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Minimize unsafety in encode_utf8 

Use slice patterns to avoid having to skip bounds checking
This commit is contained in:
Mark Rousskov 2019-12-23 15:40:20 -05:00
parent 9ae6cedb8d
commit df4d490038
2 changed files with 30 additions and 30 deletions
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59
src/libcore/char/methods.rs
View File

@ -434,36 +434,35 @@ impl char {
#[inline]
pub fn encode_utf8(self, dst: &mut [u8]) -> &mut str {
let code = self as u32;
// SAFETY: each arm checks the size of the slice and only uses `get_unchecked` unsafe ops
unsafe {
let len = if code < MAX_ONE_B && !dst.is_empty() {
*dst.get_unchecked_mut(0) = code as u8;
1
} else if code < MAX_TWO_B && dst.len() >= 2 {
*dst.get_unchecked_mut(0) = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
*dst.get_unchecked_mut(1) = (code & 0x3F) as u8 | TAG_CONT;
2
} else if code < MAX_THREE_B && dst.len() >= 3 {
*dst.get_unchecked_mut(0) = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
*dst.get_unchecked_mut(1) = (code >> 6 & 0x3F) as u8 | TAG_CONT;
*dst.get_unchecked_mut(2) = (code & 0x3F) as u8 | TAG_CONT;
3
} else if dst.len() >= 4 {
*dst.get_unchecked_mut(0) = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
*dst.get_unchecked_mut(1) = (code >> 12 & 0x3F) as u8 | TAG_CONT;
*dst.get_unchecked_mut(2) = (code >> 6 & 0x3F) as u8 | TAG_CONT;
*dst.get_unchecked_mut(3) = (code & 0x3F) as u8 | TAG_CONT;
4
} else {
panic!(
"encode_utf8: need {} bytes to encode U+{:X}, but the buffer has {}",
from_u32_unchecked(code).len_utf8(),
code,
dst.len(),
)
};
from_utf8_unchecked_mut(dst.get_unchecked_mut(..len))
}
let len = self.len_utf8();
match (len, &mut dst[..]) {
(1, [a, ..]) => {
*a = code as u8;
}
(2, [a, b, ..]) => {
*a = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
*b = (code & 0x3F) as u8 | TAG_CONT;
}
(3, [a, b, c, ..]) => {
*a = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
*b = (code >> 6 & 0x3F) as u8 | TAG_CONT;
*c = (code & 0x3F) as u8 | TAG_CONT;
}
(4, [a, b, c, d, ..]) => {
*a = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
*b = (code >> 12 & 0x3F) as u8 | TAG_CONT;
*c = (code >> 6 & 0x3F) as u8 | TAG_CONT;
*d = (code & 0x3F) as u8 | TAG_CONT;
}
_ => panic!(
"encode_utf8: need {} bytes to encode U+{:X}, but the buffer has {}",
len,
code,
dst.len(),
),
};
// SAFETY: We just wrote UTF-8 content in, so converting to str is fine.
unsafe { from_utf8_unchecked_mut(&mut dst[..len]) }
}
/// Encodes this character as UTF-16 into the provided `u16` buffer,

1
src/libcore/lib.rs
View File

@ -129,6 +129,7 @@
#![feature(associated_type_bounds)]
#![feature(const_type_id)]
#![feature(const_caller_location)]
#![feature(slice_patterns)]
#[prelude_import]
#[allow(unused)]