Switch some HashMaps to FxHashMaps

FxHashMap has a faster hashing algorithm,
at the expense of not being resistent to DOS
attacks.

Cargo.toml

@@ -18,6 +18,7 @@ description = "a ninja compatible build system"
 anyhow = "1.0"
 argh = "0.1.10"
 libc = "0.2"
+rustc-hash = "1.1.0"
 
 [target.'cfg(windows)'.dependencies.windows-sys]
 version = "0.48"

src/eval.rs

@@ -1,10 +1,11 @@
 //! Represents parsed Ninja strings with embedded variable references, e.g.
 //! `c++ $in -o $out`, and mechanisms for expanding those into plain strings.
 
+use rustc_hash::FxHashMap;
+
 use crate::smallmap::SmallMap;
 use std::borrow::Borrow;
 use std::borrow::Cow;
-use std::collections::HashMap;
 
 /// An environment providing a mapping of variable name to variable value.
 /// This represents one "frame" of evaluation context, a given EvalString may
@@ -123,7 +124,7 @@ impl EvalString<&str> {
 
 /// A single scope's worth of variable definitions.
 #[derive(Debug, Default)]
-pub struct Vars<'text>(HashMap<&'text str, String>);
+pub struct Vars<'text>(FxHashMap<&'text str, String>);
 
 impl<'text> Vars<'text> {
     pub fn insert(&mut self, key: &'text str, val: String) {

src/graph.rs

@@ -1,5 +1,7 @@
 //! The build graph, a graph between files and commands.
 
+use rustc_hash::FxHashMap;
+
 use crate::{
     densemap::{self, DenseMap},
     hash::BuildHash,
@@ -258,7 +260,7 @@ pub struct Graph {
 #[derive(Default)]
 pub struct GraphFiles {
     pub by_id: DenseMap<FileId, File>,
-    by_name: HashMap<String, FileId>,
+    by_name: FxHashMap<String, FileId>,
 }
 
 impl Graph {