Check for negative impls when finding auto traits

src/librustc/traits/auto_trait.rs

@@ -309,7 +309,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
     ) -> Option<(ty::ParamEnv<'c>, ty::ParamEnv<'c>)> {
         let tcx = infcx.tcx;
 
-        let mut select = SelectionContext::new(&infcx);
+        let mut select = SelectionContext::with_negative(&infcx, true);
 
         let mut already_visited = FxHashSet::default();
         let mut predicates = VecDeque::new();
@@ -338,6 +338,21 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
 
             match &result {
                 &Ok(Some(ref vtable)) => {
+                    // If we see an explicit negative impl (e.g. 'impl !Send for MyStruct'),
+                    // we immediately bail out, since it's impossible for us to continue.
+                    match vtable {
+                        Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => {
+                            // Blame tidy for the weird bracket placement
+                            if infcx.tcx.impl_polarity(*impl_def_id) == hir::ImplPolarity::Negative
+                            {
+                                debug!("evaluate_nested_obligations: Found explicit negative impl\
+                                        {:?}, bailing out", impl_def_id);
+                                return None;
+                            }
+                        },
+                        _ => {}
+                    }
+
                     let obligations = vtable.clone().nested_obligations().into_iter();
 
                     if !self.evaluate_nested_obligations(

src/test/rustdoc/issue-55321.rs

@@ -0,0 +1,27 @@
+// Copyright 2018 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(optin_builtin_traits)]
+
+// @has issue_55321/struct.A.html
+// @has - '//*[@id="implementations-list"]/*[@class="impl"]//*/code' "impl !Send for A"
+// @has - '//*[@id="implementations-list"]/*[@class="impl"]//*/code' "impl !Sync for A"
+pub struct A();
+
+impl !Send for A {}
+impl !Sync for A {}
+
+// @has issue_55321/struct.B.html
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//*/code' "impl<T> !Send for \
+// B<T>"
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//*/code' "impl<T> !Sync for \
+// B<T>"
+pub struct B<T: ?Sized>(A, Box<T>);