core/str/
validations.rs

1//! Operations related to UTF-8 validation.
2
3use super::Utf8Error;
4use crate::intrinsics::const_eval_select;
5
6/// Returns the initial codepoint accumulator for the first byte.
7/// The first byte is special, only want bottom 5 bits for width 2, 4 bits
8/// for width 3, and 3 bits for width 4.
9#[inline]
10const fn utf8_first_byte(byte: u8, width: u32) -> u32 {
11    (byte & (0x7F >> width)) as u32
12}
13
14/// Returns the value of `ch` updated with continuation byte `byte`.
15#[inline]
16const fn utf8_acc_cont_byte(ch: u32, byte: u8) -> u32 {
17    (ch << 6) | (byte & CONT_MASK) as u32
18}
19
20/// Checks whether the byte is a UTF-8 continuation byte (i.e., starts with the
21/// bits `10`).
22#[inline]
23pub(super) const fn utf8_is_cont_byte(byte: u8) -> bool {
24    (byte as i8) < -64
25}
26
27/// Reads the next code point out of a byte iterator (assuming a
28/// UTF-8-like encoding).
29///
30/// # Safety
31///
32/// `bytes` must produce a valid UTF-8-like (UTF-8 or WTF-8) string
33#[unstable(feature = "str_internals", issue = "none")]
34#[inline]
35pub unsafe fn next_code_point<'a, I: Iterator<Item = &'a u8>>(bytes: &mut I) -> Option<u32> {
36    // Decode UTF-8
37    let x = *bytes.next()?;
38    if x < 128 {
39        return Some(x as u32);
40    }
41
42    // Multibyte case follows
43    // Decode from a byte combination out of: [[[x y] z] w]
44    // NOTE: Performance is sensitive to the exact formulation here
45    let init = utf8_first_byte(x, 2);
46    // SAFETY: `bytes` produces an UTF-8-like string,
47    // so the iterator must produce a value here.
48    let y = unsafe { *bytes.next().unwrap_unchecked() };
49    let mut ch = utf8_acc_cont_byte(init, y);
50    if x >= 0xE0 {
51        // [[x y z] w] case
52        // 5th bit in 0xE0 .. 0xEF is always clear, so `init` is still valid
53        // SAFETY: `bytes` produces an UTF-8-like string,
54        // so the iterator must produce a value here.
55        let z = unsafe { *bytes.next().unwrap_unchecked() };
56        let y_z = utf8_acc_cont_byte((y & CONT_MASK) as u32, z);
57        ch = init << 12 | y_z;
58        if x >= 0xF0 {
59            // [x y z w] case
60            // use only the lower 3 bits of `init`
61            // SAFETY: `bytes` produces an UTF-8-like string,
62            // so the iterator must produce a value here.
63            let w = unsafe { *bytes.next().unwrap_unchecked() };
64            ch = (init & 7) << 18 | utf8_acc_cont_byte(y_z, w);
65        }
66    }
67
68    Some(ch)
69}
70
71/// Reads the last code point out of a byte iterator (assuming a
72/// UTF-8-like encoding).
73///
74/// # Safety
75///
76/// `bytes` must produce a valid UTF-8-like (UTF-8 or WTF-8) string
77#[inline]
78pub(super) unsafe fn next_code_point_reverse<'a, I>(bytes: &mut I) -> Option<u32>
79where
80    I: DoubleEndedIterator<Item = &'a u8>,
81{
82    // Decode UTF-8
83    let w = match *bytes.next_back()? {
84        next_byte if next_byte < 128 => return Some(next_byte as u32),
85        back_byte => back_byte,
86    };
87
88    // Multibyte case follows
89    // Decode from a byte combination out of: [x [y [z w]]]
90    let mut ch;
91    // SAFETY: `bytes` produces an UTF-8-like string,
92    // so the iterator must produce a value here.
93    let z = unsafe { *bytes.next_back().unwrap_unchecked() };
94    ch = utf8_first_byte(z, 2);
95    if utf8_is_cont_byte(z) {
96        // SAFETY: `bytes` produces an UTF-8-like string,
97        // so the iterator must produce a value here.
98        let y = unsafe { *bytes.next_back().unwrap_unchecked() };
99        ch = utf8_first_byte(y, 3);
100        if utf8_is_cont_byte(y) {
101            // SAFETY: `bytes` produces an UTF-8-like string,
102            // so the iterator must produce a value here.
103            let x = unsafe { *bytes.next_back().unwrap_unchecked() };
104            ch = utf8_first_byte(x, 4);
105            ch = utf8_acc_cont_byte(ch, y);
106        }
107        ch = utf8_acc_cont_byte(ch, z);
108    }
109    ch = utf8_acc_cont_byte(ch, w);
110
111    Some(ch)
112}
113
114const NONASCII_MASK: usize = usize::repeat_u8(0x80);
115
116/// Returns `true` if any byte in the word `x` is nonascii (>= 128).
117#[inline]
118const fn contains_nonascii(x: usize) -> bool {
119    (x & NONASCII_MASK) != 0
120}
121
122/// Walks through `v` checking that it's a valid UTF-8 sequence,
123/// returning `Ok(())` in that case, or, if it is invalid, `Err(err)`.
124#[inline(always)]
125#[rustc_allow_const_fn_unstable(const_eval_select)] // fallback impl has same behavior
126pub(super) const fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
127    let mut index = 0;
128    let len = v.len();
129
130    const USIZE_BYTES: usize = size_of::<usize>();
131
132    let ascii_block_size = 2 * USIZE_BYTES;
133    let blocks_end = if len >= ascii_block_size { len - ascii_block_size + 1 } else { 0 };
134    // Below, we safely fall back to a slower codepath if the offset is `usize::MAX`,
135    // so the end-to-end behavior is the same at compiletime and runtime.
136    let align = {
    #[inline]
    fn runtime(v: &[u8]) -> usize { { v.as_ptr().align_offset(USIZE_BYTES) } }
    #[inline]
    const fn compiletime(v: &[u8]) -> usize { let _ = v; { usize::MAX } }
    const_eval_select((v,), compiletime, runtime)
}const_eval_select!(
137        @capture { v: &[u8] } -> usize:
138        if const {
139            usize::MAX
140        } else {
141            v.as_ptr().align_offset(USIZE_BYTES)
142        }
143    );
144
145    while index < len {
146        let old_offset = index;
147        macro_rules! err {
148            ($error_len: expr) => {
149                return Err(Utf8Error { valid_up_to: old_offset, error_len: $error_len })
150            };
151        }
152
153        macro_rules! next {
154            () => {{
155                index += 1;
156                // we needed data, but there was none: error!
157                if index >= len {
158                    err!(None)
159                }
160                v[index]
161            }};
162        }
163
164        let first = v[index];
165        if first >= 128 {
166            let w = utf8_char_width(first);
167            // 2-byte encoding is for codepoints  \u{0080} to  \u{07ff}
168            //        first  C2 80        last DF BF
169            // 3-byte encoding is for codepoints  \u{0800} to  \u{ffff}
170            //        first  E0 A0 80     last EF BF BF
171            //   excluding surrogates codepoints  \u{d800} to  \u{dfff}
172            //               ED A0 80 to       ED BF BF
173            // 4-byte encoding is for codepoints \u{10000} to \u{10ffff}
174            //        first  F0 90 80 80  last F4 8F BF BF
175            //
176            // Use the UTF-8 syntax from the RFC
177            //
178            // https://tools.ietf.org/html/rfc3629
179            // UTF8-1      = %x00-7F
180            // UTF8-2      = %xC2-DF UTF8-tail
181            // UTF8-3      = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
182            //               %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
183            // UTF8-4      = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
184            //               %xF4 %x80-8F 2( UTF8-tail )
185            match w {
186                2 => {
187                    if {
    index += 1;
    if index >= len {
        return Err(Utf8Error { valid_up_to: old_offset, error_len: None })
    }
    v[index]
}next!() as i8 >= -64 {
188                        return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(1) })err!(Some(1))
189                    }
190                }
191                3 => {
192                    match (first, {
    index += 1;
    if index >= len {
        return Err(Utf8Error { valid_up_to: old_offset, error_len: None })
    }
    v[index]
}next!()) {
193                        (0xE0, 0xA0..=0xBF)
194                        | (0xE1..=0xEC, 0x80..=0xBF)
195                        | (0xED, 0x80..=0x9F)
196                        | (0xEE..=0xEF, 0x80..=0xBF) => {}
197                        _ => return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(1) })err!(Some(1)),
198                    }
199                    if {
    index += 1;
    if index >= len {
        return Err(Utf8Error { valid_up_to: old_offset, error_len: None })
    }
    v[index]
}next!() as i8 >= -64 {
200                        return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(2) })err!(Some(2))
201                    }
202                }
203                4 => {
204                    match (first, {
    index += 1;
    if index >= len {
        return Err(Utf8Error { valid_up_to: old_offset, error_len: None })
    }
    v[index]
}next!()) {
205                        (0xF0, 0x90..=0xBF) | (0xF1..=0xF3, 0x80..=0xBF) | (0xF4, 0x80..=0x8F) => {}
206                        _ => return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(1) })err!(Some(1)),
207                    }
208                    if {
    index += 1;
    if index >= len {
        return Err(Utf8Error { valid_up_to: old_offset, error_len: None })
    }
    v[index]
}next!() as i8 >= -64 {
209                        return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(2) })err!(Some(2))
210                    }
211                    if {
    index += 1;
    if index >= len {
        return Err(Utf8Error { valid_up_to: old_offset, error_len: None })
    }
    v[index]
}next!() as i8 >= -64 {
212                        return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(3) })err!(Some(3))
213                    }
214                }
215                _ => return Err(Utf8Error { valid_up_to: old_offset, error_len: Some(1) })err!(Some(1)),
216            }
217            index += 1;
218        } else {
219            // Ascii case, try to skip forward quickly.
220            // When the pointer is aligned, read 2 words of data per iteration
221            // until we find a word containing a non-ascii byte.
222            if align != usize::MAX && align.wrapping_sub(index).is_multiple_of(USIZE_BYTES) {
223                let ptr = v.as_ptr();
224                while index < blocks_end {
225                    // SAFETY: since `align - index` and `ascii_block_size` are
226                    // multiples of `USIZE_BYTES`, `block = ptr.add(index)` is
227                    // always aligned with a `usize` so it's safe to dereference
228                    // both `block` and `block.add(1)`.
229                    unsafe {
230                        let block = ptr.add(index) as *const usize;
231                        // break if there is a nonascii byte
232                        let zu = contains_nonascii(*block);
233                        let zv = contains_nonascii(*block.add(1));
234                        if zu || zv {
235                            break;
236                        }
237                    }
238                    index += ascii_block_size;
239                }
240                // step from the point where the wordwise loop stopped
241                while index < len && v[index] < 128 {
242                    index += 1;
243                }
244            } else {
245                index += 1;
246            }
247        }
248    }
249
250    Ok(())
251}
252
253// https://tools.ietf.org/html/rfc3629
254const UTF8_CHAR_WIDTH: &[u8; 256] = &[
255    // 1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
256    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0
257    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 1
258    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 2
259    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 3
260    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 4
261    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 5
262    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 6
263    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 7
264    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 8
265    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 9
266    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // A
267    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // B
268    0, 0, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, // C
269    2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, // D
270    3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // E
271    4, 4, 4, 4, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // F
272];
273
274/// Given a first byte, determines how many bytes are in this UTF-8 character.
275#[unstable(feature = "str_internals", issue = "none")]
276#[must_use]
277#[inline]
278pub const fn utf8_char_width(b: u8) -> usize {
279    UTF8_CHAR_WIDTH[b as usize] as usize
280}
281
282/// Mask of the value bits of a continuation byte.
283const CONT_MASK: u8 = 0b0011_1111;
core/str/validations.rs

core/str/
validations.rs
