Remove stable_hash feature and use environment variable.

CHANGELOG.md

@@ -7,7 +7,9 @@ Version 1.11.0
 New features
 ------------
 
-* A new feature flag, `stable_hash`, is added that forces hashing to be consistent using a fixed seed.
+### Stable hashing
+
+* It is now possible to specify a fixed _seed_ for use with the `ahash` hasher in order to force stable (i.e. deterministic) hashes for function signatures.
 
 Enhancements
 ------------

Cargo.toml

@@ -62,7 +62,6 @@ metadata = ["serde", "serde_json", "rhai_codegen/metadata", "smartstring/serde"]
 internals = []                  # expose internal data structures
 debugging = ["internals"]       # enable debugging
 serde = ["dep:serde", "smartstring/serde", "smallvec/serde"] # implement serde for rhai types
-stable_hash = []                # perform all hashing with fixed seed value
 
 # compiling for no-std
 no_std = ["no-std-compat", "num-traits/libm", "core-error", "libm", "ahash/compile-time-rng", "hashbrown/ahash-compile-time-rng"]

build.rs

@@ -0,0 +1,27 @@
+use std::{env, fs::File, io::Write};
+
+const WRITE_ERROR: &str = "cannot write to `config.rs`";
+
+fn main() {
+    // Tell Cargo that if the given environment variable changes, to rerun this build script.
+    println!("cargo:rerun-if-env-changed=RHAI_AHASH_SEED");
+
+    let mut f = File::create("src/config.rs").expect("cannot create `config.rs`");
+
+    f.write_fmt(format_args!(
+        "//! Configuration settings for this Rhai build
+#![allow(dead_code)]
+
+"
+    ))
+    .expect(WRITE_ERROR);
+
+    let seed = env::var("RHAI_AHASH_SEED").map_or_else(|_| "None".into(), |s| format!("Some({s})"));
+
+    f.write_fmt(format_args!(
+        "pub const AHASH_SEED: Option<[u64; 4]> = {seed};\n"
+    ))
+    .expect(WRITE_ERROR);
+
+    f.flush().expect("cannot flush `config.rs`");
+}

src/config.rs

@@ -0,0 +1,4 @@
+//! Configuration settings for this Rhai build
+#![allow(dead_code)]
+
+pub const AHASH_SEED: Option<[u64; 4]> = None;

src/func/hashing.rs

@@ -1,5 +1,6 @@
 //! Module containing utilities to hash functions and function calls.
 
+use crate::config;
 #[cfg(feature = "no_std")]
 use std::prelude::v1::*;
 use std::{
@@ -74,8 +75,8 @@ impl BuildHasher for StraightHasherBuilder {
 #[inline(always)]
 #[must_use]
 pub fn get_hasher() -> ahash::AHasher {
-    if cfg!(feature = "stable_hash") {
-        ahash::RandomState::with_seeds(42, 999, 123, 0).build_hasher()
+    if let Some([seed1, seed2, seed3, seed4]) = config::AHASH_SEED {
+        ahash::RandomState::with_seeds(seed1, seed2, seed3, seed4).build_hasher()
     } else {
         ahash::AHasher::default()
     }

src/lib.rs

@@ -83,6 +83,7 @@ use std::prelude::v1::*;
 // Internal modules
 mod api;
 mod ast;
+mod config;
 mod engine;
 mod eval;
 mod func;
@@ -224,7 +225,7 @@ pub mod debugger {
 /// An identifier in Rhai. [`SmartString`](https://crates.io/crates/smartstring) is used because most
 /// identifiers are ASCII and short, fewer than 23 characters, so they can be stored inline.
 #[cfg(not(feature = "internals"))]
-pub(crate) type Identifier = SmartString;
+type Identifier = SmartString;
 
 /// An identifier in Rhai. [`SmartString`](https://crates.io/crates/smartstring) is used because most
 /// identifiers are ASCII and short, fewer than 23 characters, so they can be stored inline.
@@ -237,7 +238,7 @@ pub use func::Shared;
 /// Alias to [`RefCell`][std::cell::RefCell] or [`RwLock`][std::sync::RwLock] depending on the `sync` feature flag.
 pub use func::Locked;
 
-pub(crate) use func::{calc_fn_hash, calc_fn_params_hash, calc_var_hash, combine_hashes};
+use func::{calc_fn_hash, calc_fn_params_hash, calc_var_hash, combine_hashes};
 
 pub use rhai_codegen::*;
 
@@ -429,7 +430,7 @@ type FnArgsVec<T> = smallvec::SmallVec<[T; 5]>;
 #[cfg(feature = "no_closure")]
 type FnArgsVec<T> = crate::StaticVec<T>;
 
-pub(crate) type SmartString = smartstring::SmartString<smartstring::LazyCompact>;
+type SmartString = smartstring::SmartString<smartstring::LazyCompact>;
 
 // Compiler guards against mutually-exclusive feature flags