auto merge of #15256 : erickt/rust/optimizations, r=alexcrichton

The bug #11084 causes `option::collect` and `result::collect` about twice as slower as it should because llvm is having some trouble optimizing away the scan closure. This gets rid of it so now those functions perform equivalent to a hand written version.

This also adds an impl of `Default` for `Rc` along the way.

2 files changed, 44 insertions, 20 deletions

diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index c0d04ee0b95..b8612ed93e0 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -587,20 +587,32 @@ impl<A> ExactSize<A> for Item<A> {}
 /// ```
 #[inline]
 pub fn collect<T, Iter: Iterator<Option<T>>, V: FromIterator<T>>(iter: Iter) -> Option<V> {
-    // FIXME(#11084): This should be twice as fast once this bug is closed.
-    let mut iter = iter.scan(false, |state, x| {
-        match x {
-            Some(x) => Some(x),
-            None => {
-                *state = true;
-                None
+    // FIXME(#11084): This could be replaced with Iterator::scan when this
+    // performance bug is closed.
+
+    struct Adapter<Iter> {
+        iter: Iter,
+        found_none: bool,
+    }
+
+    impl<T, Iter: Iterator<Option<T>>> Iterator<T> for Adapter<Iter> {
+        #[inline]
+        fn next(&mut self) -> Option<T> {
+            match self.iter.next() {
+                Some(Some(value)) => Some(value),
+                Some(None) => {
+                    self.found_none = true;
+                    None
+                }
+                None => None,
             }
         }
-    });
+    }
 
-    let v: V = FromIterator::from_iter(iter.by_ref());
+    let mut adapter = Adapter { iter: iter, found_none: false };
+    let v: V = FromIterator::from_iter(adapter.by_ref());
 
-    if iter.state {
+    if adapter.found_none {
         None
     } else {
         Some(v)
diff --git a/src/libcore/result.rs b/src/libcore/result.rs
index a104d20f4e5..5cbbf30cd36 100644
--- a/src/libcore/result.rs
+++ b/src/libcore/result.rs
@@ -585,20 +585,32 @@ impl<T: Show, E> Result<T, E> {
 /// ```
 #[inline]
 pub fn collect<T, E, Iter: Iterator<Result<T, E>>, V: FromIterator<T>>(iter: Iter) -> Result<V, E> {
-    // FIXME(#11084): This should be twice as fast once this bug is closed.
-    let mut iter = iter.scan(None, |state, x| {
-        match x {
-            Ok(x) => Some(x),
-            Err(err) => {
-                *state = Some(err);
-                None
+    // FIXME(#11084): This could be replaced with Iterator::scan when this
+    // performance bug is closed.
+
+    struct Adapter<Iter, E> {
+        iter: Iter,
+        err: Option<E>,
+    }
+
+    impl<T, E, Iter: Iterator<Result<T, E>>> Iterator<T> for Adapter<Iter, E> {
+        #[inline]
+        fn next(&mut self) -> Option<T> {
+            match self.iter.next() {
+                Some(Ok(value)) => Some(value),
+                Some(Err(err)) => {
+                    self.err = Some(err);
+                    None
+                }
+                None => None,
             }
         }
-    });
+    }
 
-    let v: V = FromIterator::from_iter(iter.by_ref());
+    let mut adapter = Adapter { iter: iter, err: None };
+    let v: V = FromIterator::from_iter(adapter.by_ref());
 
-    match iter.state {
+    match adapter.err {
         Some(err) => Err(err),
         None => Ok(v),
     }