Mmap the incremental data instead of reading it.

compiler/rustc_incremental/src/persist/file_format.rs

@@ -14,6 +14,7 @@ use std::fs;
 use std::io::{self, Read};
 use std::path::Path;
 
+use rustc_data_structures::memmap::Mmap;
 use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
 use rustc_serialize::Encoder;
 
@@ -54,14 +55,15 @@ pub fn read_file(
     report_incremental_info: bool,
     path: &Path,
     nightly_build: bool,
-) -> io::Result<Option<(Vec<u8>, usize)>> {
-    let data = match fs::read(path) {
-        Ok(data) => data,
+) -> io::Result<Option<(Mmap, usize)>> {
+    let file = match fs::File::open(path) {
+        Ok(file) => file,
         Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(None),
         Err(err) => return Err(err),
     };
+    let mmap = unsafe { Mmap::map(file) }?;
 
-    let mut file = io::Cursor::new(data);
+    let mut file = io::Cursor::new(&*mmap);
 
     // Check FILE_MAGIC
     {
@@ -103,7 +105,7 @@ pub fn read_file(
     }
 
     let post_header_start_pos = file.position() as usize;
-    Ok(Some((file.into_inner(), post_header_start_pos)))
+    Ok(Some((mmap, post_header_start_pos)))
 }
 
 fn report_format_mismatch(report_incremental_info: bool, file: &Path, message: &str) {

compiler/rustc_incremental/src/persist/load.rs

@@ -1,6 +1,7 @@
 //! Code to save/load the dep-graph from files.
 
 use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::memmap::Mmap;
 use rustc_middle::dep_graph::{SerializedDepGraph, WorkProduct, WorkProductId};
 use rustc_middle::ty::OnDiskCache;
 use rustc_serialize::opaque::Decoder;
@@ -48,7 +49,7 @@ fn load_data(
     report_incremental_info: bool,
     path: &Path,
     nightly_build: bool,
-) -> LoadResult<(Vec<u8>, usize)> {
+) -> LoadResult<(Mmap, usize)> {
     match file_format::read_file(report_incremental_info, path, nightly_build) {
         Ok(Some(data_and_pos)) => LoadResult::Ok { data: data_and_pos },
         Ok(None) => {