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Auto merge of rust-lang#35755 - SimonSapin:char_convert, r=alexcrichton
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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.
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bors authored Sep 1, 2016
2 parents 3135b78 + f040208 commit b2799a5
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Showing 5 changed files with 91 additions and 6 deletions.
69 changes: 63 additions & 6 deletions src/libcore/char.rs
Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,8 @@
#![stable(feature = "core_char", since = "1.2.0")]

use char_private::is_printable;
use convert::TryFrom;
use fmt;
use iter::FusedIterator;
use mem::transmute;

Expand Down Expand Up @@ -122,12 +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.
Expand Down Expand Up @@ -175,6 +172,66 @@ pub unsafe fn from_u32_unchecked(i: u32) -> char {
transmute(i)
}

#[stable(feature = "char_convert", since = "1.13.0")]
impl From<char> for u32 {
#[inline]
fn from(c: char) -> Self {
c as u32
}
}

/// Maps a byte in 0x00...0xFF to a `char` whose code point has the same value, in U+0000 to U+00FF.
///
/// Unicode is designed such that this effectively decodes bytes
/// with the character encoding that IANA calls ISO-8859-1.
/// This encoding is compatible with ASCII.
///
/// Note that this is different from ISO/IEC 8859-1 a.k.a. ISO 8859-1 (with one less hypen),
/// which leaves some "blanks", byte values that are not assigned to any character.
/// ISO-8859-1 (the IANA one) assigns them to the C0 and C1 control codes.
///
/// Note that this is *also* different from Windows-1252 a.k.a. code page 1252,
/// which is a superset ISO/IEC 8859-1 that assigns some (not all!) blanks
/// to punctuation and various Latin characters.
///
/// To confuse things further, [on the Web](https://encoding.spec.whatwg.org/)
/// `ascii`, `iso-8859-1`, and `windows-1252` are all aliases
/// for a superset of Windows-1252 that fills the remaining blanks with corresponding
/// C0 and C1 control codes.
#[stable(feature = "char_convert", since = "1.13.0")]
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(());

#[unstable(feature = "try_from", issue = "33417")]
impl fmt::Display for CharTryFromError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
"converted integer out of range for `char`".fmt(f)
}
}

/// Converts a digit in the given radix to a `char`.
///
/// A 'radix' here is sometimes also called a 'base'. A radix of two
Expand Down
18 changes: 18 additions & 0 deletions src/libcoretest/char.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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() {
Expand Down
2 changes: 2 additions & 0 deletions src/librustc_unicode/char.rs
Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,8 @@ 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")]
Expand Down
1 change: 1 addition & 0 deletions src/librustc_unicode/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@
#![feature(fused)]
#![feature(lang_items)]
#![feature(staged_api)]
#![feature(try_from)]
#![feature(unicode)]

mod tables;
Expand Down
7 changes: 7 additions & 0 deletions src/libstd/error.rs
Original file line number Diff line number Diff line change
Expand Up @@ -302,6 +302,13 @@ impl<'a, T: ?Sized + Reflect> Error for cell::BorrowMutError<'a, T> {
}
}

#[unstable(feature = "try_from", issue = "33417")]
impl Error for char::CharTryFromError {
fn description(&self) -> &str {
"converted integer out of range for `char`"
}
}

// copied from any.rs
impl Error + 'static {
/// Returns true if the boxed type is the same as `T`
Expand Down

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