Check if Rust lib needs rebuilding. (#13759)

This protects against the common mistake of failing to remember to rebuild Rust code after making changes.

rust/src/lib.rs

@@ -1,5 +1,13 @@
 use pyo3::prelude::*;
 
+/// Returns the hash of all the rust source files at the time it was compiled.
+///
+/// Used by python to detect if the rust library is outdated.
+#[pyfunction]
+fn get_rust_file_digest() -> &'static str {
+    env!("SYNAPSE_RUST_DIGEST")
+}
+
 /// Formats the sum of two numbers as string.
 #[pyfunction]
 #[pyo3(text_signature = "(a, b, /)")]
@@ -11,6 +19,6 @@ fn sum_as_string(a: usize, b: usize) -> PyResult<String> {
 #[pymodule]
 fn synapse_rust(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
     m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
-
+    m.add_function(wrap_pyfunction!(get_rust_file_digest, m)?)?;
     Ok(())
 }