perf(toml): Allow opting in to faster hasher (#956)

I chose `foldhash` because it is the default of `[email protected]`
(replacing ahash), see rust-lang/hashbrown#563.

Currently, `ahash` is the only hasher I noticed inside of Cargo and that
is coming in from `[email protected]`.

In the future, we could consider allowing the use of a `FixedState` so
we could offer `IndexMap` on `alloc`.

```
$ cargo bench --bench 0-cargo -Fpreserve_order -- 0_cargo::toml::document::2-features
   Compiling toml v0.5.11
   Compiling toml v0.8.23 (/home/epage/src/personal/toml/crates/toml)
   Compiling toml_benchmarks v0.0.0 (/home/epage/src/personal/toml/crates/benchmarks)
    Finished `bench` profile [optimized] target(s) in 19.13s
     Running benches/0-cargo.rs (/home/epage/src/personal/toml/target/release/deps/0_cargo-09d7e00849b1b878)
Timer precision: 11 ns
0_cargo              fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ toml                            │               │               │               │         │
   ╰─ document                     │               │               │               │         │
      ╰─ 2-features  468.8 µs      │ 1.016 ms      │ 513.5 µs      │ 543.5 µs      │ 100     │ 100


$ cargo bench --bench 0-cargo -Ffast_hash -- 0_cargo::toml::document::2-features
    Finished `bench` profile [optimized] target(s) in 0.02s
     Running benches/0-cargo.rs (/home/epage/src/personal/toml/target/release/deps/0_cargo-ea11bd9a52599c35)
Timer precision: 18 ns
0_cargo              fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ toml                            │               │               │               │         │
   ╰─ document                     │               │               │               │         │
      ╰─ 2-features  431.4 µs      │ 870.3 µs      │ 446.2 µs      │ 459.9 µs      │ 100     │ 100
```

Author: epage

Cargo.lock

@@ -13,6 +13,7 @@ default = []
 simd = ["toml_parse/simd"]
 unsafe = ["toml_parse/unsafe", "toml_edit/unsafe"]
 preserve_order = ["toml_old/preserve_order", "toml/preserve_order"]
+fast_hash = ["toml/fast_hash"]
 
 [dependencies]
 serde = { version = "1.0.197", features = ["derive"] }

crates/benchmarks/Cargo.toml

@@ -33,6 +33,7 @@ std = ["indexmap?/std"]
 parse = ["dep:toml_parse", "dep:winnow"]
 display = ["std", "dep:toml_edit", "toml_edit?/display"]
 unsafe = ["toml_parse?/unsafe"]
+fast_hash = ["preserve_order", "dep:foldhash"]
 debug = ["std", "toml_parse?/debug", "dep:anstream", "dep:anstyle"]
 
 # Provide a method disable_recursion_limit to parse arbitrarily deep structures
@@ -57,6 +58,7 @@ anstyle = { version = "1.0.8", optional = true }
 toml_edit = { version = "0.22.27", path = "../toml_edit", default-features = false, features = ["serde"], optional = true }
 toml_datetime = { version = "0.6.11", path = "../toml_datetime", default-features = false, features = ["serde", "alloc"] }
 serde_spanned = { version = "0.6.9", path = "../serde_spanned", default-features = false, features = ["serde", "alloc"] }
+foldhash = { version = "0.1.5", default-features = false, optional = true }
 
 [dev-dependencies]
 serde = { version = "1.0.199", features = ["derive"] }

crates/toml/Cargo.toml

@@ -40,8 +40,12 @@ pub struct Map<K, V> {
 
 #[cfg(not(feature = "preserve_order"))]
 type MapImpl<K, V> = BTreeMap<K, V>;
+#[cfg(all(feature = "preserve_order", not(feature = "fast_hash")))]
+type RandomState = std::collections::hash_map::RandomState;
+#[cfg(all(feature = "preserve_order", feature = "fast_hash"))]
+type RandomState = foldhash::fast::RandomState;
 #[cfg(feature = "preserve_order")]
-type MapImpl<K, V> = IndexMap<K, V>;
+type MapImpl<K, V> = IndexMap<K, V, RandomState>;
 
 impl<K, V> Map<K, V>
 where
@@ -51,6 +55,9 @@ where
     #[inline]
     pub fn new() -> Self {
         Map {
+            #[cfg(feature = "preserve_order")]
+            map: MapImpl::with_hasher(RandomState::default()),
+            #[cfg(not(feature = "preserve_order"))]
             map: MapImpl::new(),
             dotted: false,
             implicit: false,
@@ -64,20 +71,15 @@ where
     pub fn with_capacity(capacity: usize) -> Self {
         // does not support with_capacity
         let _ = capacity;
-        Map {
-            map: BTreeMap::new(),
-            dotted: false,
-            implicit: false,
-            inline: false,
-        }
+        Self::new()
     }
 
     #[cfg(feature = "preserve_order")]
     /// Makes a new empty Map with the given initial capacity.
     #[inline]
     pub fn with_capacity(capacity: usize) -> Self {
         Map {
-            map: IndexMap::with_capacity(capacity),
+            map: IndexMap::with_capacity_and_hasher(capacity, RandomState::default()),
             dotted: false,
             implicit: false,
             inline: false,
@@ -298,15 +300,13 @@ where
     }
 }
 
-impl<K, V> Default for Map<K, V> {
+impl<K, V> Default for Map<K, V>
+where
+    K: Ord + Hash,
+{
     #[inline]
     fn default() -> Self {
-        Map {
-            map: MapImpl::new(),
-            dotted: false,
-            implicit: false,
-            inline: false,
-        }
+        Self::new()
     }
 }