Expose snapshotting in FulfillmentContext

Author: soltanmm-google

src/librustc/middle/traits/fulfill.rs

@@ -11,7 +11,8 @@
 use dep_graph::DepGraph;
 use middle::infer::InferCtxt;
 use middle::ty::{self, Ty, TypeFoldable};
-use rustc_data_structures::obligation_forest::{Backtrace, ObligationForest, Error};
+use rustc_data_structures::obligation_forest::{Backtrace, Error, ObligationForest};
+use rustc_data_structures::obligation_forest::Snapshot as ForestSnapshot;
 use std::iter;
 use syntax::ast;
 use util::common::ErrorReported;
@@ -95,6 +96,8 @@ pub struct FulfillmentContext<'tcx> {
     region_obligations: NodeMap<Vec<RegionObligation<'tcx>>>,
 }
 
+pub struct FulfillmentSnapshot(ForestSnapshot);
+
 #[derive(Clone)]
 pub struct RegionObligation<'tcx> {
     pub sub_region: ty::Region,
@@ -311,6 +314,20 @@ impl<'tcx> FulfillmentContext<'tcx> {
             Err(errors)
         }
     }
+
+    // FIXME Perhaps this function (and rollback_to, and commit_from) should not be public if/when
+    // the super all powerful TypeCtxt thing appears.
+    pub fn start_snapshot(&mut self) -> FulfillmentSnapshot {
+        FulfillmentSnapshot(self.predicates.start_snapshot())
+    }
+
+    pub fn rollback_to(&mut self, snapshot: FulfillmentSnapshot) {
+        self.predicates.rollback_snapshot(snapshot.0)
+    }
+
+    pub fn commit_from(&mut self, snapshot: FulfillmentSnapshot) {
+        self.predicates.commit_snapshot(snapshot.0)
+    }
 }
 
 /// Like `process_predicate1`, but wrap result into a pending predicate.

src/librustc/middle/traits/mod.rs

@@ -35,7 +35,8 @@ pub use self::error_reporting::report_object_safety_error;
 pub use self::coherence::orphan_check;
 pub use self::coherence::overlapping_impls;
 pub use self::coherence::OrphanCheckErr;
-pub use self::fulfill::{FulfillmentContext, GlobalFulfilledPredicates, RegionObligation};
+pub use self::fulfill::{FulfillmentContext, FulfillmentSnapshot, GlobalFulfilledPredicates,
+                        RegionObligation};
 pub use self::project::MismatchedProjectionTypes;
 pub use self::project::normalize;
 pub use self::project::Normalized;