Revert "Auto merge of #42840 - arielb1:poison-smoke-and-mirrors, r=nikomatsakis"

This reverts commit 9b85e1c, reversing
changes made to 13157c4.

Author: arielb1

src/librustc/traits/fulfill.rs

@@ -8,14 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use dep_graph::DepGraph;
 use infer::{InferCtxt, InferOk};
-use ty::{self, Ty, TypeFoldable, ToPolyTraitRef, ToPredicate};
+use ty::{self, Ty, TypeFoldable, ToPolyTraitRef, TyCtxt, ToPredicate};
 use ty::error::ExpectedFound;
 use rustc_data_structures::obligation_forest::{ObligationForest, Error};
 use rustc_data_structures::obligation_forest::{ForestObligation, ObligationProcessor};
 use std::marker::PhantomData;
 use syntax::ast;
-use util::nodemap::NodeMap;
+use util::nodemap::{FxHashSet, NodeMap};
 use hir::def_id::DefId;
 
 use super::CodeAmbiguity;
@@ -33,6 +34,11 @@ impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
     fn as_predicate(&self) -> &Self::Predicate { &self.obligation.predicate }
 }
 
+pub struct GlobalFulfilledPredicates<'tcx> {
+    set: FxHashSet<ty::PolyTraitPredicate<'tcx>>,
+    dep_graph: DepGraph,
+}
+
 /// The fulfillment context is used to drive trait resolution.  It
 /// consists of a list of obligations that must be (eventually)
 /// satisfied. The job is to track which are satisfied, which yielded
@@ -177,6 +183,13 @@ impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> {
 
         assert!(!infcx.is_in_snapshot());
 
+        let tcx = infcx.tcx;
+
+        if tcx.fulfilled_predicates.borrow().check_duplicate(tcx, &obligation.predicate) {
+            debug!("register_predicate_obligation: duplicate");
+            return
+        }
+
         self.predicates.register_obligation(PendingPredicateObligation {
             obligation,
             stalled_on: vec![]
@@ -251,6 +264,13 @@ impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> {
             });
             debug!("select: outcome={:?}", outcome);
 
+            // these are obligations that were proven to be true.
+            for pending_obligation in outcome.completed {
+                let predicate = &pending_obligation.obligation.predicate;
+                selcx.tcx().fulfilled_predicates.borrow_mut()
+                           .add_if_global(selcx.tcx(), predicate);
+            }
+
             errors.extend(
                 outcome.errors.into_iter()
                               .map(|e| to_fulfillment_error(e)));
@@ -298,7 +318,7 @@ impl<'a, 'b, 'gcx, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'gcx,
                                _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>)
         where I: Clone + Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
     {
-        if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
+        if coinductive_match(self.selcx, cycle.clone()) {
             debug!("process_child_obligations: coinductive match");
         } else {
             let cycle : Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
@@ -355,31 +375,21 @@ fn process_predicate<'a, 'gcx, 'tcx>(
 
     match obligation.predicate {
         ty::Predicate::Trait(ref data) => {
-            let trait_obligation = obligation.with(data.clone());
-
-            if data.is_global() {
-                // no type variables present, can use evaluation for better caching.
-                // FIXME: consider caching errors too.
-                if
-                    // make defaulted unit go through the slow path for better warnings,
-                    // please remove this when the warnings are removed.
-                    !trait_obligation.predicate.skip_binder().self_ty().is_defaulted_unit() &&
-                    selcx.evaluate_obligation_conservatively(&obligation) {
-                    debug!("selecting trait `{:?}` at depth {} evaluated to holds",
-                           data, obligation.recursion_depth);
-                    return Ok(Some(vec![]))
-                }
+            let tcx = selcx.tcx();
+            if tcx.fulfilled_predicates.borrow().check_duplicate_trait(tcx, data) {
+                return Ok(Some(vec![]));
             }
 
+            let trait_obligation = obligation.with(data.clone());
             match selcx.select(&trait_obligation) {
                 Ok(Some(vtable)) => {
                     debug!("selecting trait `{:?}` at depth {} yielded Ok(Some)",
-                           data, obligation.recursion_depth);
+                          data, obligation.recursion_depth);
                     Ok(Some(vtable.nested_obligations()))
                 }
                 Ok(None) => {
                     debug!("selecting trait `{:?}` at depth {} yielded Ok(None)",
-                           data, obligation.recursion_depth);
+                          data, obligation.recursion_depth);
 
                     // This is a bit subtle: for the most part, the
                     // only reason we can fail to make progress on
@@ -540,6 +550,40 @@ fn process_predicate<'a, 'gcx, 'tcx>(
     }
 }
 
+/// For defaulted traits, we use a co-inductive strategy to solve, so
+/// that recursion is ok. This routine returns true if the top of the
+/// stack (`cycle[0]`):
+/// - is a defaulted trait, and
+/// - it also appears in the backtrace at some position `X`; and,
+/// - all the predicates at positions `X..` between `X` an the top are
+///   also defaulted traits.
+fn coinductive_match<'a,'c,'gcx,'tcx,I>(selcx: &mut SelectionContext<'a,'gcx,'tcx>,
+                                        cycle: I) -> bool
+    where I: Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
+          'tcx: 'c
+{
+    let mut cycle = cycle;
+    cycle
+        .all(|bt_obligation| {
+            let result = coinductive_obligation(selcx, &bt_obligation.obligation);
+            debug!("coinductive_match: bt_obligation={:?} coinductive={}",
+                   bt_obligation, result);
+            result
+        })
+}
+
+fn coinductive_obligation<'a,'gcx,'tcx>(selcx: &SelectionContext<'a,'gcx,'tcx>,
+                                          obligation: &PredicateObligation<'tcx>)
+                                          -> bool {
+    match obligation.predicate {
+        ty::Predicate::Trait(ref data) => {
+            selcx.tcx().trait_has_default_impl(data.def_id())
+        }
+        _ => {
+            false
+        }
+    }
+}
 
 fn register_region_obligation<'tcx>(t_a: Ty<'tcx>,
                                     r_b: ty::Region<'tcx>,
@@ -559,6 +603,55 @@ fn register_region_obligation<'tcx>(t_a: Ty<'tcx>,
 
 }
 
+impl<'a, 'gcx, 'tcx> GlobalFulfilledPredicates<'gcx> {
+    pub fn new(dep_graph: DepGraph) -> GlobalFulfilledPredicates<'gcx> {
+        GlobalFulfilledPredicates {
+            set: FxHashSet(),
+            dep_graph,
+        }
+    }
+
+    pub fn check_duplicate(&self, tcx: TyCtxt, key: &ty::Predicate<'tcx>) -> bool {
+        if let ty::Predicate::Trait(ref data) = *key {
+            self.check_duplicate_trait(tcx, data)
+        } else {
+            false
+        }
+    }
+
+    pub fn check_duplicate_trait(&self, tcx: TyCtxt, data: &ty::PolyTraitPredicate<'tcx>) -> bool {
+        // For the global predicate registry, when we find a match, it
+        // may have been computed by some other task, so we want to
+        // add a read from the node corresponding to the predicate
+        // processing to make sure we get the transitive dependencies.
+        if self.set.contains(data) {
+            debug_assert!(data.is_global());
+            self.dep_graph.read(data.dep_node(tcx));
+            debug!("check_duplicate: global predicate `{:?}` already proved elsewhere", data);
+
+            true
+        } else {
+            false
+        }
+    }
+
+    fn add_if_global(&mut self, tcx: TyCtxt<'a, 'gcx, 'tcx>, key: &ty::Predicate<'tcx>) {
+        if let ty::Predicate::Trait(ref data) = *key {
+            // We only add things to the global predicate registry
+            // after the current task has proved them, and hence
+            // already has the required read edges, so we don't need
+            // to add any more edges here.
+            if data.is_global() {
+                if let Some(data) = tcx.lift_to_global(data) {
+                    if self.set.insert(data.clone()) {
+                        debug!("add_if_global: global predicate `{:?}` added", data);
+                    }
+                }
+            }
+        }
+    }
+}
+
 fn to_fulfillment_error<'tcx>(
     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>)
     -> FulfillmentError<'tcx>

src/librustc/traits/mod.rs

@@ -31,7 +31,7 @@ use syntax_pos::{Span, DUMMY_SP};
 pub use self::coherence::orphan_check;
 pub use self::coherence::overlapping_impls;
 pub use self::coherence::OrphanCheckErr;
-pub use self::fulfill::{FulfillmentContext, RegionObligation};
+pub use self::fulfill::{FulfillmentContext, GlobalFulfilledPredicates, RegionObligation};
 pub use self::project::MismatchedProjectionTypes;
 pub use self::project::{normalize, normalize_projection_type, Normalized};
 pub use self::project::{ProjectionCache, ProjectionCacheSnapshot, Reveal};