Use mmap for proc macro libs

Cargo.lock

@@ -675,6 +675,16 @@ version = "2.3.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3728d817d99e5ac407411fa471ff9800a778d88a24685968b36824eaf4bee400"
 
+[[package]]
+name = "memmap"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6585fd95e7bb50d6cc31e20d4cf9afb4e2ba16c5846fc76793f11218da9c475b"
+dependencies = [
+ "libc",
+ "winapi 0.3.8",
+]
+
 [[package]]
 name = "memoffset"
 version = "0.5.4"
@@ -1112,6 +1122,7 @@ dependencies = [
  "difference",
  "goblin",
  "libloading",
+ "memmap",
  "ra_mbe",
  "ra_proc_macro",
  "ra_tt",

crates/ra_proc_macro_srv/Cargo.toml

@@ -14,6 +14,7 @@ ra_mbe = { path = "../ra_mbe" }
 ra_proc_macro = { path = "../ra_proc_macro" }
 goblin = "0.2.1"
 libloading = "0.6.0"
+memmap = "0.7"
 test_utils = { path = "../test_utils" }
 
 [dev-dependencies]

crates/ra_proc_macro_srv/src/dylib.rs

@@ -1,10 +1,12 @@
 //! Handles dynamic library loading for proc macro
 
 use crate::{proc_macro::bridge, rustc_server::TokenStream};
+use std::fs::File;
 use std::path::Path;
 
 use goblin::{mach::Mach, Object};
 use libloading::Library;
+use memmap::Mmap;
 use ra_proc_macro::ProcMacroKind;
 
 use std::io::Error as IoError;
@@ -21,7 +23,8 @@ fn is_derive_registrar_symbol(symbol: &str) -> bool {
 }
 
 fn find_registrar_symbol(file: &Path) -> Result<Option<String>, IoError> {
-    let buffer = std::fs::read(file)?;
+    let file = File::open(file)?;
+    let buffer = unsafe { Mmap::map(&file)? };
     let object = Object::parse(&buffer).map_err(invalid_data_err)?;
 
     match object {
@@ -55,7 +58,7 @@ fn find_registrar_symbol(file: &Path) -> Result<Option<String>, IoError> {
                         &s.name
                     }
                 })
-                .find(|s| is_derive_registrar_symbol(&s))
+                .find(|s| is_derive_registrar_symbol(s))
                 .map(|s| s.to_string());
             Ok(name)
         }