clear obligations-added flag with nested fulfillcx

src/librustc/infer/mod.rs

@@ -830,6 +830,33 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         result.map(move |t| InferOk { value: t, obligations: fields.obligations })
     }
 
+    // Clear the "obligations in snapshot" flag, invoke the closure,
+    // then restore the flag to its original value. This flag is a
+    // debugging measure designed to detect cases where we start a
+    // snapshot, create type variables, register obligations involving
+    // those type variables in the fulfillment cx, and then have to
+    // unroll the snapshot, leaving "dangling type variables" behind.
+    // In such cases, the flag will be set by the fulfillment cx, and
+    // an assertion will fail when rolling the snapshot back.  Very
+    // useful, much better than grovelling through megabytes of
+    // RUST_LOG output.
+    //
+    // HOWEVER, in some cases the flag is wrong. In particular, we
+    // sometimes create a "mini-fulfilment-cx" in which we enroll
+    // obligations. As long as this fulfillment cx is fully drained
+    // before we return, this is not a problem, as there won't be any
+    // escaping obligations in the main cx. In those cases, you can
+    // use this function.
+    pub fn save_and_restore_obligations_in_snapshot_flag<F, R>(&self, func: F) -> R
+        where F: FnOnce(&Self) -> R
+    {
+        let flag = self.obligations_in_snapshot.get();
+        self.obligations_in_snapshot.set(false);
+        let result = func(self);
+        self.obligations_in_snapshot.set(flag);
+        result
+    }
+
     fn start_snapshot(&self) -> CombinedSnapshot {
         debug!("start_snapshot()");
 

src/librustc/traits/specialize/mod.rs

@@ -203,32 +203,34 @@ fn fulfill_implication<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
     // attempt to prove all of the predicates for impl2 given those for impl1
     // (which are packed up in penv)
 
-    let mut fulfill_cx = FulfillmentContext::new();
-    for oblig in obligations.into_iter() {
-        fulfill_cx.register_predicate_obligation(&infcx, oblig);
-    }
-    match fulfill_cx.select_all_or_error(infcx) {
-        Err(errors) => {
-            // no dice!
-            debug!("fulfill_implication: for impls on {:?} and {:?}, could not fulfill: {:?} given \
-                    {:?}",
-                   source_trait_ref,
-                   target_trait_ref,
-                   errors,
-                   infcx.parameter_environment.caller_bounds);
-            Err(())
+    infcx.save_and_restore_obligations_in_snapshot_flag(|infcx| {
+        let mut fulfill_cx = FulfillmentContext::new();
+        for oblig in obligations.into_iter() {
+            fulfill_cx.register_predicate_obligation(&infcx, oblig);
         }
+        match fulfill_cx.select_all_or_error(infcx) {
+            Err(errors) => {
+                // no dice!
+                debug!("fulfill_implication: for impls on {:?} and {:?}, \
+                        could not fulfill: {:?} given {:?}",
+                       source_trait_ref,
+                       target_trait_ref,
+                       errors,
+                       infcx.parameter_environment.caller_bounds);
+                Err(())
+            }
 
-        Ok(()) => {
-            debug!("fulfill_implication: an impl for {:?} specializes {:?}",
-                   source_trait_ref,
-                   target_trait_ref);
+            Ok(()) => {
+                debug!("fulfill_implication: an impl for {:?} specializes {:?}",
+                       source_trait_ref,
+                       target_trait_ref);
 
-            // Now resolve the *substitution* we built for the target earlier, replacing
-            // the inference variables inside with whatever we got from fulfillment.
-            Ok(infcx.resolve_type_vars_if_possible(&target_substs))
+                // Now resolve the *substitution* we built for the target earlier, replacing
+                // the inference variables inside with whatever we got from fulfillment.
+                Ok(infcx.resolve_type_vars_if_possible(&target_substs))
+            }
         }
-    }
+    })
 }
 
 pub struct SpecializesCache {

src/test/compile-fail/issue-36053-2.rs

@@ -0,0 +1,21 @@
+// Copyright 2016 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.
+
+// Regression test for #36053. ICE was caused due to obligations
+// being added to a special, dedicated fulfillment cx during
+// a probe.
+
+use std::iter::once;
+fn main() {
+    once::<&str>("str").fuse().filter(|a: &str| true).count();
+    //~^ ERROR no method named `count`
+    //~| ERROR E0281
+    //~| ERROR E0281
+}

src/test/run-pass/issue-36053.rs

@@ -0,0 +1,32 @@
+// Copyright 2016 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.
+
+// Regression test for #36053. ICE was caused due to obligations being
+// added to a special, dedicated fulfillment cx during a
+// probe. Problem seems to be related to the particular definition of
+// `FusedIterator` in std but I was not able to isolate that into an
+// external crate.
+
+#![feature(fused)]
+use std::iter::FusedIterator;
+
+struct Thing<'a>(&'a str);
+impl<'a> Iterator for Thing<'a> {
+    type Item = &'a str;
+    fn next(&mut self) -> Option<&'a str> {
+        None
+    }
+}
+
+impl<'a> FusedIterator for Thing<'a> {}
+
+fn main() {
+    Thing("test").fuse().filter(|_| true).count();
+}