Add u16::is_utf16_surrogate

library/core/src/char/decode.rs

@@ -91,7 +91,7 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
             None => self.iter.next()?,
         };
 
-        if u < 0xD800 || 0xDFFF < u {
+        if !u.is_utf16_surrogate() {
             // SAFETY: not a surrogate
             Some(Ok(unsafe { from_u32_unchecked(u as u32) }))
         } else if u >= 0xDC00 {
@@ -125,7 +125,7 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
             // buf is empty, no additional elements from it.
             None => (0, 0),
             // `u` is a non surrogate, so it's always an additional character.
-            Some(u) if u < 0xD800 || 0xDFFF < u => (1, 1),
+            Some(u) if !u.is_utf16_surrogate() => (1, 1),
             // `u` is a leading surrogate (it can never be a trailing surrogate and
             // it's a surrogate due to the previous branch) and `self.iter` is empty.
             //

library/core/src/lib.rs

@@ -93,7 +93,7 @@
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
 //
-// Library features for const fns:
+// Library features:
 #![feature(const_align_offset)]
 #![feature(const_align_of_val)]
 #![feature(const_alloc_layout)]
@@ -146,6 +146,8 @@
 #![feature(ptr_metadata)]
 #![feature(slice_ptr_get)]
 #![feature(str_internals)]
+#![feature(utf16_extra)]
+#![feature(utf16_extra_const)]
 #![feature(variant_count)]
 #![feature(const_array_from_ref)]
 #![feature(const_slice_from_ref)]

library/core/src/num/mod.rs

@@ -820,6 +820,31 @@ impl u16 {
     uint_impl! { u16, u16, i16, NonZeroU16, 16, 65535, 4, "0xa003", "0x3a", "0x1234", "0x3412", "0x2c48",
     "[0x34, 0x12]", "[0x12, 0x34]", "", "" }
     widening_impl! { u16, u32, 16, unsigned }
+
+    /// Checks if the value is a Unicode surrogate code point, which are disallowed values for [`char`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(utf16_extra)]
+    ///
+    /// let low_non_surrogate = 0xA000u16;
+    /// let low_surrogate = 0xD800u16;
+    /// let high_surrogate = 0xDC00u16;
+    /// let high_non_surrogate = 0xE000u16;
+    ///
+    /// assert!(!low_non_surrogate.is_utf16_surrogate());
+    /// assert!(low_surrogate.is_utf16_surrogate());
+    /// assert!(high_surrogate.is_utf16_surrogate());
+    /// assert!(!high_non_surrogate.is_utf16_surrogate());
+    /// ```
+    #[must_use]
+    #[unstable(feature = "utf16_extra", issue = "94919")]
+    #[rustc_const_unstable(feature = "utf16_extra_const", issue = "94919")]
+    #[inline]
+    pub const fn is_utf16_surrogate(self) -> bool {
+        matches!(self, 0xD800..=0xDFFF)
+    }
 }
 
 #[lang = "u32"]