Rollup merge of rust-lang#35755 - SimonSapin:char_convert, r=alexcrichton

Implement std::convert traits for char

This is motivated by avoiding the `as` operator, which sometimes silently truncates, and instead use conversions that are explicitly lossless and infallible.

I’m less certain that `From<u8> for char` should be implemented: while it matches an existing behavior of `as`, it’s not necessarily the right thing to use for non-ASCII bytes. It effectively decodes bytes as ISO/IEC 8859-1 (since Unicode designed its first 256 code points to be compatible with that encoding), but that is not apparent in the API name.

Author: eddyb

src/libcore/char.rs

@@ -18,6 +18,7 @@
 use prelude::v1::*;
 
 use char_private::is_printable;
+use convert::TryFrom;
 use mem::transmute;
 
 // UTF-8 ranges and tags for encoding characters
@@ -123,12 +124,7 @@ pub const MAX: char = '\u{10ffff}';
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn from_u32(i: u32) -> Option<char> {
-    // catch out-of-bounds and surrogates
-    if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
-        None
-    } else {
-        Some(unsafe { from_u32_unchecked(i) })
-    }
+    char::try_from(i).ok()
 }
 
 /// Converts a `u32` to a `char`, ignoring validity.
@@ -176,6 +172,41 @@ pub unsafe fn from_u32_unchecked(i: u32) -> char {
     transmute(i)
 }
 
+#[stable(feature = "char_convert", since = "1.12.0")]
+impl From<char> for u32 {
+    #[inline]
+    fn from(c: char) -> Self {
+        c as u32
+    }
+}
+
+#[stable(feature = "char_convert", since = "1.12.0")]
+impl From<u8> for char {
+    #[inline]
+    fn from(i: u8) -> Self {
+        i as char
+    }
+}
+
+#[unstable(feature = "try_from", issue = "33417")]
+impl TryFrom<u32> for char {
+    type Err = CharTryFromError;
+
+    #[inline]
+    fn try_from(i: u32) -> Result<Self, Self::Err> {
+        if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
+            Err(CharTryFromError(()))
+        } else {
+            Ok(unsafe { from_u32_unchecked(i) })
+        }
+    }
+}
+
+/// The error type returned when a conversion from u32 to char fails.
+#[unstable(feature = "try_from", issue = "33417")]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct CharTryFromError(());
+
 /// Converts a digit in the given radix to a `char`.
 ///
 /// A 'radix' here is sometimes also called a 'base'. A radix of two

src/libcoretest/char.rs

@@ -9,6 +9,24 @@
 // except according to those terms.
 
 use std::char;
+use std::convert::TryFrom;
+
+#[test]
+fn test_convert() {
+    assert_eq!(u32::from('a'), 0x61);
+    assert_eq!(char::from(b'\0'), '\0');
+    assert_eq!(char::from(b'a'), 'a');
+    assert_eq!(char::from(b'\xFF'), '\u{FF}');
+    assert_eq!(char::try_from(0_u32), Ok('\0'));
+    assert_eq!(char::try_from(0x61_u32), Ok('a'));
+    assert_eq!(char::try_from(0xD7FF_u32), Ok('\u{D7FF}'));
+    assert!(char::try_from(0xD800_u32).is_err());
+    assert!(char::try_from(0xDFFF_u32).is_err());
+    assert_eq!(char::try_from(0xE000_u32), Ok('\u{E000}'));
+    assert_eq!(char::try_from(0x10FFFF_u32), Ok('\u{10FFFF}'));
+    assert!(char::try_from(0x110000_u32).is_err());
+    assert!(char::try_from(0xFFFF_FFFF_u32).is_err());
+}
 
 #[test]
 fn test_is_lowercase() {

src/librustc_unicode/char.rs

@@ -39,6 +39,8 @@ pub use core::char::{MAX, from_digit, from_u32, from_u32_unchecked};
 pub use core::char::{EncodeUtf16, EncodeUtf8, EscapeDebug, EscapeDefault, EscapeUnicode};
 
 // unstable reexports
+#[unstable(feature = "try_from", issue = "33417")]
+pub use core::char::CharTryFromError;
 #[unstable(feature = "decode_utf8", issue = "33906")]
 pub use core::char::{DecodeUtf8, decode_utf8};
 #[unstable(feature = "unicode", issue = "27783")]

src/librustc_unicode/lib.rs

@@ -37,6 +37,7 @@
 #![feature(decode_utf8)]
 #![feature(lang_items)]
 #![feature(staged_api)]
+#![feature(try_from)]
 #![feature(unicode)]
 
 mod tables;