Auto merge of #38713 - clarcharr:trusted_len, r=brson

TrustedLen for Empty and Once.

These implementations were missing, so, I went ahead and added them.

src/libcore/iter/sources.rs

@@ -12,7 +12,7 @@
 use marker;
 use usize;
 
-use super::FusedIterator;
+use super::{FusedIterator, TrustedLen};
 
 /// An iterator that repeats an element endlessly.
 ///
@@ -138,6 +138,9 @@ fn len(&self) -> usize {
     }
 }
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T> TrustedLen for Empty<T> {}
+
 #[unstable(feature = "fused", issue = "35602")]
 impl<T> FusedIterator for Empty<T> {}
 
@@ -216,6 +219,9 @@ fn len(&self) -> usize {
     }
 }
 
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T> TrustedLen for Once<T> {}
+
 #[unstable(feature = "fused", issue = "35602")]
 impl<T> FusedIterator for Once<T> {}