items_against_trait`, so here we
+                    // just return ty_err.
+                    (selcx.tcx().types.err, vec!())
                 }
             }
         }

src/test/compile-fail/associated-types-issue-17359.rs

@@ -0,0 +1,23 @@
+// Copyright 2014 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.
+
+// Test that we do not ICE when an impl is missing an associated type (and that we report
+// a useful error, of course).
+
+#![feature(associated_types)]
+
+trait Trait {
+    type Type;
+}
+
+impl Trait for int {}  //~ ERROR missing: `Type`
+
+fn main() {}
+