Deprecate Read::chars and char::decode_utf8

Per FCP:

* rust-lang#27802 (comment)
* rust-lang#33906 (comment)

src/libcore/char/decode.rs

@@ -17,22 +17,32 @@ use super::from_u32_unchecked;
 /// An iterator over an iterator of bytes of the characters the bytes represent
 /// as UTF-8
 #[unstable(feature = "decode_utf8", issue = "33906")]
+#[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+    https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
 #[derive(Clone, Debug)]
+#[allow(deprecated)]
 pub struct DecodeUtf8<I: Iterator<Item = u8>>(::iter::Peekable<I>);
 
 /// Decodes an `Iterator` of bytes as UTF-8.
 #[unstable(feature = "decode_utf8", issue = "33906")]
+#[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+    https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
+#[allow(deprecated)]
 #[inline]
 pub fn decode_utf8<I: IntoIterator<Item = u8>>(i: I) -> DecodeUtf8<I::IntoIter> {
     DecodeUtf8(i.into_iter().peekable())
 }
 
 /// `<DecodeUtf8 as Iterator>::next` returns this for an invalid input sequence.
 #[unstable(feature = "decode_utf8", issue = "33906")]
+#[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+    https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
 #[derive(PartialEq, Eq, Debug)]
+#[allow(deprecated)]
 pub struct InvalidSequence(());
 
 #[unstable(feature = "decode_utf8", issue = "33906")]
+#[allow(deprecated)]
 impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
     type Item = Result<char, InvalidSequence>;
     #[inline]
@@ -127,6 +137,7 @@ impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
 }
 
 #[unstable(feature = "decode_utf8", issue = "33906")]
+#[allow(deprecated)]
 impl<I: FusedIterator<Item = u8>> FusedIterator for DecodeUtf8<I> {}
 
 /// An iterator that decodes UTF-16 encoded code points from an iterator of `u16`s.

src/libcore/char/mod.rs

@@ -51,6 +51,9 @@ pub use unicode::tables::UNICODE_VERSION;
 #[unstable(feature = "unicode_version", issue = "49726")]
 pub use unicode::version::UnicodeVersion;
 #[unstable(feature = "decode_utf8", issue = "33906")]
+#[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+    https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
+#[allow(deprecated)]
 pub use self::decode::{decode_utf8, DecodeUtf8, InvalidSequence};
 
 use fmt::{self, Write};

src/libcore/tests/char.rs

@@ -364,6 +364,7 @@ fn eu_iterator_specializations() {
 }
 
 #[test]
+#[allow(deprecated)]
 fn test_decode_utf8() {
     macro_rules! assert_decode_utf8 {
         ($input_bytes: expr, $expected_str: expr) => {

src/libstd/io/buffered.rs

@@ -1251,13 +1251,15 @@ mod tests {
     }
 
     #[test]
+    #[allow(deprecated)]
     fn read_char_buffered() {
         let buf = [195, 159];
         let reader = BufReader::with_capacity(1, &buf[..]);
         assert_eq!(reader.chars().next().unwrap().unwrap(), 'ß');
     }
 
     #[test]
+    #[allow(deprecated)]
     fn test_chars() {
         let buf = [195, 159, b'a'];
         let reader = BufReader::with_capacity(1, &buf[..]);

src/libstd/io/cursor.rs

@@ -566,6 +566,7 @@ mod tests {
     }
 
     #[test]
+    #[allow(deprecated)]
     fn test_read_char() {
         let b = &b"Vi\xE1\xBB\x87t"[..];
         let mut c = Cursor::new(b).chars();
@@ -577,6 +578,7 @@ mod tests {
     }
 
     #[test]
+    #[allow(deprecated)]
     fn test_read_bad_char() {
         let b = &b"\x80"[..];
         let mut c = Cursor::new(b).chars();

src/libstd/io/mod.rs

@@ -840,6 +840,9 @@ pub trait Read {
                                          of where errors happen is currently \
                                          unclear and may change",
                issue = "27802")]
+    #[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+        https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
+    #[allow(deprecated)]
     fn chars(self) -> Chars<Self> where Self: Sized {
         Chars { inner: self }
     }
@@ -2010,16 +2013,22 @@ impl<R: Read> Iterator for Bytes<R> {
 /// [chars]: trait.Read.html#method.chars
 #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
            issue = "27802")]
+#[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+    https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
 #[derive(Debug)]
+#[allow(deprecated)]
 pub struct Chars<R> {
     inner: R,
 }
 
 /// An enumeration of possible errors that can be generated from the `Chars`
 /// adapter.
-#[derive(Debug)]
 #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
            issue = "27802")]
+#[rustc_deprecated(since = "1.27.0", reason = "Use str::from_utf8 instead:
+    https://doc.rust-lang.org/nightly/std/str/struct.Utf8Error.html#examples")]
+#[derive(Debug)]
+#[allow(deprecated)]
 pub enum CharsError {
     /// Variant representing that the underlying stream was read successfully
     /// but it did not contain valid utf8 data.
@@ -2031,6 +2040,7 @@ pub enum CharsError {
 
 #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
            issue = "27802")]
+#[allow(deprecated)]
 impl<R: Read> Iterator for Chars<R> {
     type Item = result::Result<char, CharsError>;
 
@@ -2063,6 +2073,7 @@ impl<R: Read> Iterator for Chars<R> {
 
 #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
            issue = "27802")]
+#[allow(deprecated)]
 impl std_error::Error for CharsError {
     fn description(&self) -> &str {
         match *self {
@@ -2080,6 +2091,7 @@ impl std_error::Error for CharsError {
 
 #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
            issue = "27802")]
+#[allow(deprecated)]
 impl fmt::Display for CharsError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {