Ensure that Rusdoc discovers all necessary auto trait bounds

Fixes #50159

This commit makes several improvements to AutoTraitFinder:

* Call infcx.resolve_type_vars_if_possible before processing new
predicates. This ensures that we eliminate inference variables wherever
possible.
* Process all nested obligations we get from a vtable, not just ones
with depth=1.
  * The 'depth=1' check was a hack to work around issues processing
certain predicates. The other changes in this commit allow us to
properly process all predicates that we encounter, so the check is no
longer necessary,
* Ensure that we only display predicates *without* inference variables
to the user, and only attempt to unify predicates that *have* an
inference variable as their type.

Additionally, the internal helper method is_of_param now operates
directly on a type, rather than taking a Substs. This allows us to use
the 'self_ty' method, rather than directly dealing with Substs.

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_of_param(pred.skip_binder().self_ty()) {
                         already_visited.remove(&pred);
                         self.add_user_pred(
                             &mut user_computed_preds,
@@ -631,14 +636,10 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
         finished_map
     }
 
-    pub fn is_of_param(&self, substs: &Substs<'_>) -> bool {
-        if substs.is_noop() {
-            return false;
-        }
-
-        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,
         };
     }
@@ -661,28 +662,61 @@ 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_of_param(p.skip_binder().self_ty())
+                        && !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 {
+                    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_of_param(p.skip_binder().projection_ty.self_ty())
+                        && !p.ty().skip_binder().is_ty_infer()
+                        && is_new_pred {
+                            debug!("evaluate_nested_obligations: adding projection predicate\
+                            to computed_preds: {:?}", predicate);
+
                         self.add_user_pred(computed_preds, predicate);
-                    } else {
+                    }
+
+                    // 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,
+}