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]
24pub(super) const unsafe fn from_u32_unchecked(i: u32) -> char {
25    // SAFETY: the caller must guarantee that `i` is a valid char value.
26    unsafe {
27        assert_unsafe_precondition!(
28            check_language_ub,
29            "invalid value for `char`",
30            (i: u32 = i) => char_try_from_u32(i).is_ok()
31        );
32        transmute(i)
33    }
34}
35
36#[stable(feature = "char_convert", since = "1.13.0")]
37impl From<char> for u32 {
38    /// Converts a [`char`] into a [`u32`].
39    ///
40    /// # Examples
41    ///
42    /// ```
43    /// use std::mem;
44    ///
45    /// let c = 'c';
46    /// let u = u32::from(c);
47    /// assert!(4 == mem::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    /// use std::mem;
63    ///
64    /// let c = '👤';
65    /// let u = u64::from(c);
66    /// assert!(8 == mem::size_of_val(&u))
67    /// ```
68    #[inline]
69    fn from(c: char) -> Self {
70        // The char is casted to the value of the code point, then zero-extended to 64 bit.
71        // See [https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics]
72        c as u64
73    }
74}
75
76#[stable(feature = "more_char_conversions", since = "1.51.0")]
77impl From<char> for u128 {
78    /// Converts a [`char`] into a [`u128`].
79    ///
80    /// # Examples
81    ///
82    /// ```
83    /// use std::mem;
84    ///
85    /// let c = '⚙';
86    /// let u = u128::from(c);
87    /// assert!(16 == mem::size_of_val(&u))
88    /// ```
89    #[inline]
90    fn from(c: char) -> Self {
91        // The char is casted to the value of the code point, then zero-extended to 128 bit.
92        // See [https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics]
93        c as u128
94    }
95}
96
97/// Maps a `char` with code point in U+0000..=U+00FF to a byte in 0x00..=0xFF with same value,
98/// failing if the code point is greater than U+00FF.
99///
100/// See [`impl From<u8> for char`](char#impl-From<u8>-for-char) for details on the encoding.
101#[stable(feature = "u8_from_char", since = "1.59.0")]
102impl TryFrom<char> for u8 {
103    type Error = TryFromCharError;
104
105    /// Tries to convert a [`char`] into a [`u8`].
106    ///
107    /// # Examples
108    ///
109    /// ```
110    /// let a = 'ÿ'; // U+00FF
111    /// let b = 'Ā'; // U+0100
112    /// assert_eq!(u8::try_from(a), Ok(0xFF_u8));
113    /// assert!(u8::try_from(b).is_err());
114    /// ```
115    #[inline]
116    fn try_from(c: char) -> Result<u8, Self::Error> {
117        u8::try_from(u32::from(c)).map_err(|_| TryFromCharError(()))
118    }
119}
120
121/// Maps a `char` with code point in U+0000..=U+FFFF to a `u16` in 0x0000..=0xFFFF with same value,
122/// failing if the code point is greater than U+FFFF.
123///
124/// This corresponds to the UCS-2 encoding, as specified in ISO/IEC 10646:2003.
125#[stable(feature = "u16_from_char", since = "1.74.0")]
126impl TryFrom<char> for u16 {
127    type Error = TryFromCharError;
128
129    /// Tries to convert a [`char`] into a [`u16`].
130    ///
131    /// # Examples
132    ///
133    /// ```
134    /// let trans_rights = '⚧'; // U+26A7
135    /// let ninjas = '🥷'; // U+1F977
136    /// assert_eq!(u16::try_from(trans_rights), Ok(0x26A7_u16));
137    /// assert!(u16::try_from(ninjas).is_err());
138    /// ```
139    #[inline]
140    fn try_from(c: char) -> Result<u16, Self::Error> {
141        u16::try_from(u32::from(c)).map_err(|_| TryFromCharError(()))
142    }
143}
144
145/// Maps a byte in 0x00..=0xFF to a `char` whose code point has the same value, in U+0000..=U+00FF.
146///
147/// Unicode is designed such that this effectively decodes bytes
148/// with the character encoding that IANA calls ISO-8859-1.
149/// This encoding is compatible with ASCII.
150///
151/// Note that this is different from ISO/IEC 8859-1 a.k.a. ISO 8859-1 (with one less hyphen),
152/// which leaves some "blanks", byte values that are not assigned to any character.
153/// ISO-8859-1 (the IANA one) assigns them to the C0 and C1 control codes.
154///
155/// Note that this is *also* different from Windows-1252 a.k.a. code page 1252,
156/// which is a superset ISO/IEC 8859-1 that assigns some (not all!) blanks
157/// to punctuation and various Latin characters.
158///
159/// To confuse things further, [on the Web](https://encoding.spec.whatwg.org/)
160/// `ascii`, `iso-8859-1`, and `windows-1252` are all aliases
161/// for a superset of Windows-1252 that fills the remaining blanks with corresponding
162/// C0 and C1 control codes.
163#[stable(feature = "char_convert", since = "1.13.0")]
164impl From<u8> for char {
165    /// Converts a [`u8`] into a [`char`].
166    ///
167    /// # Examples
168    ///
169    /// ```
170    /// use std::mem;
171    ///
172    /// let u = 32 as u8;
173    /// let c = char::from(u);
174    /// assert!(4 == mem::size_of_val(&c))
175    /// ```
176    #[inline]
177    fn from(i: u8) -> Self {
178        i as char
179    }
180}
181
182/// An error which can be returned when parsing a char.
183///
184/// This `struct` is created when using the [`char::from_str`] method.
185#[stable(feature = "char_from_str", since = "1.20.0")]
186#[derive(Clone, Debug, PartialEq, Eq)]
187pub struct ParseCharError {
188    kind: CharErrorKind,
189}
190
191#[derive(Copy, Clone, Debug, PartialEq, Eq)]
192enum CharErrorKind {
193    EmptyString,
194    TooManyChars,
195}
196
197#[stable(feature = "char_from_str", since = "1.20.0")]
198impl Error for ParseCharError {
199    #[allow(deprecated)]
200    fn description(&self) -> &str {
201        match self.kind {
202            CharErrorKind::EmptyString => "cannot parse char from empty string",
203            CharErrorKind::TooManyChars => "too many characters in string",
204        }
205    }
206}
207
208#[stable(feature = "char_from_str", since = "1.20.0")]
209impl fmt::Display for ParseCharError {
210    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
211        #[allow(deprecated)]
212        self.description().fmt(f)
213    }
214}
215
216#[stable(feature = "char_from_str", since = "1.20.0")]
217impl FromStr for char {
218    type Err = ParseCharError;
219
220    #[inline]
221    fn from_str(s: &str) -> Result<Self, Self::Err> {
222        let mut chars = s.chars();
223        match (chars.next(), chars.next()) {
224            (None, _) => Err(ParseCharError { kind: CharErrorKind::EmptyString }),
225            (Some(c), None) => Ok(c),
226            _ => Err(ParseCharError { kind: CharErrorKind::TooManyChars }),
227        }
228    }
229}
230
231#[inline]
232const fn char_try_from_u32(i: u32) -> Result<char, CharTryFromError> {
233    // This is an optimized version of the check
234    // (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF),
235    // which can also be written as
236    // i >= 0x110000 || (i >= 0xD800 && i < 0xE000).
237    //
238    // The XOR with 0xD800 permutes the ranges such that 0xD800..0xE000 is
239    // mapped to 0x0000..0x0800, while keeping all the high bits outside 0xFFFF the same.
240    // In particular, numbers >= 0x110000 stay in this range.
241    //
242    // Subtracting 0x800 causes 0x0000..0x0800 to wrap, meaning that a single
243    // unsigned comparison against 0x110000 - 0x800 will detect both the wrapped
244    // surrogate range as well as the numbers originally larger than 0x110000.
245    //
246    if (i ^ 0xD800).wrapping_sub(0x800) >= 0x110000 - 0x800 {
247        Err(CharTryFromError(()))
248    } else {
249        // SAFETY: checked that it's a legal unicode value
250        Ok(unsafe { transmute(i) })
251    }
252}
253
254#[stable(feature = "try_from", since = "1.34.0")]
255impl TryFrom<u32> for char {
256    type Error = CharTryFromError;
257
258    #[inline]
259    fn try_from(i: u32) -> Result<Self, Self::Error> {
260        char_try_from_u32(i)
261    }
262}
263
264/// The error type returned when a conversion from [`prim@u32`] to [`prim@char`] fails.
265///
266/// This `struct` is created by the [`char::try_from<u32>`](char#impl-TryFrom<u32>-for-char) method.
267/// See its documentation for more.
268#[stable(feature = "try_from", since = "1.34.0")]
269#[derive(Copy, Clone, Debug, PartialEq, Eq)]
270pub struct CharTryFromError(());
271
272#[stable(feature = "try_from", since = "1.34.0")]
273impl fmt::Display for CharTryFromError {
274    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
275        "converted integer out of range for `char`".fmt(f)
276    }
277}
278
279/// Converts a digit in the given radix to a `char`. See [`char::from_digit`].
280#[inline]
281#[must_use]
282pub(super) const fn from_digit(num: u32, radix: u32) -> Option<char> {
283    if radix > 36 {
284        panic!("from_digit: radix is too high (maximum 36)");
285    }
286    if num < radix {
287        let num = num as u8;
288        if num < 10 { Some((b'0' + num) as char) } else { Some((b'a' + num - 10) as char) }
289    } else {
290        None
291    }
292}