Auto merge of #27808 - SimonSapin:utf16decoder, r=alexcrichton

* Rename `Utf16Items` to `Utf16Decoder`. "Items" is meaningless.
* Generalize it to any `u16` iterator, not just `[u16].iter()`
* Make it yield `Result` instead of a custom `Utf16Item` enum that was isomorphic to `Result`. This enable using the `FromIterator for Result` impl.
* Replace `Utf16Item::to_char_lossy` with a `Utf16Decoder::lossy` iterator adaptor.

This is a [breaking change], but only for users of the unstable `rustc_unicode` crate.

I’d like this functionality to be stabilized and re-exported in `std` eventually, as the "low-level equivalent" of `String::from_utf16` and `String::from_utf16_lossy` like #27784 is the low-level equivalent of #27714.

CC @aturon, @alexcrichton 

2 files changed, 10 insertions, 0 deletions

diff --git a/src/libcoretest/char.rs b/src/libcoretest/char.rs
index f03f8e0ee78..d23442379bc 100644
--- a/src/libcoretest/char.rs
+++ b/src/libcoretest/char.rs
@@ -207,3 +207,12 @@ fn test_len_utf16() {
     assert!('\u{a66e}'.len_utf16() == 1);
     assert!('\u{1f4a9}'.len_utf16() == 2);
 }
+
+#[test]
+fn test_decode_utf16() {
+    fn check(s: &[u16], expected: &[Result<char, u16>]) {
+        assert_eq!(::std::char::decode_utf16(s.iter().cloned()).collect::<Vec<_>>(), expected);
+    }
+    check(&[0xD800, 0x41, 0x42], &[Err(0xD800), Ok('A'), Ok('B')]);
+    check(&[0xD800, 0], &[Err(0xD800), Ok('\0')]);
+}
diff --git a/src/libcoretest/lib.rs b/src/libcoretest/lib.rs
index 6313e42e0ed..dda1b096e88 100644
--- a/src/libcoretest/lib.rs
+++ b/src/libcoretest/lib.rs
@@ -19,6 +19,7 @@
 #![feature(float_from_str_radix)]
 #![feature(flt2dec)]
 #![feature(dec2flt)]
+#![feature(decode_utf16)]
 #![feature(fmt_radix)]
 #![feature(iter_arith)]
 #![feature(iter_arith)]