new companion method to next_code_point() that accepts Iterator<Item=u8>

library/alloc/tests/lib.rs

@@ -40,6 +40,7 @@
 #![feature(panic_update_hook)]
 #![feature(slice_flatten)]
 #![feature(thin_box)]
+#![feature(str_internals)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};

library/alloc/tests/str.rs

@@ -1,3 +1,4 @@
+use core::str::next_code_point_val;
 use std::assert_matches::assert_matches;
 use std::borrow::Cow;
 use std::cmp::Ordering::{Equal, Greater, Less};
@@ -2367,3 +2368,71 @@ fn ceil_char_boundary() {
 fn ceil_char_boundary_above_len_panic() {
     let _ = "x".ceil_char_boundary(2);
 }
+
+fn check_decoded_string<I: Iterator<Item = u8>>(mut iter: &mut I, expected: &str) {
+    for char in expected.chars() {
+        assert_eq!(Some(char as u32), unsafe { next_code_point_val(&mut iter) });
+    }
+    assert_eq!(None, unsafe { next_code_point_val(&mut iter) });
+}
+
+#[test]
+pub fn dirt_simple_code_point() {
+    unsafe {
+        let src = b"banana";
+        let mut iter = src.iter().copied();
+        for char in "banana".chars() {
+            assert_eq!(Some(char as u32), next_code_point_val(&mut iter));
+        }
+
+        let tmp = next_code_point_val(&mut iter);
+        assert_eq!(None, tmp);
+    }
+    {
+        let src = [
+            b'd', b'a', b'i', b' ', 229, 164, 167, 232, 179, 162, 232, 128, 133, b' ', b'k', b'e',
+            b'n', b'j', b'a',
+        ];
+
+        let mut iter = src.into_iter();
+
+        check_decoded_string(&mut iter, "dai 大賢者 kenja");
+    }
+}
+
+struct Shenanigans {
+    compressed: Vec<u8>,
+    cursor: usize,
+}
+
+// This provides an example of a u8 iterator which can not use Item=&u8.
+// A real-world case is a string stored in progmem on an AVR, which can
+// not be incorporated into a unit test
+impl Iterator for Shenanigans {
+    type Item = u8;
+    fn next(&mut self) -> Option<<Self as Iterator>::Item> {
+        let end = self.cursor + 6;
+        let i1 = self.cursor / 8;
+        let i2 = (end - 1) / 8;
+        if i2 >= self.compressed.len() {
+            return None;
+        }
+        let base64 = if i1 == i2 {
+            self.compressed[i1] >> (2 - self.cursor % 8) & 0x3f
+        } else {
+            0x3f & ((self.compressed[i1] << (self.cursor % 8 - 2))
+                | (self.compressed[i2] >> (10 - self.cursor % 8)))
+        };
+        self.cursor += 6;
+        Some(base64 + b' ')
+    }
+}
+
+#[test]
+pub fn fancy_code_point() {
+    let mut iter =
+        Shenanigans { compressed: vec![142, 139, 236, 228, 10, 238, 166, 122, 52], cursor: 0 };
+    for char in "CHOLY KNIGHT".chars() {
+        assert_eq!(Some(char as u32), unsafe { next_code_point_val(&mut iter) });
+    }
+}

library/core/src/str/mod.rs

@@ -70,7 +70,7 @@ pub use iter::SplitAsciiWhitespace;
 pub use iter::SplitInclusive;
 
 #[unstable(feature = "str_internals", issue = "none")]
-pub use validations::{next_code_point, utf8_char_width};
+pub use validations::{next_code_point, next_code_point_val, utf8_char_width};
 
 use iter::MatchIndicesInternal;
 use iter::SplitInternal;

library/core/src/str/validations.rs

@@ -34,8 +34,21 @@ pub(super) const fn utf8_is_cont_byte(byte: u8) -> bool {
 #[unstable(feature = "str_internals", issue = "none")]
 #[inline]
 pub unsafe fn next_code_point<'a, I: Iterator<Item = &'a u8>>(bytes: &mut I) -> Option<u32> {
+    // SAFETY: `bytes` must produce a valid UTF-8-like (UTF-8 or WTF-8) string
+    unsafe { next_code_point_val(&mut bytes.copied()) }
+}
+
+/// Reads the next code point out of a byte iterator (assuming a
+/// UTF-8-like encoding).
+///
+/// # Safety
+///
+/// `bytes` must produce a valid UTF-8-like (UTF-8 or WTF-8) string
+#[unstable(feature = "str_internals", issue = "none")]
+#[inline]
+pub unsafe fn next_code_point_val<I: Iterator<Item = u8>>(bytes: &mut I) -> Option<u32> {
     // Decode UTF-8
-    let x = *bytes.next()?;
+    let x = bytes.next()?;
     if x < 128 {
         return Some(x as u32);
     }
@@ -46,22 +59,22 @@ pub unsafe fn next_code_point<'a, I: Iterator<Item = &'a u8>>(bytes: &mut I) ->
     let init = utf8_first_byte(x, 2);
     // SAFETY: `bytes` produces an UTF-8-like string,
     // so the iterator must produce a value here.
-    let y = unsafe { *bytes.next().unwrap_unchecked() };
+    let y = unsafe { bytes.next().unwrap_unchecked() };
     let mut ch = utf8_acc_cont_byte(init, y);
     if x >= 0xE0 {
         // [[x y z] w] case
         // 5th bit in 0xE0 .. 0xEF is always clear, so `init` is still valid
         // SAFETY: `bytes` produces an UTF-8-like string,
         // so the iterator must produce a value here.
-        let z = unsafe { *bytes.next().unwrap_unchecked() };
+        let z = unsafe { bytes.next().unwrap_unchecked() };
         let y_z = utf8_acc_cont_byte((y & CONT_MASK) as u32, z);
         ch = init << 12 | y_z;
         if x >= 0xF0 {
             // [x y z w] case
             // use only the lower 3 bits of `init`
             // SAFETY: `bytes` produces an UTF-8-like string,
             // so the iterator must produce a value here.
-            let w = unsafe { *bytes.next().unwrap_unchecked() };
+            let w = unsafe { bytes.next().unwrap_unchecked() };
             ch = (init & 7) << 18 | utf8_acc_cont_byte(y_z, w);
         }
     }