Reimplement FileEncoder with a small-write optimization

Author: saethlin

compiler/rustc_metadata/src/rmeta/table.rs

@@ -5,7 +5,6 @@ use rustc_hir::def::{CtorKind, CtorOf};
 use rustc_index::Idx;
 use rustc_middle::ty::{ParameterizedOverTcx, UnusedGenericParams};
 use rustc_serialize::opaque::FileEncoder;
-use rustc_serialize::Encoder as _;
 use rustc_span::hygiene::MacroKind;
 use std::marker::PhantomData;
 use std::num::NonZeroUsize;
@@ -468,7 +467,10 @@ impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]>> TableBui
 
         let width = self.width;
         for block in &self.blocks {
-            buf.emit_raw_bytes(&block[..width]);
+            buf.write_with(|dest| {
+                *dest = *block;
+                width
+            });
         }
 
         LazyTable::from_position_and_encoded_size(

compiler/rustc_query_system/src/dep_graph/serialized.rs

@@ -394,16 +394,21 @@ struct NodeInfo<K: DepKind> {
 impl<K: DepKind> Encodable<FileEncoder> for NodeInfo<K> {
     fn encode(&self, e: &mut FileEncoder) {
         let header = SerializedNodeHeader::new(self);
-        e.emit_raw_bytes(&header.bytes);
+        e.write_with(|dest| {
+            *dest = header.bytes;
+            header.bytes.len()
+        });
 
         if header.len().is_none() {
             e.emit_usize(self.edges.len());
         }
 
         let bytes_per_index = header.bytes_per_index();
         for node_index in self.edges.iter() {
-            let bytes = node_index.as_u32().to_le_bytes();
-            e.emit_raw_bytes(&bytes[..bytes_per_index]);
+            e.write_with(|dest| {
+                *dest = node_index.as_u32().to_le_bytes();
+                bytes_per_index
+            });
         }
     }
 }

compiler/rustc_serialize/src/leb128.rs

@@ -15,31 +15,28 @@ pub const fn largest_max_leb128_len() -> usize {
 macro_rules! impl_write_unsigned_leb128 {
     ($fn_name:ident, $int_ty:ty) => {
         #[inline]
-        pub fn $fn_name(
-            out: &mut [::std::mem::MaybeUninit<u8>; max_leb128_len::<$int_ty>()],
-            mut value: $int_ty,
-        ) -> &[u8] {
+        pub fn $fn_name(out: &mut [u8; max_leb128_len::<$int_ty>()], mut value: $int_ty) -> usize {
             let mut i = 0;
 
             loop {
                 if value < 0x80 {
                     unsafe {
-                        *out.get_unchecked_mut(i).as_mut_ptr() = value as u8;
+                        *out.get_unchecked_mut(i) = value as u8;
                     }
 
                     i += 1;
                     break;
                 } else {
                     unsafe {
-                        *out.get_unchecked_mut(i).as_mut_ptr() = ((value & 0x7f) | 0x80) as u8;
+                        *out.get_unchecked_mut(i) = ((value & 0x7f) | 0x80) as u8;
                     }
 
                     value >>= 7;
                     i += 1;
                 }
             }
 
-            unsafe { ::std::mem::MaybeUninit::slice_assume_init_ref(&out.get_unchecked(..i)) }
+            i
         }
     };
 }
@@ -87,10 +84,7 @@ impl_read_unsigned_leb128!(read_usize_leb128, usize);
 macro_rules! impl_write_signed_leb128 {
     ($fn_name:ident, $int_ty:ty) => {
         #[inline]
-        pub fn $fn_name(
-            out: &mut [::std::mem::MaybeUninit<u8>; max_leb128_len::<$int_ty>()],
-            mut value: $int_ty,
-        ) -> &[u8] {
+        pub fn $fn_name(out: &mut [u8; max_leb128_len::<$int_ty>()], mut value: $int_ty) -> usize {
             let mut i = 0;
 
             loop {
@@ -104,7 +98,7 @@ macro_rules! impl_write_signed_leb128 {
                 }
 
                 unsafe {
-                    *out.get_unchecked_mut(i).as_mut_ptr() = byte;
+                    *out.get_unchecked_mut(i) = byte;
                 }
 
                 i += 1;
@@ -114,7 +108,7 @@ macro_rules! impl_write_signed_leb128 {
                 }
             }
 
-            unsafe { ::std::mem::MaybeUninit::slice_assume_init_ref(&out.get_unchecked(..i)) }
+            i
         }
     };
 }

compiler/rustc_serialize/src/lib.rs

@@ -17,6 +17,9 @@ Core encoding and decoding interfaces.
 #![feature(new_uninit)]
 #![feature(allocator_api)]
 #![feature(ptr_sub_ptr)]
+#![feature(slice_first_last_chunk)]
+#![feature(inline_const)]
+#![feature(const_option)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]
 #![deny(rustc::untranslatable_diagnostic)]