Ensure that Rustdoc discovers all necessary auto trait bounds

src/librustc/traits/auto_trait.rs

@@ -334,7 +334,12 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                 continue;
             }
 
-            let result = select.select(&Obligation::new(dummy_cause.clone(), new_env, pred));
+            // Call infcx.resolve_type_vars_if_possible to see if we can
+            // get rid of any inference variables.
+            let obligation = infcx.resolve_type_vars_if_possible(
+                &Obligation::new(dummy_cause.clone(), new_env, pred)
+            );
+            let result = select.select(&obligation);
 
             match &result {
                 &Ok(Some(ref vtable)) => {
@@ -369,7 +374,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                 }
                 &Ok(None) => {}
                 &Err(SelectionError::Unimplemented) => {
-                    if self.is_of_param(pred.skip_binder().trait_ref.substs) {
+                    if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
                         already_visited.remove(&pred);
                         self.add_user_pred(
                             &mut user_computed_preds,
@@ -631,18 +636,28 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
         finished_map
     }
 
-    pub fn is_of_param(&self, substs: &Substs<'_>) -> bool {
-        if substs.is_noop() {
-            return false;
-        }
+    fn is_param_no_infer(&self, substs: &Substs<'_>) -> bool {
+        return self.is_of_param(substs.type_at(0)) &&
+            !substs.types().any(|t| t.has_infer_types());
+    }
 
-        return match substs.type_at(0).sty {
+    pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
+        return match ty.sty {
             ty::Param(_) => true,
-            ty::Projection(p) => self.is_of_param(p.substs),
+            ty::Projection(p) => self.is_of_param(p.self_ty()),
             _ => false,
         };
     }
 
+    fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
+        match p.ty().skip_binder().sty {
+            ty::Projection(proj) if proj == p.skip_binder().projection_ty => {
+                true
+            },
+            _ => false
+        }
+    }
+
     pub fn evaluate_nested_obligations<
         'b,
         'c,
@@ -661,28 +676,77 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
     ) -> bool {
         let dummy_cause = ObligationCause::misc(DUMMY_SP, ast::DUMMY_NODE_ID);
 
-        for (obligation, predicate) in nested
-            .filter(|o| o.recursion_depth == 1)
+        for (obligation, mut predicate) in nested
             .map(|o| (o.clone(), o.predicate.clone()))
         {
             let is_new_pred =
                 fresh_preds.insert(self.clean_pred(select.infcx(), predicate.clone()));
 
+            // Resolve any inference variables that we can, to help selection succeed
+            predicate = select.infcx().resolve_type_vars_if_possible(&predicate);
+
+            // We only add a predicate as a user-displayable bound if
+            // it involves a generic parameter, and doesn't contain
+            // any inference variables.
+            //
+            // Displaying a bound involving a concrete type (instead of a generic
+            // parameter) would be pointless, since it's always true
+            // (e.g. u8: Copy)
+            // Displaying an inference variable is impossible, since they're
+            // an internal compiler detail without a defined visual representation
+            //
+            // We check this by calling is_of_param on the relevant types
+            // from the various possible predicates
             match &predicate {
                 &ty::Predicate::Trait(ref p) => {
-                    let substs = &p.skip_binder().trait_ref.substs;
+                    if self.is_param_no_infer(p.skip_binder().trait_ref.substs)
+                        && !only_projections
+                        && is_new_pred {
 
-                    if self.is_of_param(substs) && !only_projections && is_new_pred {
                         self.add_user_pred(computed_preds, predicate);
                     }
                     predicates.push_back(p.clone());
                 }
                 &ty::Predicate::Projection(p) => {
-                    // If the projection isn't all type vars, then
-                    // we don't want to add it as a bound
-                    if self.is_of_param(p.skip_binder().projection_ty.substs) && is_new_pred {
-                        self.add_user_pred(computed_preds, predicate);
-                    } else {
+                    debug!("evaluate_nested_obligations: examining projection predicate {:?}",
+                           predicate);
+
+                    // As described above, we only want to display
+                    // bounds which include a generic parameter but don't include
+                    // an inference variable.
+                    // Additionally, we check if we've seen this predicate before,
+                    // to avoid rendering duplicate bounds to the user.
+                    if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
+                        && !p.ty().skip_binder().is_ty_infer()
+                        && is_new_pred {
+                            debug!("evaluate_nested_obligations: adding projection predicate\
+                            to computed_preds: {:?}", predicate);
+
+                            // Under unusual circumstances, we can end up with a self-refeential
+                            // projection predicate. For example:
+                            // <T as MyType>::Value == <T as MyType>::Value
+                            // Not only is displaying this to the user pointless,
+                            // having it in the ParamEnv will cause an issue if we try to call
+                            // poly_project_and_unify_type on the predicate, since this kind of
+                            // predicate will normally never end up in a ParamEnv.
+                            //
+                            // For these reasons, we ignore these weird predicates,
+                            // ensuring that we're able to properly synthesize an auto trait impl
+                            if self.is_self_referential_projection(p) {
+                                debug!("evaluate_nested_obligations: encountered a projection
+                                 predicate equating a type with itself! Skipping");
+
+                            } else {
+                                self.add_user_pred(computed_preds, predicate);
+                            }
+                    }
+
+                    // We can only call poly_project_and_unify_type when our predicate's
+                    // Ty is an inference variable - otherwise, there won't be anything to
+                    // unify
+                    if p.ty().skip_binder().is_ty_infer() {
+                        debug!("Projecting and unifying projection predicate {:?}",
+                               predicate);
                         match poly_project_and_unify_type(select, &obligation.with(p.clone())) {
                             Err(e) => {
                                 debug!(

src/test/rustdoc/issue-50159.rs

@@ -0,0 +1,31 @@
+// 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.
+
+
+pub trait Signal {
+    type Item;
+}
+
+pub trait Signal2 {
+    type Item2;
+}
+
+impl<B, C> Signal2 for B where B: Signal<Item = C> {
+    type Item2 = C;
+}
+
+// @has issue_50159/struct.Switch.html
+// @has - '//code' 'impl<B> Send for Switch<B> where <B as Signal>::Item: Send'
+// @has - '//code' 'impl<B> Sync for Switch<B> where <B as Signal>::Item: Sync'
+// @count - '//*[@id="implementations-list"]/*[@class="impl"]' 0
+// @count - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]' 2
+pub struct Switch<B: Signal> {
+    pub inner: <B as Signal2>::Item2,
+}

src/test/rustdoc/synthetic_auto/self-referential.rs

@@ -0,0 +1,40 @@
+// 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.
+
+// Some unusual code minimized from
+// /~https://github.com/sile/handy_async/tree/7b619b762c06544fc67792c8ff8ebc24a88fdb98
+
+pub trait Pattern {
+    type Value;
+}
+
+pub struct Constrain<A, B = A, C = A>(A, B, C);
+
+impl<A, B, C> Pattern for Constrain<A, B, C>
+    where A: Pattern,
+          B: Pattern<Value = A::Value>,
+          C: Pattern<Value = A::Value>,
+{
+    type Value = A::Value;
+}
+
+pub struct Wrapper<T>(T);
+
+impl<T> Pattern for Wrapper<T> {
+    type Value = T;
+}
+
+
+// @has self_referential/struct.WriteAndThen.html
+// @has - '//*[@id="synthetic-implementations-list"]/*[@class="impl"]//*/code' "impl<P1> Send for \
+// WriteAndThen<P1>  where  <P1 as Pattern>::Value: Send"
+pub struct WriteAndThen<P1>(pub P1::Value,pub <Constrain<P1, Wrapper<P1::Value>> as Pattern>::Value)
+    where P1: Pattern;
+