Use memmap for reading files

Cargo.lock

@@ -2170,6 +2170,7 @@ dependencies = [
  "indoc",
  "inlinable_string",
  "maplit",
+ "memmap",
  "pretty_assertions",
  "quickcheck",
  "quickcheck_macros",

compiler/load/Cargo.toml

@@ -19,6 +19,7 @@ roc_parse = { path = "../parse" }
 roc_solve = { path = "../solve" }
 bumpalo = { version = "3.2", features = ["collections"] }
 inlinable_string = "0.1"
+memmap = "0.7"
 tokio = { version = "0.2", features = ["blocking", "fs", "sync", "rt-threaded"] }
 
 [dev-dependencies]

compiler/load/src/file.rs

@@ -1,4 +1,5 @@
 use bumpalo::Bump;
+use memmap::MmapOptions;
 use roc_builtins::std::{Mode, StdLib};
 use roc_can::constraint::Constraint;
 use roc_can::def::Declaration;
@@ -19,9 +20,10 @@ use roc_solve::solve;
 use roc_types::solved_types::Solved;
 use roc_types::subs::{Subs, VarStore, Variable};
 use std::collections::{HashMap, HashSet};
-use std::fs::read_to_string;
+use std::fs::File;
 use std::io;
 use std::path::{Path, PathBuf};
+use std::str::from_utf8_unchecked;
 use std::sync::{Arc, Mutex};
 use tokio::sync::mpsc;
 use tokio::task::spawn_blocking;
@@ -526,16 +528,14 @@ fn load_module(
     load_filename(filename, msg_tx, module_ids)
 }
 
-/// Load a module by its filename
+fn parse_src(
-fn load_filename(
     filename: PathBuf,
     msg_tx: MsgSender,
     module_ids: SharedModules<'_, '_>,
+    src: &str
 ) -> Result<ModuleId, LoadingProblem> {
-    match read_to_string(&filename) {
+    let state = State::new(src, Attempting::Module);
-        Ok(src) => {
     let arena = Bump::new();
-            let state = State::new(&src, Attempting::Module);
 
     // TODO figure out if there's a way to address this clippy error
     // without introducing a borrow error. ("let and return" is literally
@@ -577,6 +577,32 @@ fn load_filename(
     };
 
     answer
+}
+
+/// Load a module by its filename
+///
+/// This has two unsafe calls:
+///
+/// * memory map the filename instead of doing a buffered read
+/// * assume the contents of the file are valid utf8
+fn load_filename(
+    filename: PathBuf,
+    msg_tx: MsgSender,
+    module_ids: SharedModules<'_, '_>,
+) -> Result<ModuleId, LoadingProblem> {
+    match File::open(&filename) {
+        Ok(file) => {
+            match unsafe { MmapOptions::new().map(&file) } {
+                Ok(mmap) => {
+                    let src = unsafe { from_utf8_unchecked(mmap.as_ref()) };
+
+                    parse_src(filename, msg_tx, module_ids, src)
+                }
+                Err(err) => Err(LoadingProblem::FileProblem {
+                    filename,
+                    error: err.kind(),
+                }),
+            }
         }
         Err(err) => Err(LoadingProblem::FileProblem {
             filename,