Avoid SmallVec::collect() in Result::intern_with().

This commit reduces instruction counts for several benchmarks by up to
5%.

Author: nnethercote

src/librustc/ty/context.rs

@@ -2848,8 +2848,29 @@ impl<'a, T, R> InternIteratorElement<T, R> for &'a T
 
 impl<T, R, E> InternIteratorElement<T, R> for Result<T, E> {
     type Output = Result<R, E>;
-    fn intern_with<I: Iterator<Item=Self>, F: FnOnce(&[T]) -> R>(iter: I, f: F) -> Self::Output {
-        Ok(f(&iter.collect::<Result<SmallVec<[_; 8]>, _>>()?))
+    fn intern_with<I: Iterator<Item=Self>, F: FnOnce(&[T]) -> R>(mut iter: I, f: F)
+            -> Self::Output {
+        // This code is hot enough that it's worth specializing for the most
+        // common length lists, to avoid the overhead of `SmallVec` creation.
+        // The match arms are in order of frequency. The 1, 2, and 0 cases are
+        // typically hit in ~95% of cases. We assume that if the upper and
+        // lower bounds from `size_hint` agree they are correct.
+        Ok(match iter.size_hint() {
+            (1, Some(1)) => {
+                f(&[iter.next().unwrap()?])
+            }
+            (2, Some(2)) => {
+                let t0 = iter.next().unwrap()?;
+                let t1 = iter.next().unwrap()?;
+                f(&[t0, t1])
+            }
+            (0, Some(0)) => {
+                f(&[])
+            }
+            _ => {
+                f(&iter.collect::<Result<SmallVec<[_; 8]>, _>>()?)
+            }
+        })
     }
 }