Rollup merge of #47463 - bluss:fused-iterator, r=alexcrichton

Stabilize FusedIterator

FusedIterator is a marker trait that promises that the implementing
iterator continues to return `None` from `.next()` once it has returned
`None` once (and/or `.next_back()`, if implemented).

The effects of FusedIterator are already widely available through
`.fuse()`, but with stable `FusedIterator`, stable Rust users can
implement this trait for their iterators when appropriate.

Closes #35602

src/libstd_unicode/char.rs

@@ -70,7 +70,7 @@ impl Iterator for ToLowercase {
     }
 }
 
-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for ToLowercase {}
 
 /// Returns an iterator that yields the uppercase equivalent of a `char`.
@@ -92,7 +92,7 @@ impl Iterator for ToUppercase {
     }
 }
 
-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for ToUppercase {}
 
 #[derive(Debug, Clone)]

src/libstd_unicode/lib.rs

@@ -36,7 +36,6 @@
 #![feature(str_internals)]
 #![feature(decode_utf8)]
 #![feature(fn_traits)]
-#![feature(fused)]
 #![feature(lang_items)]
 #![feature(non_exhaustive)]
 #![feature(staged_api)]

src/libstd_unicode/u_str.rs

@@ -127,7 +127,6 @@ impl<I> Iterator for Utf16Encoder<I>
     }
 }
 
-#[unstable(feature = "fused", issue = "35602")]
 impl<I> FusedIterator for Utf16Encoder<I>
     where I: FusedIterator<Item = char> {}
 
@@ -186,5 +185,5 @@ impl<'a> DoubleEndedIterator for SplitWhitespace<'a> {
     }
 }
 
-#[unstable(feature = "fused", issue = "35602")]
+#[stable(feature = "fused", since = "1.26.0")]
 impl<'a> FusedIterator for SplitWhitespace<'a> {}