Rollup merge of #42271 - tinaun:charfromstr, r=alexcrichton

add `FromStr` Impl for `char`

fixes #24939.

is it possible to use pub(restricted) instead of using a stability attribute for the internal error representation? is it needed at all?

src/libcore/char.rs

@@ -19,7 +19,7 @@ use char_private::is_printable;
 use convert::TryFrom;
 use fmt::{self, Write};
 use slice;
-use str::from_utf8_unchecked_mut;
+use str::{from_utf8_unchecked_mut, FromStr};
 use iter::FusedIterator;
 use mem::transmute;
 
@@ -208,6 +208,63 @@ impl From<u8> for char {
     }
 }
 
+
+/// An error which can be returned when parsing a char.
+#[stable(feature = "char_from_str", since = "1.19.0")]
+#[derive(Clone, Debug)]
+pub struct ParseCharError {
+    kind: CharErrorKind,
+}
+
+impl ParseCharError {
+    #[unstable(feature = "char_error_internals",
+               reason = "this method should not be available publicly",
+               issue = "0")]
+    #[doc(hidden)]
+    pub fn __description(&self) -> &str {
+        match self.kind {
+            CharErrorKind::EmptyString => {
+                "cannot parse char from empty string"
+            },
+            CharErrorKind::TooManyChars => "too many characters in string"
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+enum CharErrorKind {
+    EmptyString,
+    TooManyChars,
+}
+
+#[stable(feature = "char_from_str", since = "1.19.0")]
+impl fmt::Display for ParseCharError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.__description().fmt(f)
+    }
+}
+
+
+#[stable(feature = "char_from_str", since = "1.19.0")]
+impl FromStr for char {
+    type Err = ParseCharError;
+
+    #[inline]
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut chars = s.chars();
+        match (chars.next(), chars.next()) {
+            (None, _) => {
+                Err(ParseCharError { kind: CharErrorKind::EmptyString })
+            },
+            (Some(c), None) => Ok(c),
+            _ => {
+                Err(ParseCharError { kind: CharErrorKind::TooManyChars })
+            }
+        }
+    }
+}
+
+
 #[unstable(feature = "try_from", issue = "33417")]
 impl TryFrom<u32> for char {
     type Error = CharTryFromError;

src/libcore/tests/char.rs

@@ -10,6 +10,7 @@
 
 use std::{char,str};
 use std::convert::TryFrom;
+use std::str::FromStr;
 
 #[test]
 fn test_convert() {
@@ -28,6 +29,16 @@ fn test_convert() {
     assert!(char::try_from(0xFFFF_FFFF_u32).is_err());
 }
 
+#[test]
+fn test_from_str() {
+    assert_eq!(char::from_str("a").unwrap(), 'a');
+    assert_eq!(char::try_from("a").unwrap(), 'a');
+    assert_eq!(char::from_str("\0").unwrap(), '\0');
+    assert_eq!(char::from_str("\u{D7FF}").unwrap(), '\u{d7FF}');
+    assert!(char::from_str("").is_err());
+    assert!(char::from_str("abc").is_err());
+}
+
 #[test]
 fn test_is_lowercase() {
     assert!('a'.is_lowercase());