Refactor how opaque::Decoder represents its contents

Instead of (&[u8], position) simply store the &[u8] and reslice.

This was originally written for #58475, to see if removing the arithmetic helped with avoiding integer overflow checks, however I think the result is slightly more readable in general -- specifically the removal of set_position is a nice win. I think this might be a hair faster even without the changes in #58475, but I haven't measured that.

src/librustc/ty/query/on_disk_cache.rs

@@ -105,22 +105,21 @@ impl AbsoluteBytePos {
 
 impl<'sess> OnDiskCache<'sess> {
     /// Creates a new OnDiskCache instance from the serialized data in `data`.
-    pub fn new(sess: &'sess Session, data: Vec<u8>, start_pos: usize) -> OnDiskCache<'sess> {
+    pub fn new(sess: &'sess Session, data: Vec<u8>) -> OnDiskCache<'sess> {
         debug_assert!(sess.opts.incremental.is_some());
 
         // Wrapping in a scope so we can borrow `data`
         let footer: Footer = {
-            let mut decoder = opaque::Decoder::new(&data[..], start_pos);
-
             // Decode the *position* of the footer which can be found in the
             // last 8 bytes of the file.
-            decoder.set_position(data.len() - IntEncodedWithFixedSize::ENCODED_SIZE);
+            let mut decoder = opaque::Decoder::new(
+                &data, data.len() - IntEncodedWithFixedSize::ENCODED_SIZE);
             let query_result_index_pos = IntEncodedWithFixedSize::decode(&mut decoder)
                 .expect("Error while trying to decode query result index position.")
                 .0 as usize;
 
             // Decoder the file footer which contains all the lookup tables, etc.
-            decoder.set_position(query_result_index_pos);
+            decoder = opaque::Decoder::new(&data, query_result_index_pos);
             decode_tagged(&mut decoder, TAG_FILE_FOOTER)
                 .expect("Error while trying to decode query result index position.")
         };
@@ -540,7 +539,7 @@ impl<'a, 'tcx: 'a, 'x> ty_codec::TyDecoder<'a, 'tcx> for CacheDecoder<'a, 'tcx,
 
     #[inline]
     fn peek_byte(&self) -> u8 {
-        self.opaque.data[self.opaque.position()]
+        self.opaque.data()[0]
     }
 
     fn cached_ty_for_shorthand<F>(&mut self,
@@ -569,9 +568,9 @@ impl<'a, 'tcx: 'a, 'x> ty_codec::TyDecoder<'a, 'tcx> for CacheDecoder<'a, 'tcx,
     fn with_position<F, R>(&mut self, pos: usize, f: F) -> R
         where F: FnOnce(&mut Self) -> R
     {
-        debug_assert!(pos < self.opaque.data.len());
+        debug_assert!(pos < self.opaque.original_data.len());
 
-        let new_opaque = opaque::Decoder::new(self.opaque.data, pos);
+        let new_opaque = opaque::Decoder::new(&self.opaque.original_data, pos);
         let old_opaque = mem::replace(&mut self.opaque, new_opaque);
         let r = f(self);
         self.opaque = old_opaque;

src/librustc_incremental/persist/load.rs

@@ -200,7 +200,7 @@ pub fn load_query_result_cache<'sess>(sess: &'sess Session) -> OnDiskCache<'sess
     }
 
     match load_data(sess.opts.debugging_opts.incremental_info, &query_cache_path(sess)) {
-        LoadResult::Ok{ data: (bytes, start_pos) } => OnDiskCache::new(sess, bytes, start_pos),
+        LoadResult::Ok{ data: (bytes, _) } => OnDiskCache::new(sess, bytes),
         _ => OnDiskCache::new_empty(sess.source_map())
     }
 }

src/librustc_metadata/decoder.rs

@@ -179,7 +179,7 @@ impl<'a, 'tcx: 'a> TyDecoder<'a, 'tcx> for DecodeContext<'a, 'tcx> {
 
     #[inline]
     fn peek_byte(&self) -> u8 {
-        self.opaque.data[self.opaque.position()]
+        self.opaque.data()[0]
     }
 
     #[inline]
@@ -212,7 +212,7 @@ impl<'a, 'tcx: 'a> TyDecoder<'a, 'tcx> for DecodeContext<'a, 'tcx> {
     fn with_position<F, R>(&mut self, pos: usize, f: F) -> R
         where F: FnOnce(&mut Self) -> R
     {
-        let new_opaque = opaque::Decoder::new(self.opaque.data, pos);
+        let new_opaque = opaque::Decoder::new(self.opaque.original_data, pos);
         let old_opaque = mem::replace(&mut self.opaque, new_opaque);
         let old_state = mem::replace(&mut self.lazy_state, LazyState::NoNode);
         let r = f(self);

src/libserialize/leb128.rs

@@ -114,10 +114,10 @@ pub fn write_signed_leb128(out: &mut Vec<u8>, value: i128) {
 }
 
 #[inline]
-pub fn read_signed_leb128(data: &[u8], start_position: usize) -> (i128, usize) {
+pub fn read_signed_leb128(data: &[u8]) -> (i128, usize) {
     let mut result = 0;
     let mut shift = 0;
-    let mut position = start_position;
+    let mut position = 0;
     let mut byte;
 
     loop {
@@ -136,7 +136,7 @@ pub fn read_signed_leb128(data: &[u8], start_position: usize) -> (i128, usize) {
         result |= -(1 << shift);
     }
 
-    (result, position - start_position)
+    (result, position)
 }
 
 macro_rules! impl_test_unsigned_leb128 {
@@ -176,7 +176,7 @@ fn test_signed_leb128() {
     }
     let mut pos = 0;
     for &x in &values {
-        let (value, bytes_read) = read_signed_leb128(&mut stream, pos);
+        let (value, bytes_read) = read_signed_leb128(&mut stream[pos..]);
         pos += bytes_read;
         assert_eq!(x, value);
     }

src/libserialize/lib.rs

@@ -15,6 +15,7 @@ Core encoding and decoding interfaces.
 #![feature(specialization)]
 #![feature(never_type)]
 #![feature(nll)]
+#![feature(ptr_wrapping_offset_from)]
 #![cfg_attr(test, feature(test))]
 
 pub use self::serialize::{Decoder, Encoder, Decodable, Encodable};

src/libserialize/opaque.rs

@@ -157,59 +157,55 @@ impl Encoder {
 // -----------------------------------------------------------------------------
 
 pub struct Decoder<'a> {
-    pub data: &'a [u8],
-    position: usize,
+    pub original_data: &'a [u8],
+    data: &'a [u8],
 }
 
 impl<'a> Decoder<'a> {
     #[inline]
-    pub fn new(data: &'a [u8], position: usize) -> Decoder<'a> {
+    pub fn new(data: &'a [u8], pos: usize) -> Decoder<'a> {
         Decoder {
-            data,
-            position,
+            original_data: data,
+            data: &data[pos..],
         }
     }
 
     #[inline]
-    pub fn position(&self) -> usize {
-        self.position
+    pub fn data(&self) -> &[u8] {
+        self.data
     }
 
     #[inline]
-    pub fn set_position(&mut self, pos: usize) {
-        self.position = pos
+    pub fn position(&self) -> usize {
+        self.data.as_ptr().wrapping_offset_from(self.original_data.as_ptr()) as usize
     }
 
     #[inline]
     pub fn advance(&mut self, bytes: usize) {
-        self.position += bytes;
+        self.data = &self.data[bytes..];
     }
 
     #[inline]
     pub fn read_raw_bytes(&mut self, s: &mut [u8]) -> Result<(), String> {
-        let start = self.position;
-        let end = start + s.len();
-
-        s.copy_from_slice(&self.data[start..end]);
-
-        self.position = end;
+        s.copy_from_slice(&self.data[..s.len()]);
+        self.advance(s.len());
 
         Ok(())
     }
 }
 
 macro_rules! read_uleb128 {
     ($dec:expr, $t:ty, $fun:ident) => ({
-        let (value, bytes_read) = leb128::$fun(&$dec.data[$dec.position ..]);
-        $dec.position += bytes_read;
+        let (value, bytes_read) = leb128::$fun(&$dec.data);
+        $dec.advance(bytes_read);
         Ok(value)
     })
 }
 
 macro_rules! read_sleb128 {
     ($dec:expr, $t:ty) => ({
-        let (value, bytes_read) = read_signed_leb128($dec.data, $dec.position);
-        $dec.position += bytes_read;
+        let (value, bytes_read) = read_signed_leb128($dec.data);
+        $dec.advance(bytes_read);
         Ok(value as $t)
     })
 }
@@ -245,8 +241,8 @@ impl<'a> serialize::Decoder for Decoder<'a> {
 
     #[inline]
     fn read_u8(&mut self) -> Result<u8, Self::Error> {
-        let value = self.data[self.position];
-        self.position += 1;
+        let value = self.data[0];
+        self.advance(1);
         Ok(value)
     }
 
@@ -277,8 +273,8 @@ impl<'a> serialize::Decoder for Decoder<'a> {
 
     #[inline]
     fn read_i8(&mut self) -> Result<i8, Self::Error> {
-        let as_u8 = self.data[self.position];
-        self.position += 1;
+        let as_u8 = self.data[0];
+        self.advance(1);
         unsafe { Ok(::std::mem::transmute(as_u8)) }
     }
 
@@ -314,8 +310,8 @@ impl<'a> serialize::Decoder for Decoder<'a> {
     #[inline]
     fn read_str(&mut self) -> Result<Cow<'_, str>, Self::Error> {
         let len = self.read_usize()?;
-        let s = ::std::str::from_utf8(&self.data[self.position..self.position + len]).unwrap();
-        self.position += len;
+        let s = ::std::str::from_utf8(&self.data[..len]).unwrap();
+        self.advance(len);
         Ok(Cow::Borrowed(s))
     }