core/char/
convert.rs

1//! Character conversions.
2
3use crate::char::TryFromCharError;
4use crate::error::Error;
5use crate::fmt;
6use crate::mem::transmute;
7use crate::str::FromStr;
8use crate::ub_checks::assert_unsafe_precondition;
9
10/// Converts a `u32` to a `char`. See [`char::from_u32`].
11#[must_use]
12#[inline]
13pub(super) const fn from_u32(i: u32) -> Option<char> {
14    // FIXME(const-hack): once Result::ok is const fn, use it here
15    match char_try_from_u32(i) {
16        Ok(c) => Some(c),
17        Err(_) => None,
18    }
19}
20
21/// Converts a `u32` to a `char`, ignoring validity. See [`char::from_u32_unchecked`].
22#[inline]
23#[must_use]
24#[allow(unnecessary_transmutes)]
25#[track_caller]
26pub(super) const unsafe fn from_u32_unchecked(i: u32) -> char {
27    // SAFETY: the caller must guarantee that `i` is a valid char value.
28    unsafe {
29        assert_unsafe_precondition!(
30            check_language_ub,
31            "invalid value for `char`",
32            (i: u32 = i) => char_try_from_u32(i).is_ok()
33        );
34        transmute(i)
35    }
36}
37
38#[stable(feature = "char_convert", since = "1.13.0")]
39impl From<char> for u32 {
40    /// Converts a [`char`] into a [`u32`].
41    ///
42    /// # Examples
43    ///
44    /// ```
45    /// let c = 'c';
46    /// let u = u32::from(c);
47    /// assert!(4 == size_of_val(&u))
48    /// ```
49    #[inline]
50    fn from(c: char) -> Self {
51        c as u32
52    }
53}
54
55#[stable(feature = "more_char_conversions", since = "1.51.0")]
56impl From<char> for u64 {
57    /// Converts a [`char`] into a [`u64`].
58    ///
59    /// # Examples
60    ///
61    /// ```
62    /// let c = '👤';
63    /// let u = u64::from(c);
64    /// assert!(8 == size_of_val(&u))
65    /// ```
66    #[inline]
67    fn from(c: char) -> Self {
68        // The char is casted to the value of the code point, then zero-extended to 64 bit.
69        // See [https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics]
70        c as u64
71    }
72}
73
74#[stable(feature = "more_char_conversions", since = "1.51.0")]
75impl From<char> for u128 {
76    /// Converts a [`char`] into a [`u128`].
77    ///
78    /// # Examples
79    ///
80    /// ```
81    /// let c = '⚙';
82    /// let u = u128::from(c);
83    /// assert!(16 == size_of_val(&u))
84    /// ```
85    #[inline]
86    fn from(c: char) -> Self {
87        // The char is casted to the value of the code point, then zero-extended to 128 bit.
88        // See [https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics]
89        c as u128
90    }
91}
92
93/// Maps a `char` with code point in U+0000..=U+00FF to a byte in 0x00..=0xFF with same value,
94/// failing if the code point is greater than U+00FF.
95///
96/// See [`impl From<u8> for char`](char#impl-From<u8>-for-char) for details on the encoding.
97#[stable(feature = "u8_from_char", since = "1.59.0")]
98impl TryFrom<char> for u8 {
99    type Error = TryFromCharError;
100
101    /// Tries to convert a [`char`] into a [`u8`].
102    ///
103    /// # Examples
104    ///
105    /// ```
106    /// let a = 'ÿ'; // U+00FF
107    /// let b = 'Ā'; // U+0100
108    /// assert_eq!(u8::try_from(a), Ok(0xFF_u8));
109    /// assert!(u8::try_from(b).is_err());
110    /// ```
111    #[inline]
112    fn try_from(c: char) -> Result<u8, Self::Error> {
113        u8::try_from(u32::from(c)).map_err(|_| TryFromCharError(()))
114    }
115}
116
117/// Maps a `char` with code point in U+0000..=U+FFFF to a `u16` in 0x0000..=0xFFFF with same value,
118/// failing if the code point is greater than U+FFFF.
119///
120/// This corresponds to the UCS-2 encoding, as specified in ISO/IEC 10646:2003.
121#[stable(feature = "u16_from_char", since = "1.74.0")]
122impl TryFrom<char> for u16 {
123    type Error = TryFromCharError;
124
125    /// Tries to convert a [`char`] into a [`u16`].
126    ///
127    /// # Examples
128    ///
129    /// ```
130    /// let trans_rights = '⚧'; // U+26A7
131    /// let ninjas = '🥷'; // U+1F977
132    /// assert_eq!(u16::try_from(trans_rights), Ok(0x26A7_u16));
133    /// assert!(u16::try_from(ninjas).is_err());
134    /// ```
135    #[inline]
136    fn try_from(c: char) -> Result<u16, Self::Error> {
137        u16::try_from(u32::from(c)).map_err(|_| TryFromCharError(()))
138    }
139}
140
141/// Maps a byte in 0x00..=0xFF to a `char` whose code point has the same value, in U+0000..=U+00FF.
142///
143/// Unicode is designed such that this effectively decodes bytes
144/// with the character encoding that IANA calls ISO-8859-1.
145/// This encoding is compatible with ASCII.
146///
147/// Note that this is different from ISO/IEC 8859-1 a.k.a. ISO 8859-1 (with one less hyphen),
148/// which leaves some "blanks", byte values that are not assigned to any character.
149/// ISO-8859-1 (the IANA one) assigns them to the C0 and C1 control codes.
150///
151/// Note that this is *also* different from Windows-1252 a.k.a. code page 1252,
152/// which is a superset ISO/IEC 8859-1 that assigns some (not all!) blanks
153/// to punctuation and various Latin characters.
154///
155/// To confuse things further, [on the Web](https://encoding.spec.whatwg.org/)
156/// `ascii`, `iso-8859-1`, and `windows-1252` are all aliases
157/// for a superset of Windows-1252 that fills the remaining blanks with corresponding
158/// C0 and C1 control codes.
159#[stable(feature = "char_convert", since = "1.13.0")]
160impl From<u8> for char {
161    /// Converts a [`u8`] into a [`char`].
162    ///
163    /// # Examples
164    ///
165    /// ```
166    /// let u = 32 as u8;
167    /// let c = char::from(u);
168    /// assert!(4 == size_of_val(&c))
169    /// ```
170    #[inline]
171    fn from(i: u8) -> Self {
172        i as char
173    }
174}
175
176/// An error which can be returned when parsing a char.
177///
178/// This `struct` is created when using the [`char::from_str`] method.
179#[stable(feature = "char_from_str", since = "1.20.0")]
180#[derive(Clone, Debug, PartialEq, Eq)]
181pub struct ParseCharError {
182    kind: CharErrorKind,
183}
184
185#[derive(Copy, Clone, Debug, PartialEq, Eq)]
186enum CharErrorKind {
187    EmptyString,
188    TooManyChars,
189}
190
191#[stable(feature = "char_from_str", since = "1.20.0")]
192impl Error for ParseCharError {
193    #[allow(deprecated)]
194    fn description(&self) -> &str {
195        match self.kind {
196            CharErrorKind::EmptyString => "cannot parse char from empty string",
197            CharErrorKind::TooManyChars => "too many characters in string",
198        }
199    }
200}
201
202#[stable(feature = "char_from_str", since = "1.20.0")]
203impl fmt::Display for ParseCharError {
204    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
205        #[allow(deprecated)]
206        self.description().fmt(f)
207    }
208}
209
210#[stable(feature = "char_from_str", since = "1.20.0")]
211impl FromStr for char {
212    type Err = ParseCharError;
213
214    #[inline]
215    fn from_str(s: &str) -> Result<Self, Self::Err> {
216        let mut chars = s.chars();
217        match (chars.next(), chars.next()) {
218            (None, _) => Err(ParseCharError { kind: CharErrorKind::EmptyString }),
219            (Some(c), None) => Ok(c),
220            _ => Err(ParseCharError { kind: CharErrorKind::TooManyChars }),
221        }
222    }
223}
224
225#[inline]
226#[allow(unnecessary_transmutes)]
227const fn char_try_from_u32(i: u32) -> Result<char, CharTryFromError> {
228    // This is an optimized version of the check
229    // (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF),
230    // which can also be written as
231    // i >= 0x110000 || (i >= 0xD800 && i < 0xE000).
232    //
233    // The XOR with 0xD800 permutes the ranges such that 0xD800..0xE000 is
234    // mapped to 0x0000..0x0800, while keeping all the high bits outside 0xFFFF the same.
235    // In particular, numbers >= 0x110000 stay in this range.
236    //
237    // Subtracting 0x800 causes 0x0000..0x0800 to wrap, meaning that a single
238    // unsigned comparison against 0x110000 - 0x800 will detect both the wrapped
239    // surrogate range as well as the numbers originally larger than 0x110000.
240    //
241    if (i ^ 0xD800).wrapping_sub(0x800) >= 0x110000 - 0x800 {
242        Err(CharTryFromError(()))
243    } else {
244        // SAFETY: checked that it's a legal unicode value
245        Ok(unsafe { transmute(i) })
246    }
247}
248
249#[stable(feature = "try_from", since = "1.34.0")]
250impl TryFrom<u32> for char {
251    type Error = CharTryFromError;
252
253    #[inline]
254    fn try_from(i: u32) -> Result<Self, Self::Error> {
255        char_try_from_u32(i)
256    }
257}
258
259/// The error type returned when a conversion from [`prim@u32`] to [`prim@char`] fails.
260///
261/// This `struct` is created by the [`char::try_from<u32>`](char#impl-TryFrom<u32>-for-char) method.
262/// See its documentation for more.
263#[stable(feature = "try_from", since = "1.34.0")]
264#[derive(Copy, Clone, Debug, PartialEq, Eq)]
265pub struct CharTryFromError(());
266
267#[stable(feature = "try_from", since = "1.34.0")]
268impl fmt::Display for CharTryFromError {
269    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
270        "converted integer out of range for `char`".fmt(f)
271    }
272}
273
274/// Converts a digit in the given radix to a `char`. See [`char::from_digit`].
275#[inline]
276#[must_use]
277pub(super) const fn from_digit(num: u32, radix: u32) -> Option<char> {
278    if radix > 36 {
279        panic!("from_digit: radix is too high (maximum 36)");
280    }
281    if num < radix {
282        let num = num as u8;
283        if num < 10 { Some((b'0' + num) as char) } else { Some((b'a' + num - 10) as char) }
284    } else {
285        None
286    }
287}
core/char/convert.rs

core/char/
convert.rs
