Replace get_split with split_first_chunk builtin

[`split_first_chunk`][0] is a nightly only builtin. It looks like it
will be [stabillized soon][1], so I decided to use it and copy over the
implementation in the meantime.

Amusingly, benchmark show a consistent 3% improvement to throughput
compared to using `get_split`.

[0]: https://doc.rust-lang.org/std/primitive.slice.html#method.split_first_chunk
[1]: rust-lang/rust#117561

Author: nickbabcock

src/binary/de.rs

@@ -2,7 +2,7 @@ use super::{tokens::*, Rgb};
 use crate::{
     binary::{BinaryFlavor, FailedResolveStrategy, TokenResolver},
     de::ColorSequence,
-    util::get_split,
+    util::split_first_chunk,
     BinaryTape, BinaryToken, DeserializeError, DeserializeErrorKind, Error, ErrorKind,
 };
 use serde::de::{self, Deserialize, DeserializeSeed, MapAccess, SeqAccess, Visitor};
@@ -24,8 +24,8 @@ impl<'data> OndemandParser<'data> {
 
     #[inline]
     pub fn next(&mut self) -> Option<u16> {
-        let (data, token) =
-            get_split::<2>(self.data).map(|(head, rest)| (rest, u16::from_le_bytes(head)))?;
+        let (data, token) = split_first_chunk::<2>(self.data)
+            .map(|(head, rest)| (rest, u16::from_le_bytes(*head)))?;
         self.data = data;
         Some(token)
     }
@@ -37,8 +37,8 @@ impl<'data> OndemandParser<'data> {
 
     #[inline]
     pub fn read_string(&mut self) -> Result<&'data [u8], Error> {
-        let (head, rest) = get_split::<2>(self.data).ok_or_else(Error::eof)?;
-        let text_len = usize::from(u16::from_le_bytes(head));
+        let (head, rest) = split_first_chunk::<2>(self.data).ok_or_else(Error::eof)?;
+        let text_len = usize::from(u16::from_le_bytes(*head));
         if text_len <= rest.len() {
             let (text, rest) = rest.split_at(text_len);
             self.data = rest;
@@ -57,44 +57,44 @@ impl<'data> OndemandParser<'data> {
 
     #[inline]
     fn read_u32(&mut self) -> Result<u32, Error> {
-        let (head, rest) = get_split::<4>(self.data).ok_or_else(Error::eof)?;
+        let (head, rest) = split_first_chunk::<4>(self.data).ok_or_else(Error::eof)?;
         self.data = rest;
-        Ok(u32::from_le_bytes(head))
+        Ok(u32::from_le_bytes(*head))
     }
 
     #[inline]
     fn read_u64(&mut self) -> Result<u64, Error> {
-        let (head, rest) = get_split::<8>(self.data).ok_or_else(Error::eof)?;
+        let (head, rest) = split_first_chunk::<8>(self.data).ok_or_else(Error::eof)?;
         self.data = rest;
-        Ok(u64::from_le_bytes(head))
+        Ok(u64::from_le_bytes(*head))
     }
 
     #[inline]
     fn read_i64(&mut self) -> Result<i64, Error> {
-        let (head, rest) = get_split::<8>(self.data).ok_or_else(Error::eof)?;
+        let (head, rest) = split_first_chunk::<8>(self.data).ok_or_else(Error::eof)?;
         self.data = rest;
-        Ok(i64::from_le_bytes(head))
+        Ok(i64::from_le_bytes(*head))
     }
 
     #[inline]
     fn read_i32(&mut self) -> Result<i32, Error> {
-        let (head, rest) = get_split::<4>(self.data).ok_or_else(Error::eof)?;
+        let (head, rest) = split_first_chunk::<4>(self.data).ok_or_else(Error::eof)?;
         self.data = rest;
-        Ok(i32::from_le_bytes(head))
+        Ok(i32::from_le_bytes(*head))
     }
 
     #[inline]
     fn read_f32(&mut self) -> Result<[u8; 4], Error> {
-        let (head, rest) = get_split::<4>(self.data).ok_or_else(Error::eof)?;
+        let (head, rest) = split_first_chunk::<4>(self.data).ok_or_else(Error::eof)?;
         self.data = rest;
-        Ok(head)
+        Ok(*head)
     }
 
     #[inline]
     fn read_f64(&mut self) -> Result<[u8; 8], Error> {
-        let (head, rest) = get_split::<8>(self.data).ok_or_else(Error::eof)?;
+        let (head, rest) = split_first_chunk::<8>(self.data).ok_or_else(Error::eof)?;
         self.data = rest;
-        Ok(head)
+        Ok(*head)
     }
 
     #[inline]

src/util.rs

@@ -5,6 +5,32 @@ fn take<const N: usize>(data: &[u8]) -> [u8; N] {
     unsafe { *(data.as_ptr() as *const [u8; N]) }
 }
 
+#[inline]
+pub const fn split_first_chunk<const N: usize>(data: &[u8]) -> Option<(&[u8; N], &[u8])> {
+    #[inline]
+    const unsafe fn split_at_unchecked(data: &[u8], mid: usize) -> (&[u8], &[u8]) {
+        let len = data.len();
+        let ptr = data.as_ptr();
+        unsafe {
+            (
+                core::slice::from_raw_parts(ptr, mid),
+                core::slice::from_raw_parts(ptr.add(mid), len - mid),
+            )
+        }
+    }
+
+    if data.len() < N {
+        None
+    } else {
+        // SAFETY: We manually verified the bounds of the split.
+        let (first, tail) = unsafe { split_at_unchecked(data, N) };
+
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   and do not let the references outlive the slice.
+        Some((unsafe { &*(first.as_ptr().cast::<[u8; N]>()) }, tail))
+    }
+}
+
 #[inline]
 pub(crate) fn get_split<const N: usize>(data: &[u8]) -> Option<([u8; N], &[u8])> {
     data.get(N..).map(|d| (take::<N>(data), d))