Do not use stable hashing with all-zero seed.

CHANGELOG.md

@@ -9,7 +9,8 @@ New features
 
 ### 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.
+* It is now possible to specify a fixed _seed_ for use with the `ahash` hasher, via an environment variable, in order to force stable (i.e. deterministic) hashes for function signatures. This is necessary when using Rhai across shared-library boundaries.
+* A build script is now used to extract the environment variable (`RHAI_AHASH_SEED`) and splice it into the source code before compilation.
 
 Enhancements
 ------------

build.rs

@@ -10,7 +10,6 @@ fn main() {
 
     f.write_fmt(format_args!(
         "//! Configuration settings for this Rhai build
-#![allow(dead_code)]
 
 "
     ))

src/config.rs

@@ -1,4 +1,3 @@
 //! Configuration settings for this Rhai build
-#![allow(dead_code)]
 
 pub const AHASH_SEED: Option<[u64; 4]> = None;

src/func/hashing.rs

@@ -76,10 +76,14 @@ impl BuildHasher for StraightHasherBuilder {
 #[must_use]
 pub fn get_hasher() -> ahash::AHasher {
     if let Some([seed1, seed2, seed3, seed4]) = config::AHASH_SEED {
+        if seed1 | seed2 | seed3 | seed4 != 0 {
             ahash::RandomState::with_seeds(seed1, seed2, seed3, seed4).build_hasher()
         } else {
             ahash::AHasher::default()
         }
+    } else {
+        ahash::AHasher::default()
+    }
 }
 
 /// Calculate a non-zero [`u64`] hash key from a namespace-qualified variable name.