diff --git a/crates/fayalite/Cargo.toml b/crates/fayalite/Cargo.toml
index 2652792..f176698 100644
--- a/crates/fayalite/Cargo.toml
+++ b/crates/fayalite/Cargo.toml
@@ -40,6 +40,7 @@ fayalite-visit-gen.workspace = true
 
 [features]
 unstable-doc = []
+unstable-test-hasher = []
 
 [package.metadata.docs.rs]
 features = ["unstable-doc"]
diff --git a/crates/fayalite/src/util.rs b/crates/fayalite/src/util.rs
index 8d90135..ebc3f6d 100644
--- a/crates/fayalite/src/util.rs
+++ b/crates/fayalite/src/util.rs
@@ -8,8 +8,12 @@ mod const_usize;
 mod misc;
 mod scoped_ref;
 pub(crate) mod streaming_read_utf8;
+mod test_hasher;
 
 // allow easily switching the hasher crate-wide for testing
+#[cfg(feature = "unstable-test-hasher")]
+pub type DefaultBuildHasher = test_hasher::DefaultBuildHasher;
+#[cfg(not(feature = "unstable-test-hasher"))]
 pub(crate) type DefaultBuildHasher = hashbrown::hash_map::DefaultHashBuilder;
 
 pub(crate) type HashMap<K, V> = hashbrown::HashMap<K, V, DefaultBuildHasher>;
diff --git a/crates/fayalite/src/util/test_hasher.rs b/crates/fayalite/src/util/test_hasher.rs
new file mode 100644
index 0000000..2a0cdd4
--- /dev/null
+++ b/crates/fayalite/src/util/test_hasher.rs
@@ -0,0 +1,240 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+#![cfg(feature = "unstable-test-hasher")]
+
+use std::{
+    fmt::Write as _,
+    hash::{BuildHasher, Hash, Hasher},
+    io::Write as _,
+    marker::PhantomData,
+    sync::LazyLock,
+};
+
+type BoxDynHasher = Box<dyn Hasher + Send + Sync>;
+type BoxDynBuildHasher = Box<dyn DynBuildHasherTrait + Send + Sync>;
+type BoxDynMakeBuildHasher = Box<dyn Fn() -> BoxDynBuildHasher + Send + Sync>;
+
+trait TryGetDynBuildHasher: Copy {
+    type Type;
+    fn try_get_make_build_hasher(self) -> Option<BoxDynMakeBuildHasher>;
+}
+
+impl<T> TryGetDynBuildHasher for PhantomData<T> {
+    type Type = T;
+    fn try_get_make_build_hasher(self) -> Option<BoxDynMakeBuildHasher> {
+        None
+    }
+}
+
+impl<T: Default + BuildHasher<Hasher: Send + Sync + 'static> + Send + Sync + 'static + Clone>
+    TryGetDynBuildHasher for &'_ PhantomData<T>
+{
+    type Type = T;
+    fn try_get_make_build_hasher(self) -> Option<BoxDynMakeBuildHasher> {
+        Some(Box::new(|| Box::<DynBuildHasher<T>>::default()))
+    }
+}
+
+#[derive(Default, Clone)]
+struct DynBuildHasher<T>(T);
+
+trait DynBuildHasherTrait: BuildHasher<Hasher = BoxDynHasher> {
+    fn clone_dyn_build_hasher(&self) -> BoxDynBuildHasher;
+}
+
+impl<BH: BuildHasher<Hasher: Send + Sync + 'static>> BuildHasher for DynBuildHasher<BH> {
+    type Hasher = BoxDynHasher;
+
+    fn build_hasher(&self) -> Self::Hasher {
+        Box::new(self.0.build_hasher())
+    }
+
+    fn hash_one<T: Hash>(&self, x: T) -> u64 {
+        self.0.hash_one(x)
+    }
+}
+
+impl<BH> DynBuildHasherTrait for DynBuildHasher<BH>
+where
+    Self: Clone + BuildHasher<Hasher = BoxDynHasher> + Send + Sync + 'static,
+{
+    fn clone_dyn_build_hasher(&self) -> BoxDynBuildHasher {
+        Box::new(self.clone())
+    }
+}
+
+pub struct DefaultBuildHasher(BoxDynBuildHasher);
+
+impl Clone for DefaultBuildHasher {
+    fn clone(&self) -> Self {
+        DefaultBuildHasher(self.0.clone_dyn_build_hasher())
+    }
+}
+
+const ENV_VAR_NAME: &'static str = "FAYALITE_TEST_HASHER";
+
+struct EnvVarValue {
+    key: &'static str,
+    try_get_make_build_hasher: fn() -> Option<BoxDynMakeBuildHasher>,
+    description: &'static str,
+}
+
+macro_rules! env_var_value {
+    (
+        key: $key:literal,
+        build_hasher: $build_hasher:ty,
+        description: $description:literal,
+    ) => {
+        EnvVarValue {
+            key: $key,
+            try_get_make_build_hasher: || {
+                // use rust method resolution to detect if $build_hasher is usable
+                // (e.g. hashbrown's hasher won't be usable without the right feature enabled)
+                (&PhantomData::<DynBuildHasher<$build_hasher>>).try_get_make_build_hasher()
+            },
+            description: $description,
+        }
+    };
+}
+
+#[derive(Default)]
+struct AlwaysZeroHasher;
+
+impl Hasher for AlwaysZeroHasher {
+    fn write(&mut self, _bytes: &[u8]) {}
+    fn finish(&self) -> u64 {
+        0
+    }
+}
+
+const ENV_VAR_VALUES: &'static [EnvVarValue] = &[
+    env_var_value! {
+        key: "std",
+        build_hasher: std::hash::RandomState,
+        description: "use std::hash::RandomState",
+    },
+    env_var_value! {
+        key: "hashbrown",
+        build_hasher: hashbrown::hash_map::DefaultHashBuilder,
+        description: "use hashbrown's DefaultHashBuilder",
+    },
+    env_var_value! {
+        key: "always_zero",
+        build_hasher: std::hash::BuildHasherDefault<AlwaysZeroHasher>,
+        description: "use a hasher that always returns 0 for all hashes,\n    \
+        this is useful for checking that PartialEq impls are correct",
+    },
+];
+
+fn report_bad_env_var(msg: impl std::fmt::Display) -> ! {
+    let mut msg = format!("{ENV_VAR_NAME}: {msg}\n");
+    for &EnvVarValue {
+        key,
+        try_get_make_build_hasher,
+        description,
+    } in ENV_VAR_VALUES
+    {
+        let availability = match try_get_make_build_hasher() {
+            Some(_) => "available",
+            None => "unavailable",
+        };
+        writeln!(msg, "{key}: ({availability})\n    {description}").expect("can't fail");
+    }
+    std::io::stderr()
+        .write_all(msg.as_bytes())
+        .expect("should be able to write to stderr");
+    std::process::abort();
+}
+
+impl Default for DefaultBuildHasher {
+    fn default() -> Self {
+        static DEFAULT_FN: LazyLock<BoxDynMakeBuildHasher> = LazyLock::new(|| {
+            let var = std::env::var_os(ENV_VAR_NAME);
+            let var = var.as_deref().unwrap_or("std".as_ref());
+            for &EnvVarValue {
+                key,
+                try_get_make_build_hasher,
+                description: _,
+            } in ENV_VAR_VALUES
+            {
+                if var.as_encoded_bytes().eq_ignore_ascii_case(key.as_bytes()) {
+                    return try_get_make_build_hasher().unwrap_or_else(|| {
+                        report_bad_env_var(format_args!(
+                            "unavailable hasher: {key} (is the appropriate feature enabled?)"
+                        ));
+                    });
+                }
+            }
+            report_bad_env_var(format_args!("unrecognized hasher: {var:?}"));
+        });
+        Self(DEFAULT_FN())
+    }
+}
+
+pub struct DefaultHasher(BoxDynHasher);
+
+impl BuildHasher for DefaultBuildHasher {
+    type Hasher = DefaultHasher;
+
+    fn build_hasher(&self) -> Self::Hasher {
+        DefaultHasher(self.0.build_hasher())
+    }
+}
+
+impl Hasher for DefaultHasher {
+    fn finish(&self) -> u64 {
+        self.0.finish()
+    }
+
+    fn write(&mut self, bytes: &[u8]) {
+        self.0.write(bytes)
+    }
+
+    fn write_u8(&mut self, i: u8) {
+        self.0.write_u8(i)
+    }
+
+    fn write_u16(&mut self, i: u16) {
+        self.0.write_u16(i)
+    }
+
+    fn write_u32(&mut self, i: u32) {
+        self.0.write_u32(i)
+    }
+
+    fn write_u64(&mut self, i: u64) {
+        self.0.write_u64(i)
+    }
+
+    fn write_u128(&mut self, i: u128) {
+        self.0.write_u128(i)
+    }
+
+    fn write_usize(&mut self, i: usize) {
+        self.0.write_usize(i)
+    }
+
+    fn write_i8(&mut self, i: i8) {
+        self.0.write_i8(i)
+    }
+
+    fn write_i16(&mut self, i: i16) {
+        self.0.write_i16(i)
+    }
+
+    fn write_i32(&mut self, i: i32) {
+        self.0.write_i32(i)
+    }
+
+    fn write_i64(&mut self, i: i64) {
+        self.0.write_i64(i)
+    }
+
+    fn write_i128(&mut self, i: i128) {
+        self.0.write_i128(i)
+    }
+
+    fn write_isize(&mut self, i: isize) {
+        self.0.write_isize(i)
+    }
+}