Check full in fold & find_any consume_iter, remove for filter*

These adaptors consume may many elements before deferring to a base
folder's fullness checks, and so they need to be performed
manually. For the `filter`s, there's no way to do it manually (rayon-rs#632),
so the specialisations just have to be removed. For `fold` and
`find_any` this can be done with a `take_while`.

This extends the octillion tests to confirm this behaviour.

This makes a program like the following slightly slower compared to
the direct `consume_iter` without a check, but it's still faster than
the non-specialized form.

```
extern crate test;
extern crate rayon;
use rayon::prelude::*;

fn main() {
    let count = (0..std::u32::MAX)
        .into_par_iter()
        .map(test::black_box)
        .find_any(|_| test::black_box(false));
    println!("{:?}", count);
}
```

```
$ hyperfine ./find-original ./find-no-check ./find-check
Benchmark #1: ./find-original
  Time (mean ± σ):     627.6 ms ±  25.7 ms    [User: 7.130 s, System: 0.014 s]
  Range (min … max):   588.4 ms … 656.4 ms    10 runs

Benchmark #2: ./find-no-check
  Time (mean ± σ):     481.5 ms ±  10.8 ms    [User: 5.415 s, System: 0.013 s]
  Range (min … max):   468.9 ms … 498.2 ms    10 runs

Benchmark #3: ./find-check
  Time (mean ± σ):     562.3 ms ±  11.8 ms    [User: 6.363 s, System: 0.013 s]
  Range (min … max):   542.5 ms … 578.2 ms    10 runs
```

(find-original = without specialization, find-no-check = custom
`consume_iter` without `take_while`, find-check = this commit)

src/iter/filter.rs

@@ -136,14 +136,9 @@ where
         }
     }
 
-    fn consume_iter<I>(mut self, iter: I) -> Self
-    where
-        I: IntoIterator<Item = T>
-    {
-        self.base = self.base.consume_iter(iter.into_iter().filter(self.filter_op));
-        self
-    }
-
+    // This cannot easily specialize `consume_iter` to be better than
+    // the default, because that requires checking `self.base.full()`
+    // during a call to `self.base.consume_iter()`. (#632)
 
     fn complete(self) -> Self::Result {
         self.base.complete()

src/iter/filter_map.rs

@@ -139,13 +139,10 @@ where
             self
         }
     }
-    fn consume_iter<I>(mut self, iter: I) -> Self
-    where
-        I: IntoIterator<Item = T>
-    {
-        self.base = self.base.consume_iter(iter.into_iter().filter_map(self.filter_op));
-        self
-    }
+
+    // This cannot easily specialize `consume_iter` to be better than
+    // the default, because that requires checking `self.base.full()`
+    // during a call to `self.base.consume_iter()`. (#632)
 
     fn complete(self) -> C::Result {
         self.base.complete()

src/iter/find.rs

@@ -89,7 +89,11 @@ where
     fn consume_iter<I>(mut self, iter: I) -> Self
     where I: IntoIterator<Item = T>
     {
-        self.item = iter.into_iter().find(self.find_op);
+        self.item = iter
+            .into_iter()
+            // stop iterating if another thread has found something
+            .take_while(|_| !self.full())
+            .find(self.find_op);
         if self.item.is_some() {
             self.found.store(true, Ordering::Relaxed)
         }

src/iter/fold.rs

@@ -141,12 +141,18 @@ where
         }
     }
 
-    fn consume_iter<I>(mut self, iter: I) -> Self
+    fn consume_iter<I>(self, iter: I) -> Self
     where
         I: IntoIterator<Item = T>
     {
-        self.item = iter.into_iter().fold(self.item, self.fold_op);
-        self
+        let base = self.base;
+        let item = iter
+            .into_iter()
+            // stop iterating if another thread has finished
+            .take_while(|_| !base.full())
+            .fold(self.item, self.fold_op);
+
+        FoldFolder { base: base, item: item, fold_op: self.fold_op }
     }
 
     fn complete(self) -> C::Result {

tests/octillion.rs

@@ -42,27 +42,57 @@ fn find_first_octillion_flat() {
     assert_eq!(x, Some(0));
 }
 
-#[test]
-fn find_last_octillion() {
+fn two_threads<F: Send + FnOnce() -> R, R: Send>(f: F) -> R {
     // FIXME: If we don't use at least two threads, then we end up walking
     // through the entire iterator sequentially, without the benefit of any
     // short-circuiting.  We probably don't want testing to wait that long. ;)
-    // It would be nice if `find_last` could prioritize the later splits,
-    // basically flipping the `join` args, without needing indexed `rev`.
-    // (or could we have an unindexed `rev`?)
     let builder = rayon::ThreadPoolBuilder::new().num_threads(2);
     let pool = builder.build().unwrap();
 
-    let x = pool.install(|| octillion().find_last(|_| true));
+    pool.install(f)
+}
+
+#[test]
+fn find_last_octillion() {
+    // It would be nice if `find_last` could prioritize the later splits,
+    // basically flipping the `join` args, without needing indexed `rev`.
+    // (or could we have an unindexed `rev`?)
+    let x = two_threads(|| octillion().find_last(|_| true));
     assert_eq!(x, Some(OCTILLION - 1));
 }
 
 #[test]
 fn find_last_octillion_flat() {
-    // FIXME: Ditto, need two threads.
-    let builder = rayon::ThreadPoolBuilder::new().num_threads(2);
-    let pool = builder.build().unwrap();
-
-    let x = pool.install(|| octillion_flat().find_last(|_| true));
+    let x = two_threads(|| octillion_flat().find_last(|_| true));
     assert_eq!(x, Some(OCTILLION - 1));
 }
+
+#[test]
+fn find_any_octillion() {
+    let x = two_threads(|| octillion().find_any(|x| *x > OCTILLION / 2));
+    assert!(x.is_some());
+}
+
+#[test]
+fn find_any_octillion_flat() {
+    let x = two_threads(|| octillion_flat().find_any(|x| *x > OCTILLION / 2));
+    assert!(x.is_some());
+}
+
+#[test]
+fn filter_find_any_octillion() {
+    let x = two_threads(|| octillion().filter(|x| *x > OCTILLION / 2).find_any(|_| true));
+    assert!(x.is_some());
+}
+
+#[test]
+fn filter_find_any_octillion_flat() {
+    let x = two_threads(|| octillion_flat().filter(|x| *x > OCTILLION / 2).find_any(|_| true));
+    assert!(x.is_some());
+}
+
+#[test]
+fn fold_find_any_octillion_flat() {
+    let x = two_threads(|| octillion_flat().fold(|| (), |_, _| ()).find_any(|_| true));
+    assert!(x.is_some());
+}