Implement PeekableIterator for Chars

Author: wmstack

library/core/src/str/iter.rs

@@ -8,8 +8,8 @@ use super::{
 };
 use crate::fmt::{self, Write};
 use crate::iter::{
-    Chain, Copied, Filter, FlatMap, Flatten, FusedIterator, Map, TrustedLen, TrustedRandomAccess,
-    TrustedRandomAccessNoCoerce,
+    Chain, Copied, Filter, FlatMap, Flatten, FusedIterator, Map, PeekableIterator, TrustedLen,
+    TrustedRandomAccess, TrustedRandomAccessNoCoerce,
 };
 use crate::num::NonZero;
 use crate::ops::Try;
@@ -132,6 +132,18 @@ impl<'a> DoubleEndedIterator for Chars<'a> {
     }
 }
 
+#[unstable(feature = "peekable_iterator", issue = "132973")]
+impl<'a> PeekableIterator for Chars<'a> {
+    fn peek_with<T>(&mut self, func: impl for<'b> FnOnce(Option<&'b Self::Item>) -> T) -> T {
+        // SAFETY: `str` invariant says `self.iter` is a valid UTF-8 string and
+        // the resulting `ch` is a valid Unicode Scalar Value.
+        let tmp = unsafe {
+            next_code_point(&mut self.iter.clone()).map(|ch| char::from_u32_unchecked(ch))
+        };
+        func(&tmp)
+    }
+}
+
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for Chars<'_> {}