Fix Iter exhaustion in prove_predicates when debug is on

[spastorino]

Fixes the issue noted in this comment https://github.com/rust-lang/rust/pull/49885/files#r182560268

r? @pnkfelix
/cc @nikomatsakis

[tamird]

yikes, this seems excessive. why not just duplicate the iterator?

[spastorino]

that was the first thing I've tried but I can't ...

error[E0599]: no method named `clone` found for type `impl IntoIterator<Item = ty::Predicate<'tcx>>` in the current scope

[spastorino]

We were talking with @pnkfelix to maybe do that thing only in the debug case and avoid the extra cost when debug is off. He said that it may not worth, I'm ok with changing this to whatever is better :).

[tamird]

In fact, I don't see why making this function take an iterator is better - I think just reverting 0e2e179 is the right thing here 🤷‍♂️

[spastorino]

@tamird yeah we were originally discussing this with @nikomatsakis and agreed to move that to an iterator. I agree with you now though that reverting would be better.

[spastorino]

Ok, changed the commit in this PR to a revert + a fix to another call to prove_predicates that arrived in another commit.

[tamird]

I have a slightly nicer patch, but I won't lie, it took some battles with the compiler. The TL;DR is that you want IntoIterator::IntoIter to be Clone:

diff --git a/src/librustc_mir/borrow_check/nll/type_check/mod.rs b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
index acd246b703..e9c86ea99b 100644
--- a/src/librustc_mir/borrow_check/nll/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/type_check/mod.rs
@@ -275,7 +275,7 @@ impl<'a, 'b, 'gcx, 'tcx> TypeVerifier<'a, 'b, 'gcx, 'tcx> {
                         tcx.predicates_of(def_id).instantiate(tcx, substs);
                     let predicates =
                         type_checker.normalize(&instantiated_predicates.predicates, location);
-                    type_checker.prove_predicates(predicates.iter().cloned(), location);
+                    type_checker.prove_predicates(predicates, location);
                 }
 
                 value.ty
@@ -1516,34 +1516,33 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
 
         let predicates = self.normalize(&instantiated_predicates.predicates, location);
         debug!("prove_aggregate_predicates: predicates={:?}", predicates);
-        self.prove_predicates(predicates.iter().cloned(), location);
+        self.prove_predicates(predicates, location);
     }
 
     fn prove_trait_ref(&mut self, trait_ref: ty::TraitRef<'tcx>, location: Location) {
         self.prove_predicates(
-            [ty::Predicate::Trait(
+            Some(ty::Predicate::Trait(
                 trait_ref.to_poly_trait_ref().to_poly_trait_predicate(),
-            )].iter()
-                .cloned(),
+            )),
             location,
         );
     }
 
-    fn prove_predicates(
-        &mut self,
-        predicates: impl IntoIterator<Item = ty::Predicate<'tcx>>,
-        location: Location,
-    ) {
-        let mut predicates_iter = predicates.into_iter();
+    fn prove_predicates<T>(&mut self, predicates: T, location: Location)
+        where T: IntoIterator<Item = ty::Predicate<'tcx>>,
+              T::IntoIter: Clone,
+    {
+        let predicates = predicates.into_iter();
 
         debug!(
             "prove_predicates(predicates={:?}, location={:?})",
-            predicates_iter.by_ref().collect::<Vec<_>>(),
-            location
+            predicates.clone().collect::<Vec<_>>(),
+            location,
         );
         self.fully_perform_op(location.at_self(), |this| {
             let cause = this.misc(this.last_span);
-            let obligations = predicates_iter
+            let obligations = predicates
+                .into_iter()
                 .map(|p| traits::Obligation::new(cause.clone(), this.param_env, p))
                 .collect();
             Ok(InferOk {

Feel free to use this, or lmk and I'll open a new PR.

[spastorino]

Very nice trick @tamird

[spastorino]

pushed again, thanks @tamird

[tamird]

Nit: I updated my diff after posting; this can just be Clone; Iterator is implied by IntoIterator

[spastorino]

Done!

[tamird]

FYI changing this to Option can be reverted when/if #49000 is merged.

[spastorino]

👍

[pnkfelix]

@bors r+

[bors]

📌 Commit 55054ee has been approved by pnkfelix

[bors]

⌛ Testing commit 55054ee with merge b12e96f3cc3875b0ec5cc183b5fbd5fc9981330f...

[bors]

💔 Test failed - status-appveyor

[tamird]

Looks like a spurious linking failure.

[kennytm]

@bors retry #50140

[bors]

⌛ Testing commit 55054ee with merge e59f78f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing e59f78f to master...