Merging upstream version 1.71.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

4 files changed, 223 insertions, 376 deletions

diff --git a/compiler/rustc_serialize/src/leb128.rs b/compiler/rustc_serialize/src/leb128.rs
index 7dad9aa01..e568b9e67 100644
--- a/compiler/rustc_serialize/src/leb128.rs
+++ b/compiler/rustc_serialize/src/leb128.rs
@@ -1,3 +1,6 @@
+use crate::opaque::MemDecoder;
+use crate::serialize::Decoder;
+
 /// Returns the length of the longest LEB128 encoding for `T`, assuming `T` is an integer type
 pub const fn max_leb128_len<T>() -> usize {
     // The longest LEB128 encoding for an integer uses 7 bits per byte.
@@ -50,21 +53,19 @@ impl_write_unsigned_leb128!(write_usize_leb128, usize);
 macro_rules! impl_read_unsigned_leb128 {
     ($fn_name:ident, $int_ty:ty) => {
         #[inline]
-        pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
+        pub fn $fn_name(decoder: &mut MemDecoder<'_>) -> $int_ty {
             // The first iteration of this loop is unpeeled. This is a
             // performance win because this code is hot and integer values less
             // than 128 are very common, typically occurring 50-80% or more of
             // the time, even for u64 and u128.
-            let byte = slice[*position];
-            *position += 1;
+            let byte = decoder.read_u8();
             if (byte & 0x80) == 0 {
                 return byte as $int_ty;
             }
             let mut result = (byte & 0x7F) as $int_ty;
             let mut shift = 7;
             loop {
-                let byte = slice[*position];
-                *position += 1;
+                let byte = decoder.read_u8();
                 if (byte & 0x80) == 0 {
                     result |= (byte as $int_ty) << shift;
                     return result;
@@ -127,14 +128,13 @@ impl_write_signed_leb128!(write_isize_leb128, isize);
 macro_rules! impl_read_signed_leb128 {
     ($fn_name:ident, $int_ty:ty) => {
         #[inline]
-        pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
+        pub fn $fn_name(decoder: &mut MemDecoder<'_>) -> $int_ty {
             let mut result = 0;
             let mut shift = 0;
             let mut byte;
 
             loop {
-                byte = slice[*position];
-                *position += 1;
+                byte = decoder.read_u8();
                 result |= <$int_ty>::from(byte & 0x7F) << shift;
                 shift += 7;
 
diff --git a/compiler/rustc_serialize/src/lib.rs b/compiler/rustc_serialize/src/lib.rs
index 1f8d2336c..ce8503918 100644
--- a/compiler/rustc_serialize/src/lib.rs
+++ b/compiler/rustc_serialize/src/lib.rs
@@ -16,6 +16,7 @@ Core encoding and decoding interfaces.
 #![feature(maybe_uninit_slice)]
 #![feature(new_uninit)]
 #![feature(allocator_api)]
+#![feature(ptr_sub_ptr)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]
 #![deny(rustc::untranslatable_diagnostic)]
diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index 53e5c8967..0ffc537ee 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -1,8 +1,10 @@
-use crate::leb128::{self, largest_max_leb128_len};
+use crate::leb128;
 use crate::serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::fs::File;
 use std::io::{self, Write};
+use std::marker::PhantomData;
 use std::mem::MaybeUninit;
+use std::ops::Range;
 use std::path::Path;
 use std::ptr;
 
@@ -10,147 +12,17 @@ use std::ptr;
 // Encoder
 // -----------------------------------------------------------------------------
 
-pub struct MemEncoder {
-    pub data: Vec<u8>,
-}
-
-impl MemEncoder {
-    pub fn new() -> MemEncoder {
-        MemEncoder { data: vec![] }
-    }
-
-    #[inline]
-    pub fn position(&self) -> usize {
-        self.data.len()
-    }
-
-    pub fn finish(self) -> Vec<u8> {
-        self.data
-    }
-}
-
-macro_rules! write_leb128 {
-    ($enc:expr, $value:expr, $int_ty:ty, $fun:ident) => {{
-        const MAX_ENCODED_LEN: usize = $crate::leb128::max_leb128_len::<$int_ty>();
-        let old_len = $enc.data.len();
-
-        if MAX_ENCODED_LEN > $enc.data.capacity() - old_len {
-            $enc.data.reserve(MAX_ENCODED_LEN);
-        }
-
-        // SAFETY: The above check and `reserve` ensures that there is enough
-        // room to write the encoded value to the vector's internal buffer.
-        unsafe {
-            let buf = &mut *($enc.data.as_mut_ptr().add(old_len)
-                as *mut [MaybeUninit<u8>; MAX_ENCODED_LEN]);
-            let encoded = leb128::$fun(buf, $value);
-            $enc.data.set_len(old_len + encoded.len());
-        }
-    }};
-}
-
-/// A byte that [cannot occur in UTF8 sequences][utf8]. Used to mark the end of a string.
-/// This way we can skip validation and still be relatively sure that deserialization
-/// did not desynchronize.
-///
-/// [utf8]: https://en.wikipedia.org/w/index.php?title=UTF-8&oldid=1058865525#Codepage_layout
-const STR_SENTINEL: u8 = 0xC1;
-
-impl Encoder for MemEncoder {
-    #[inline]
-    fn emit_usize(&mut self, v: usize) {
-        write_leb128!(self, v, usize, write_usize_leb128)
-    }
-
-    #[inline]
-    fn emit_u128(&mut self, v: u128) {
-        write_leb128!(self, v, u128, write_u128_leb128);
-    }
-
-    #[inline]
-    fn emit_u64(&mut self, v: u64) {
-        write_leb128!(self, v, u64, write_u64_leb128);
-    }
-
-    #[inline]
-    fn emit_u32(&mut self, v: u32) {
-        write_leb128!(self, v, u32, write_u32_leb128);
-    }
-
-    #[inline]
-    fn emit_u16(&mut self, v: u16) {
-        self.data.extend_from_slice(&v.to_le_bytes());
-    }
-
-    #[inline]
-    fn emit_u8(&mut self, v: u8) {
-        self.data.push(v);
-    }
-
-    #[inline]
-    fn emit_isize(&mut self, v: isize) {
-        write_leb128!(self, v, isize, write_isize_leb128)
-    }
-
-    #[inline]
-    fn emit_i128(&mut self, v: i128) {
-        write_leb128!(self, v, i128, write_i128_leb128)
-    }
-
-    #[inline]
-    fn emit_i64(&mut self, v: i64) {
-        write_leb128!(self, v, i64, write_i64_leb128)
-    }
-
-    #[inline]
-    fn emit_i32(&mut self, v: i32) {
-        write_leb128!(self, v, i32, write_i32_leb128)
-    }
-
-    #[inline]
-    fn emit_i16(&mut self, v: i16) {
-        self.data.extend_from_slice(&v.to_le_bytes());
-    }
-
-    #[inline]
-    fn emit_i8(&mut self, v: i8) {
-        self.emit_u8(v as u8);
-    }
-
-    #[inline]
-    fn emit_bool(&mut self, v: bool) {
-        self.emit_u8(if v { 1 } else { 0 });
-    }
-
-    #[inline]
-    fn emit_char(&mut self, v: char) {
-        self.emit_u32(v as u32);
-    }
-
-    #[inline]
-    fn emit_str(&mut self, v: &str) {
-        self.emit_usize(v.len());
-        self.emit_raw_bytes(v.as_bytes());
-        self.emit_u8(STR_SENTINEL);
-    }
-
-    #[inline]
-    fn emit_raw_bytes(&mut self, s: &[u8]) {
-        self.data.extend_from_slice(s);
-    }
-}
-
 pub type FileEncodeResult = Result<usize, io::Error>;
 
+/// The size of the buffer in `FileEncoder`.
+const BUF_SIZE: usize = 8192;
+
 /// `FileEncoder` encodes data to file via fixed-size buffer.
 ///
-/// When encoding large amounts of data to a file, using `FileEncoder` may be
-/// preferred over using `MemEncoder` to encode to a `Vec`, and then writing the
-/// `Vec` to file, as the latter uses as much memory as there is encoded data,
-/// while the former uses the fixed amount of memory allocated to the buffer.
-/// `FileEncoder` also has the advantage of not needing to reallocate as data
-/// is appended to it, but the disadvantage of requiring more error handling,
-/// which has some runtime overhead.
+/// There used to be a `MemEncoder` type that encoded all the data into a
+/// `Vec`. `FileEncoder` is better because its memory use is determined by the
+/// size of the buffer, rather than the full length of the encoded data, and
+/// because it doesn't need to reallocate memory along the way.
 pub struct FileEncoder {
     /// The input buffer. For adequate performance, we need more control over
     /// buffering than `BufWriter` offers. If `BufWriter` ever offers a raw
@@ -166,26 +38,12 @@ pub struct FileEncoder {
 
 impl FileEncoder {
     pub fn new<P: AsRef<Path>>(path: P) -> io::Result<Self> {
-        const DEFAULT_BUF_SIZE: usize = 8192;
-        FileEncoder::with_capacity(path, DEFAULT_BUF_SIZE)
-    }
-
-    pub fn with_capacity<P: AsRef<Path>>(path: P, capacity: usize) -> io::Result<Self> {
-        // Require capacity at least as large as the largest LEB128 encoding
-        // here, so that we don't have to check or handle this on every write.
-        assert!(capacity >= largest_max_leb128_len());
-
-        // Require capacity small enough such that some capacity checks can be
-        // done using guaranteed non-overflowing add rather than sub, which
-        // shaves an instruction off those code paths (on x86 at least).
-        assert!(capacity <= usize::MAX - largest_max_leb128_len());
-
         // Create the file for reading and writing, because some encoders do both
         // (e.g. the metadata encoder when -Zmeta-stats is enabled)
         let file = File::options().read(true).write(true).create(true).truncate(true).open(path)?;
 
         Ok(FileEncoder {
-            buf: Box::new_uninit_slice(capacity),
+            buf: Box::new_uninit_slice(BUF_SIZE),
             buffered: 0,
             flushed: 0,
             file,
@@ -291,18 +149,10 @@ impl FileEncoder {
     }
 
     #[inline]
-    fn capacity(&self) -> usize {
-        self.buf.len()
-    }
-
-    #[inline]
     fn write_one(&mut self, value: u8) {
-        // We ensure this during `FileEncoder` construction.
-        debug_assert!(self.capacity() >= 1);
-
         let mut buffered = self.buffered;
 
-        if std::intrinsics::unlikely(buffered >= self.capacity()) {
+        if std::intrinsics::unlikely(buffered + 1 > BUF_SIZE) {
             self.flush();
             buffered = 0;
         }
@@ -318,13 +168,12 @@ impl FileEncoder {
 
     #[inline]
     fn write_all(&mut self, buf: &[u8]) {
-        let capacity = self.capacity();
         let buf_len = buf.len();
 
-        if std::intrinsics::likely(buf_len <= capacity) {
+        if std::intrinsics::likely(buf_len <= BUF_SIZE) {
             let mut buffered = self.buffered;
 
-            if std::intrinsics::unlikely(buf_len > capacity - buffered) {
+            if std::intrinsics::unlikely(buffered + buf_len > BUF_SIZE) {
                 self.flush();
                 buffered = 0;
             }
@@ -396,52 +245,39 @@ impl Drop for FileEncoder {
     }
 }
 
-macro_rules! file_encoder_write_leb128 {
-    ($enc:expr, $value:expr, $int_ty:ty, $fun:ident) => {{
-        const MAX_ENCODED_LEN: usize = $crate::leb128::max_leb128_len::<$int_ty>();
-
-        // We ensure this during `FileEncoder` construction.
-        debug_assert!($enc.capacity() >= MAX_ENCODED_LEN);
+macro_rules! write_leb128 {
+    ($this_fn:ident, $int_ty:ty, $write_leb_fn:ident) => {
+        #[inline]
+        fn $this_fn(&mut self, v: $int_ty) {
+            const MAX_ENCODED_LEN: usize = $crate::leb128::max_leb128_len::<$int_ty>();
 
-        let mut buffered = $enc.buffered;
+            let mut buffered = self.buffered;
 
-        // This can't overflow. See assertion in `FileEncoder::with_capacity`.
-        if std::intrinsics::unlikely(buffered + MAX_ENCODED_LEN > $enc.capacity()) {
-            $enc.flush();
-            buffered = 0;
-        }
+            // This can't overflow because BUF_SIZE and MAX_ENCODED_LEN are both
+            // quite small.
+            if std::intrinsics::unlikely(buffered + MAX_ENCODED_LEN > BUF_SIZE) {
+                self.flush();
+                buffered = 0;
+            }
 
-        // SAFETY: The above check and flush ensures that there is enough
-        // room to write the encoded value to the buffer.
-        let buf = unsafe {
-            &mut *($enc.buf.as_mut_ptr().add(buffered) as *mut [MaybeUninit<u8>; MAX_ENCODED_LEN])
-        };
+            // SAFETY: The above check and flush ensures that there is enough
+            // room to write the encoded value to the buffer.
+            let buf = unsafe {
+                &mut *(self.buf.as_mut_ptr().add(buffered)
+                    as *mut [MaybeUninit<u8>; MAX_ENCODED_LEN])
+            };
 
-        let encoded = leb128::$fun(buf, $value);
-        $enc.buffered = buffered + encoded.len();
-    }};
+            let encoded = leb128::$write_leb_fn(buf, v);
+            self.buffered = buffered + encoded.len();
+        }
+    };
 }
 
 impl Encoder for FileEncoder {
-    #[inline]
-    fn emit_usize(&mut self, v: usize) {
-        file_encoder_write_leb128!(self, v, usize, write_usize_leb128)
-    }
-
-    #[inline]
-    fn emit_u128(&mut self, v: u128) {
-        file_encoder_write_leb128!(self, v, u128, write_u128_leb128)
-    }
-
-    #[inline]
-    fn emit_u64(&mut self, v: u64) {
-        file_encoder_write_leb128!(self, v, u64, write_u64_leb128)
-    }
-
-    #[inline]
-    fn emit_u32(&mut self, v: u32) {
-        file_encoder_write_leb128!(self, v, u32, write_u32_leb128)
-    }
+    write_leb128!(emit_usize, usize, write_usize_leb128);
+    write_leb128!(emit_u128, u128, write_u128_leb128);
+    write_leb128!(emit_u64, u64, write_u64_leb128);
+    write_leb128!(emit_u32, u32, write_u32_leb128);
 
     #[inline]
     fn emit_u16(&mut self, v: u16) {
@@ -453,25 +289,10 @@ impl Encoder for FileEncoder {
         self.write_one(v);
     }
 
-    #[inline]
-    fn emit_isize(&mut self, v: isize) {
-        file_encoder_write_leb128!(self, v, isize, write_isize_leb128)
-    }
-
-    #[inline]
-    fn emit_i128(&mut self, v: i128) {
-        file_encoder_write_leb128!(self, v, i128, write_i128_leb128)
-    }
-
-    #[inline]
-    fn emit_i64(&mut self, v: i64) {
-        file_encoder_write_leb128!(self, v, i64, write_i64_leb128)
-    }
-
-    #[inline]
-    fn emit_i32(&mut self, v: i32) {
-        file_encoder_write_leb128!(self, v, i32, write_i32_leb128)
-    }
+    write_leb128!(emit_isize, isize, write_isize_leb128);
+    write_leb128!(emit_i128, i128, write_i128_leb128);
+    write_leb128!(emit_i64, i64, write_i64_leb128);
+    write_leb128!(emit_i32, i32, write_i32_leb128);
 
     #[inline]
     fn emit_i16(&mut self, v: i16) {
@@ -479,28 +300,6 @@ impl Encoder for FileEncoder {
     }
 
     #[inline]
-    fn emit_i8(&mut self, v: i8) {
-        self.emit_u8(v as u8);
-    }
-
-    #[inline]
-    fn emit_bool(&mut self, v: bool) {
-        self.emit_u8(if v { 1 } else { 0 });
-    }
-
-    #[inline]
-    fn emit_char(&mut self, v: char) {
-        self.emit_u32(v as u32);
-    }
-
-    #[inline]
-    fn emit_str(&mut self, v: &str) {
-        self.emit_usize(v.len());
-        self.emit_raw_bytes(v.as_bytes());
-        self.emit_u8(STR_SENTINEL);
-    }
-
-    #[inline]
     fn emit_raw_bytes(&mut self, s: &[u8]) {
         self.write_all(s);
     }
@@ -510,137 +309,155 @@ impl Encoder for FileEncoder {
 // Decoder
 // -----------------------------------------------------------------------------
 
+// Conceptually, `MemDecoder` wraps a `&[u8]` with a cursor into it that is always valid.
+// This is implemented with three pointers, two which represent the original slice and a
+// third that is our cursor.
+// It is an invariant of this type that start <= current <= end.
+// Additionally, the implementation of this type never modifies start and end.
 pub struct MemDecoder<'a> {
-    pub data: &'a [u8],
-    position: usize,
+    start: *const u8,
+    current: *const u8,
+    end: *const u8,
+    _marker: PhantomData<&'a u8>,
 }
 
 impl<'a> MemDecoder<'a> {
     #[inline]
     pub fn new(data: &'a [u8], position: usize) -> MemDecoder<'a> {
-        MemDecoder { data, position }
+        let Range { start, end } = data.as_ptr_range();
+        MemDecoder { start, current: data[position..].as_ptr(), end, _marker: PhantomData }
     }
 
     #[inline]
-    pub fn position(&self) -> usize {
-        self.position
+    pub fn data(&self) -> &'a [u8] {
+        // SAFETY: This recovers the original slice, only using members we never modify.
+        unsafe { std::slice::from_raw_parts(self.start, self.len()) }
     }
 
     #[inline]
-    pub fn set_position(&mut self, pos: usize) {
-        self.position = pos
+    pub fn len(&self) -> usize {
+        // SAFETY: This recovers the length of the original slice, only using members we never modify.
+        unsafe { self.end.sub_ptr(self.start) }
     }
 
     #[inline]
-    pub fn advance(&mut self, bytes: usize) {
-        self.position += bytes;
+    pub fn remaining(&self) -> usize {
+        // SAFETY: This type guarantees current <= end.
+        unsafe { self.end.sub_ptr(self.current) }
     }
-}
-
-macro_rules! read_leb128 {
-    ($dec:expr, $fun:ident) => {{ leb128::$fun($dec.data, &mut $dec.position) }};
-}
 
-impl<'a> Decoder for MemDecoder<'a> {
-    #[inline]
-    fn read_u128(&mut self) -> u128 {
-        read_leb128!(self, read_u128_leb128)
+    #[cold]
+    #[inline(never)]
+    fn decoder_exhausted() -> ! {
+        panic!("MemDecoder exhausted")
     }
 
     #[inline]
-    fn read_u64(&mut self) -> u64 {
-        read_leb128!(self, read_u64_leb128)
+    fn read_array<const N: usize>(&mut self) -> [u8; N] {
+        self.read_raw_bytes(N).try_into().unwrap()
     }
 
+    /// While we could manually expose manipulation of the decoder position,
+    /// all current users of that method would need to reset the position later,
+    /// incurring the bounds check of set_position twice.
     #[inline]
-    fn read_u32(&mut self) -> u32 {
-        read_leb128!(self, read_u32_leb128)
-    }
+    pub fn with_position<F, T>(&mut self, pos: usize, func: F) -> T
+    where
+        F: Fn(&mut MemDecoder<'a>) -> T,
+    {
+        struct SetOnDrop<'a, 'guarded> {
+            decoder: &'guarded mut MemDecoder<'a>,
+            current: *const u8,
+        }
+        impl Drop for SetOnDrop<'_, '_> {
+            fn drop(&mut self) {
+                self.decoder.current = self.current;
+            }
+        }
 
-    #[inline]
-    fn read_u16(&mut self) -> u16 {
-        let bytes = [self.data[self.position], self.data[self.position + 1]];
-        let value = u16::from_le_bytes(bytes);
-        self.position += 2;
-        value
+        if pos >= self.len() {
+            Self::decoder_exhausted();
+        }
+        let previous = self.current;
+        // SAFETY: We just checked if this add is in-bounds above.
+        unsafe {
+            self.current = self.start.add(pos);
+        }
+        let guard = SetOnDrop { current: previous, decoder: self };
+        func(guard.decoder)
     }
+}
 
-    #[inline]
-    fn read_u8(&mut self) -> u8 {
-        let value = self.data[self.position];
-        self.position += 1;
-        value
-    }
+macro_rules! read_leb128 {
+    ($this_fn:ident, $int_ty:ty, $read_leb_fn:ident) => {
+        #[inline]
+        fn $this_fn(&mut self) -> $int_ty {
+            leb128::$read_leb_fn(self)
+        }
+    };
+}
 
-    #[inline]
-    fn read_usize(&mut self) -> usize {
-        read_leb128!(self, read_usize_leb128)
-    }
+impl<'a> Decoder for MemDecoder<'a> {
+    read_leb128!(read_usize, usize, read_usize_leb128);
+    read_leb128!(read_u128, u128, read_u128_leb128);
+    read_leb128!(read_u64, u64, read_u64_leb128);
+    read_leb128!(read_u32, u32, read_u32_leb128);
 
     #[inline]
-    fn read_i128(&mut self) -> i128 {
-        read_leb128!(self, read_i128_leb128)
+    fn read_u16(&mut self) -> u16 {
+        u16::from_le_bytes(self.read_array())
     }
 
     #[inline]
-    fn read_i64(&mut self) -> i64 {
-        read_leb128!(self, read_i64_leb128)
+    fn read_u8(&mut self) -> u8 {
+        if self.current == self.end {
+            Self::decoder_exhausted();
+        }
+        // SAFETY: This type guarantees current <= end, and we just checked current == end.
+        unsafe {
+            let byte = *self.current;
+            self.current = self.current.add(1);
+            byte
+        }
     }
 
-    #[inline]
-    fn read_i32(&mut self) -> i32 {
-        read_leb128!(self, read_i32_leb128)
-    }
+    read_leb128!(read_isize, isize, read_isize_leb128);
+    read_leb128!(read_i128, i128, read_i128_leb128);
+    read_leb128!(read_i64, i64, read_i64_leb128);
+    read_leb128!(read_i32, i32, read_i32_leb128);
 
     #[inline]
     fn read_i16(&mut self) -> i16 {
-        let bytes = [self.data[self.position], self.data[self.position + 1]];
-        let value = i16::from_le_bytes(bytes);
-        self.position += 2;
-        value
-    }
-
-    #[inline]
-    fn read_i8(&mut self) -> i8 {
-        let value = self.data[self.position];
-        self.position += 1;
-        value as i8
-    }
-
-    #[inline]
-    fn read_isize(&mut self) -> isize {
-        read_leb128!(self, read_isize_leb128)
+        i16::from_le_bytes(self.read_array())
     }
 
     #[inline]
-    fn read_bool(&mut self) -> bool {
-        let value = self.read_u8();
-        value != 0
-    }
-
-    #[inline]
-    fn read_char(&mut self) -> char {
-        let bits = self.read_u32();
-        std::char::from_u32(bits).unwrap()
+    fn read_raw_bytes(&mut self, bytes: usize) -> &'a [u8] {
+        if bytes > self.remaining() {
+            Self::decoder_exhausted();
+        }
+        // SAFETY: We just checked if this range is in-bounds above.
+        unsafe {
+            let slice = std::slice::from_raw_parts(self.current, bytes);
+            self.current = self.current.add(bytes);
+            slice
+        }
     }
 
     #[inline]
-    fn read_str(&mut self) -> &'a str {
-        let len = self.read_usize();
-        let sentinel = self.data[self.position + len];
-        assert!(sentinel == STR_SENTINEL);
-        let s = unsafe {
-            std::str::from_utf8_unchecked(&self.data[self.position..self.position + len])
-        };
-        self.position += len + 1;
-        s
+    fn peek_byte(&self) -> u8 {
+        if self.current == self.end {
+            Self::decoder_exhausted();
+        }
+        // SAFETY: This type guarantees current is inbounds or one-past-the-end, which is end.
+        // Since we just checked current == end, the current pointer must be inbounds.
+        unsafe { *self.current }
     }
 
     #[inline]
-    fn read_raw_bytes(&mut self, bytes: usize) -> &'a [u8] {
-        let start = self.position;
-        self.position += bytes;
-        &self.data[start..self.position]
+    fn position(&self) -> usize {
+        // SAFETY: This type guarantees start <= current
+        unsafe { self.current.sub_ptr(self.start) }
     }
 }
 
@@ -651,13 +468,6 @@ impl<'a> Decoder for MemDecoder<'a> {
 
 // Specialize encoding byte slices. This specialization also applies to encoding `Vec<u8>`s, etc.,
 // since the default implementations call `encode` on their slices internally.
-impl Encodable<MemEncoder> for [u8] {
-    fn encode(&self, e: &mut MemEncoder) {
-        Encoder::emit_usize(e, self.len());
-        e.emit_raw_bytes(self);
-    }
-}
-
 impl Encodable<FileEncoder> for [u8] {
     fn encode(&self, e: &mut FileEncoder) {
         Encoder::emit_usize(e, self.len());
@@ -681,16 +491,6 @@ impl IntEncodedWithFixedSize {
     pub const ENCODED_SIZE: usize = 8;
 }
 
-impl Encodable<MemEncoder> for IntEncodedWithFixedSize {
-    #[inline]
-    fn encode(&self, e: &mut MemEncoder) {
-        let _start_pos = e.position();
-        e.emit_raw_bytes(&self.0.to_le_bytes());
-        let _end_pos = e.position();
-        debug_assert_eq!((_end_pos - _start_pos), IntEncodedWithFixedSize::ENCODED_SIZE);
-    }
-}
-
 impl Encodable<FileEncoder> for IntEncodedWithFixedSize {
     #[inline]
     fn encode(&self, e: &mut FileEncoder) {
@@ -704,12 +504,7 @@ impl Encodable<FileEncoder> for IntEncodedWithFixedSize {
 impl<'a> Decodable<MemDecoder<'a>> for IntEncodedWithFixedSize {
     #[inline]
     fn decode(decoder: &mut MemDecoder<'a>) -> IntEncodedWithFixedSize {
-        let _start_pos = decoder.position();
-        let bytes = decoder.read_raw_bytes(IntEncodedWithFixedSize::ENCODED_SIZE);
-        let value = u64::from_le_bytes(bytes.try_into().unwrap());
-        let _end_pos = decoder.position();
-        debug_assert_eq!((_end_pos - _start_pos), IntEncodedWithFixedSize::ENCODED_SIZE);
-
-        IntEncodedWithFixedSize(value)
+        let bytes = decoder.read_array::<{ IntEncodedWithFixedSize::ENCODED_SIZE }>();
+        IntEncodedWithFixedSize(u64::from_le_bytes(bytes))
     }
 }
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index 527abc237..06166cabc 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -1,9 +1,5 @@
 //! Support code for encoding and decoding types.
 
-/*
-Core encoding and decoding interfaces.
-*/
-
 use std::alloc::Allocator;
 use std::borrow::Cow;
 use std::cell::{Cell, RefCell};
@@ -12,6 +8,13 @@ use std::path;
 use std::rc::Rc;
 use std::sync::Arc;
 
+/// A byte that [cannot occur in UTF8 sequences][utf8]. Used to mark the end of a string.
+/// This way we can skip validation and still be relatively sure that deserialization
+/// did not desynchronize.
+///
+/// [utf8]: https://en.wikipedia.org/w/index.php?title=UTF-8&oldid=1058865525#Codepage_layout
+const STR_SENTINEL: u8 = 0xC1;
+
 /// A note about error handling.
 ///
 /// Encoders may be fallible, but in practice failure is rare and there are so
@@ -28,22 +31,41 @@ use std::sync::Arc;
 /// really makes sense to store floating-point values at all.
 /// (If you need it, revert <https://github.com/rust-lang/rust/pull/109984>.)
 pub trait Encoder {
-    // Primitive types:
     fn emit_usize(&mut self, v: usize);
     fn emit_u128(&mut self, v: u128);
     fn emit_u64(&mut self, v: u64);
     fn emit_u32(&mut self, v: u32);
     fn emit_u16(&mut self, v: u16);
     fn emit_u8(&mut self, v: u8);
+
     fn emit_isize(&mut self, v: isize);
     fn emit_i128(&mut self, v: i128);
     fn emit_i64(&mut self, v: i64);
     fn emit_i32(&mut self, v: i32);
     fn emit_i16(&mut self, v: i16);
-    fn emit_i8(&mut self, v: i8);
-    fn emit_bool(&mut self, v: bool);
-    fn emit_char(&mut self, v: char);
-    fn emit_str(&mut self, v: &str);
+
+    #[inline]
+    fn emit_i8(&mut self, v: i8) {
+        self.emit_u8(v as u8);
+    }
+
+    #[inline]
+    fn emit_bool(&mut self, v: bool) {
+        self.emit_u8(if v { 1 } else { 0 });
+    }
+
+    #[inline]
+    fn emit_char(&mut self, v: char) {
+        self.emit_u32(v as u32);
+    }
+
+    #[inline]
+    fn emit_str(&mut self, v: &str) {
+        self.emit_usize(v.len());
+        self.emit_raw_bytes(v.as_bytes());
+        self.emit_u8(STR_SENTINEL);
+    }
+
     fn emit_raw_bytes(&mut self, s: &[u8]);
 
     fn emit_enum_variant<F>(&mut self, v_id: usize, f: F)
@@ -67,23 +89,52 @@ pub trait Encoder {
 /// really makes sense to store floating-point values at all.
 /// (If you need it, revert <https://github.com/rust-lang/rust/pull/109984>.)
 pub trait Decoder {
-    // Primitive types:
     fn read_usize(&mut self) -> usize;
     fn read_u128(&mut self) -> u128;
     fn read_u64(&mut self) -> u64;
     fn read_u32(&mut self) -> u32;
     fn read_u16(&mut self) -> u16;
     fn read_u8(&mut self) -> u8;
+
     fn read_isize(&mut self) -> isize;
     fn read_i128(&mut self) -> i128;
     fn read_i64(&mut self) -> i64;
     fn read_i32(&mut self) -> i32;
     fn read_i16(&mut self) -> i16;
-    fn read_i8(&mut self) -> i8;
-    fn read_bool(&mut self) -> bool;
-    fn read_char(&mut self) -> char;
-    fn read_str(&mut self) -> &str;
+
+    #[inline]
+    fn read_i8(&mut self) -> i8 {
+        self.read_u8() as i8
+    }
+
+    #[inline]
+    fn read_bool(&mut self) -> bool {
+        let value = self.read_u8();
+        value != 0
+    }
+
+    #[inline]
+    fn read_char(&mut self) -> char {
+        let bits = self.read_u32();
+        std::char::from_u32(bits).unwrap()
+    }
+
+    #[inline]
+    fn read_str(&mut self) -> &str {
+        let len = self.read_usize();
+        let bytes = self.read_raw_bytes(len + 1);
+        assert!(bytes[len] == STR_SENTINEL);
+        unsafe { std::str::from_utf8_unchecked(&bytes[..len]) }
+    }
+
     fn read_raw_bytes(&mut self, len: usize) -> &[u8];
+
+    // Although there is an `emit_enum_variant` method in `Encoder`, the code
+    // patterns in decoding are different enough to encoding that there is no
+    // need for a corresponding `read_enum_variant` method here.
+
+    fn peek_byte(&self) -> u8;
+    fn position(&self) -> usize;
 }
 
 /// Trait for types that can be serialized